Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Repurposing anti-cancer porphyrin derivative drugs to target SARS-CoV-2 envelope

Antiviral medicines to treat COVID-19 are still scarce. Porphyrins and porphyrin derivatives (PDs) usually present broad-spectrum antiviral activity with low risk of resistance development. In fact, some PDs are clinically approved to be used in anti-cancer photodynamic therapy and repurposing clinically approved PDs might be an alternative to treat COVID-19. Here, we characterize the ability of temoporfin, verteporfin, talaporfin and redaporfin to inactivate SARS-CoV-2 infectious particles. PDs light-dependent and -independent effect on SARS-CoV-2 infectivity were evaluated. PDs photoactivation successfully inactivated SARS-CoV-2 with very low concentrations and light dose. However, only temoporfin and verteporfin inactivated SARS-CoV-2 in the dark, being verteporfin the most effective. PDs treatment reduced viral load in infected Caco-2 cells, while not inducing cytotoxicity. Furthermore, light-independent treatment with temoporfin and verteporfin act on early stages of viral infection. Using lipid vehicles as membrane models, we characterized PDs interaction to the viral envelope. Verteporfin presented the lowest IC50 for viral inactivation and the highest partition coefficients (Kp) towards lipid bilayers. Curiously, although temoporfin and redaporfin presented similar Kps, redaporfin did not present light-independent antiviral activity, and only temoporfin and verteporfin caused lipid membrane disorder. In fact, redaporfin is located closer to the bilayer surface, while temoporfin and verteporfin are located closer to the centre. Our results suggest that viral envelope affinity, with penetration and destabilization of the lipid bilayer, seems critical to mediate PDs antiviral activity. Altogether, these findings open new avenues for the off-label application of temoporfin and verteporfin in the systemic treatment of COVID-19.

Interaction of the antifungal ketoconazole and its diphenylphosphine derivatives with lipid bilayers: Insights into their antifungal action

Ketoconazole (Ke) is an important antifungal drug, and two of its diphenylphosphinemethyl derivatives (KeP: Ph2PCH2-Ke and KeOP: Ph2P(O)CH2-Ke) have shown improved antifungal activity, namely against a yeast strain lacking ergosterol, suggesting alternative modes of action for azole compounds. In this context, the interactions of these compounds with a model of the cell membrane were investigated, using POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) large unilamellar vesicles and taking advantage of the intrinsic fluorescence of Ke, KeP and KeOP. Steady-state fluorescence spectra and anisotropy, including partition and aggregation studies, as well as fluorescence lifetime measurements, were carried out. In addition, the ability of the compounds to increase membrane permeability was assessed through carboxyfluorescein leakage. The membrane/water mole fraction partition coefficients (Kp,x): (3.31 ± 0.36) x105, (8.31 ± 1.60) x105 and (4.66 ± 0.72) x106, for Ke, KeP and KeOP, respectively, show that all three compounds have moderate to high affinity for the lipid bilayer. Moreover, KeP, and particularly KeOP interact more efficiently with POPC bilayers than Ke, which correlates well with their in vitro antifungal activity. Furthermore, although the three compounds disturb the lipid bilayer, KeOP is the quickest and most efficient one. Hence, the higher affinity and ability to permeabilize the membrane of KeOP when compared to that of KeP, despite the higher lipophilicity of the latter, points to an important role of Ph2P(O)CH2- oxygen. Overall, this work suggests that membrane interactions are important for the antifungal activity of these azoles and should be considered in the design of new therapeutic agents.

Real-Time Monitoring of Mitochondrial Damage Using Conjugated Oligoelectrolytes

Conjugated oligoelectrolytes (COEs) comprise a class of fluorescent reporters with tunable optical properties and lipid bilayer affinity. These molecules have proven effective in a range of bioimaging applications; however, their use in characterizing specific subcellular structures remains restricted. Such capabilities would broaden COE applications to understand cellular dysfunction, cell communication, and the targets of different pharmaceutical agents. Here, we disclose a novel COE derivative, COE-CN, which enables the visualization of mitochondria, including morphological changes and lysosomal fusion upon treatment with depolarizing agents. COE-CN is characterized by the presence of imidazolium solubilizing groups and an optically active cyanovinyl-linked distyrylbenzene core with intramolecular charge-transfer characteristics. Our current understanding is that the relatively shorter molecular length of COE-CN leads to weaker binding within lipid bilayer membranes, which allows sampling of internal cellular structures and ultimately to different localization relative to elongated COEs. As a means of practical demonstration, COE-CN can be used to diagnose cells with damaged mitochondria via flow cytometry. Coupled with an elongated COE that does not translocate upon depolarization, changes in ratiometric fluorescence intensity can be used to monitor mitochondrial membrane potential disruption, demonstrating the potential for use in diagnostic assays.

Lipophilicity Modulations by Fluorination Correlate with Membrane Partitioning

Bioactive compounds generally need to cross membranes to arrive at their site of action. The octanol-water partition coefficient (lipophilicity, logPOW ) has proven to be an excellent proxy for membrane permeability. In modern drug discovery, logPOW and bioactivity are optimized simultaneously, for which fluorination is one of the relevant strategies. The question arises as to which extent the often subtle logP modifications resulting from different aliphatic fluorine-motif introductions also lead to concomitant membrane permeability changes, given the difference in molecular environment between octanol and (anisotropic) membranes. It was found that for a given compound class, there is excellent correlation between logPOW values with the corresponding membrane molar partitioning coefficients (logKp ); a study enabled by novel solid-state 19 F NMR MAS methodology using lipid vesicles. Our results show that the factors that cause modulation of octanol-water partition coefficients similarly affect membrane permeability.

Water Maintains the UV-Vis Spectral Features During the Insertion of Anionic Naproxen and Ibuprofen into Model Cell Membranes

UV-vis spectra of anionic ibuprofen and naproxen in a model lipid bilayer of the cell membrane are investigated using computational techniques in combination with a comparative analysis of drug spectra in purely aqueous environments. The simulations aim at elucidating the intricacies behind the negligible changes in the maximum absorption wavelength in the experimental spectra. A set of configurations of the systems constituted by lipid, water, and drugs or just water and drugs are obtained from classical Molecular Dynamics simulations. UV-vis spectra are computed in the framework of atomistic Quantum Mechanical/Molecular Mechanics (QM/MM) approaches together with Time-Dependent Density Functional Theory (TD-DFT). Our results suggest that the molecular orbitals involved in the electronic transitions are the same, regardless of the chemical environment. A thorough analysis of the contacts between the drug and water molecules reveals that no significant changes in UV-vis spectra are a consequence of ibuprofen and naproxen molecules being permanently microsolvated by water molecules, despite the presence of lipid molecules. Water molecules microsolvate the charged carboxylate group as expected but also microsolvate the aromatic regions of the drugs.

Quantitative characterization of membrane-protein reversible association using FCS

Functionally meaningful reversible protein-membrane interactions mediate many biological events. Fluorescence correlation spectroscopy (FCS) is increasingly used to quantitatively study the non-reversible binding of proteins to membranes using lipid vesicles in solution. However, the lack of a complete description of the phase and statistical equilibria in the case of reversible protein-membrane partitioning has hampered the application of FCS to quantify the partition coefficient (K_x). In this work, we further extend the theory that describes membrane-protein partitioning to account for spontaneous protein-membrane dissociation and reassociation to the same or a different lipid vesicle. We derive the probability distribution of proteins on lipid vesicles for reversible binding and demonstrate that FCS is a suitable technique for accurate K_x quantification of membrane-protein reversible association. We also establish the limits to K_x determination by FCS studying the Cramer-Rao bound on the variance of the retrieved parameters. We validate the mathematical formulation against reaction-diffusion simulations to study phase and statistical equilibria and compare the K_x obtained from a computational FCS titration experiment with the experimental ground truth. Finally, we demonstrate the application of our methodology studying the association of anti-HIV broadly neutralizing antibody (10E8-3R) to the membrane.

Interaction of GC376, a SARS-COV-2 MPRO inhibitor, with model lipid membranes

Partitioning and effect of antiviral GC376, a potential SARS-CoV-2 inhibitor, on model lipid membranes was studied using dynamic light scattering (DLS), UV–VIS spectrometry, Excimer fluorescence, Differential scanning calorimetry (DSC) and Small- and Wide-angle X-ray scattering (SAXS/WAXS). Partition coefficient of GC376 between lipid and water phase was found to be low, reaching K_P = 46.8 ± 18.2. Results suggest that GC376 partitions into lipid bilayers at the level of lipid head-groups, close to the polar/hydrophobic interface. Changes in structural and thermodynamic properties strongly depend on the GC376/lipid mole ratio. Already at lowest mole ratios GC376 induces increase of lateral pressures, mainly in the interfacial region of the bilayer. Hereby, the pre- and main-transition temperature of the lipid system increases, what is attributed to tighter packing of acyl chains induced by GC376. At GC376/DPPC ≥ 0.03 mol/mol we detected formation of domains with different GC376 content resulting in the lateral phase separation and changes in both, main transition temperature and enthalpy. The observed changes are attributed to the response of the system on the increased lateral stresses induced by partitioning of GC376. Obtained results are discussed in context of liposome-based drug delivery systems for GC376 and in context of indirect mechanism of virus replication inhibition.

The Antimicrobial Peptide 1018-K6 Interacts Distinctly with Eukaryotic and Bacterial Membranes, the Basis of Its Specificity and Bactericidal Activity

Since penicillin was discovered, antibiotics have been critical in the fight against infections. However, antibiotic misuse has led to drug resistance, which now constitutes a serious health problem. In this context, antimicrobial peptides (AMPs) constitute a natural group of short proteins, varying in structure and length, that act against certain types of bacterial pathogens. The antimicrobial peptide 1018-K6 (VRLIVKVRIWRR- NH2) has significant bactericidal and antibiofilm activity against Listeria monocytogenes isolates, and against different strains and serotypes of Salmonella. Here, the mechanism of action of 1018-K6 was explored further to understand the peptide-membrane interactions relevant to its activity, and to define their determinants. We combined studies with model synthetic membranes (liposomes) and model biological membranes, assessing the absorption maximum and the quenching of 1018-K6 fluorescence in aqueous and lipid environments, the self-quenching of carboxyfluorescein, as well as performing lipid sedimentation assays. The data obtained reflect the differential interactions of the 1018-K6 peptide with eukaryotic and prokaryotic membranes, and the specific interactions and mechanisms of action in the three prokaryotic species studied: Salmonella Typhimurium^2GN, Escherichia coli^3GN, and Staphylococcus aureus^3GP. The AMP 1018-K6 is a candidate to prevent (food preservation) or treat (antibiotic use) infections caused by certain pathogenic bacteria, especially some that are resistant to current antibiotics.

Analysis of the Equilibrium Distribution of Ligands in Heterogeneous Media–Approaches and Pitfalls

The equilibrium distribution of small molecules (ligands) between binding agents in heterogeneous media is an important property that determines their activity. Heterogeneous systems containing proteins and lipid membranes are particularly relevant due to their prevalence in biological systems, and their importance to ligand distribution, which, in turn, is crucial to ligand’s availability and biological activity. In this work, we review several approaches and formalisms for the analysis of the equilibrium distribution of ligands in the presence of proteins, lipid membranes, or both. Special attention is given to common pitfalls in the analysis, with the establishment of the validity limits for the distinct approaches. Due to its widespread use, special attention is given to the characterization of ligand binding through the analysis of Stern–Volmer plots of protein fluorescence quenching. Systems of increasing complexity are considered, from proteins with single to multiple binding sites, from ligands interacting with proteins only to biomembranes containing lipid bilayers and membrane proteins. A new formalism is proposed, in which ligand binding is treated as a partition process, while considering the saturation of protein binding sites. This formalism is particularly useful for the characterization of interaction with membrane proteins.

Investigations into the membrane activity of arenicin antimicrobial peptide AA139

Arenicin-3 is an amphipathic β-hairpin antimicrobial peptide that is produced by the lugworm Arenicola marina. In this study, we have investigated the mechanism of action of arenicin-3 and an optimized synthetic analogue, AA139, by studying their effects on lipid bilayer model membranes and Escherichia coli bacterial cells. The results show that simple amino acid changes can lead to subtle variations in their interaction with membranes and therefore alter their pre-clinical potency, selectivity and toxicity. While the mechanism of action of arenicin-3 is primarily dependent on universal membrane permeabilization, our data suggest that the analogue AA139 relies on more specific binding and insertion properties to elicit its improved antibacterial activity and lower toxicity, as exemplified by greater selectivity between lipid composition when inserting into model membranes i.e. the N-terminus of AA139 seems to insert deeper into lipid bilayers than arenicin-3 does, with a clear distinction between zwitterionic and negatively charged lipid bilayer vesicles, and AA139 demonstrates a cytoplasmic permeabilization dose response profile that is consistent with its greater antibacterial potency against E. coli cells compared to arenicin-3.

Parainfluenza Fusion Peptide Promotes Membrane Fusion by Assembling into Oligomeric Porelike Structures

Paramyxoviruses are enveloped viruses harboring a negative-sense RNA genome that must enter the host's cells to replicate. In the case of the parainfluenza virus, the cell entry process starts with the recognition and attachment to target receptors, followed by proteolytic cleavage of the fusion glycoprotein (F) protein, exposing the fusion peptide (FP) region. The FP is responsible for binding to the target membrane, and it is believed to play a crucial role in the fusion process, but the mechanism by which the parainfluenza FP (PIFP) promotes membrane fusion is still unclear. To elucidate this matter, we performed biophysical experimentation of the PIFP in membranes, together with coarse grain (CG) and atomistic (AA) molecular dynamics (MD) simulations. The simulation results led to the pinpointing of the most important PIFP amino acid residues for membrane fusion and show that, at high concentrations, the peptide induces the formation of a water-permeable porelike structure. This structure promotes lipid head intrusion and lipid tail protrusion, which facilitates membrane fusion. Biophysical experimental results validate these findings, showing that, depending on the peptide/lipid ratio, the PIFP can promote fusion and/or membrane leakage. Our work furthers the understanding of the PIFP-induced membrane fusion process, which might help foster development in the field of viral entry inhibition.

Partition of antimicrobial D-L-α-cyclic peptides into bacterial model membranes

Fluorescence spectroscopy is used to characterize the partition of three second-generation D,L-α-cyclic peptides to two lipid model membranes. The peptides have proven antimicrobial activity, particularly against Gram positive bacteria, and the model membranes are formed of either with 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG) or its mixture with 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE), at a molar ratio of (1:1). The peptide's intrinsic fluorescence was used in the Steady State and/or Time Resolved Fluorescence Spectroscopy experiments, showing that the peptides bind to the membranes, and the extent of their partition is thereof quantified. The peptide-induced membrane leakage was followed using an encapsulated fluorescent dye. Overall, the partition is mainly driven by electrostatics, but also involves hydrophobic interactions. The introduction of a hydrocarbon tail in one of the residues of the parent peptide, CPR, adjacent to the tryptophan (Trp) residue, significantly improves the partition of the modified peptides, CPRT10 and CPRT14, to both membrane systems. Further, we show that the length of the tail is the main distinguishing factor for the extension of the partition process. The parent peptide induces very limited leakage, at odds with the peptides with tail, that promote fast leakage, increasing in most cases with peptide concentration, and being almost complete for the highest peptide concentration and negatively charged membranes. Overall, the results help the unravelling of the antimicrobial action of these peptides and are well in line with their proven high antimicrobial activity.

What different physical techniques can disclose about disruptions on membrane structure caused by the antimicrobial peptide Hylin a1 and a more positively charged analogue

The present work monitors structural changes in anionic membranes (DPPG; 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol)) caused by the native antimicrobial peptide (AMP) Hylin a1 (Hya1; IFGAILPLALGALKNLIK-NH₂) and its synthetic analogue K⁰Hya1 (KIFGAILPLALGALKNLIK-NH₂), with an extra positive residue of lysine at the N-terminus of the peptide chain. Anionic membranes were used to mimic anionic lipids in bacteria membranes. Differential scanning calorimetry (DSC) evinced that both peptides strongly disrupt the lipid bilayers. However, whereas the native peptide (+3) induces a space-average and/or time-average disruption on DPPG bilayers, the more charged, K⁰Hya1 (+4), appears to be strongly attached to the membrane, clearly giving rise to the coexistence of two different lipid regions, one depleted of peptide and another one peptide-disrupted. The membrane fluorescent probe Laurdan indicates that, in average, the peptides increase the bilayer packing of fluid DPPG (above the lipid gel-fluid transition temperature) and/or decrease its polarity. Spin labels, incorporated into DPPG membrane, confirm, and extend the results obtained with Laurdan, indicating that the peptides increase the lipid packing both in gel and fluid DPPG bilayers. Therefore, our results confirm that Laurdan is often unable to monitor structural modifications induced on gel membranes by exogenous molecules. Through the measurement of the leakage of entrapped carboxyfluorescein (CF), a fluorescent dye, in DPPG large unilamellar vesicles it was possible to show that both peptides induce pore formation in DPPG bilayers. Furthermore, CF experiments show that Hylin peptides are strongly bound to DPPG bilayers in the gel phase, not being able to migrate to other DPPG vesicles. Here we discuss the complementarity of different techniques in monitoring structural alterations caused on lipid bilayers by Hylin peptides, and how it could be used to help in the understanding of the action of other exogenous molecules on biological membranes.

The pH-sensitive action of cholesterol-conjugated peptide inhibitors of influenza virus

Influenza viruses are major human pathogens, responsible for respiratory diseases affecting millions of people worldwide, with high morbidity and significant mortality. Infections by influenza can be controlled by vaccines and antiviral drugs. However, this virus is constantly under mutations, limiting the effectiveness of these clinical antiviral strategies. It is therefore urgent to develop new ones. Influenza hemagglutinin (HA) is involved in receptor binding and promotes the pH-dependent fusion of viral and cell endocytic membranes. HA-targeted peptides may emerge as a novel antiviral option to block this viral entry step. In this study, we evaluated three HA-derived (lipo)peptides using fluorescence spectroscopy. Peptide membrane interaction assays were performed at neutral and acidic pH to better resemble the natural conditions in which influenza fusion occurs. We found that peptide affinity towards membranes decreases upon the acidification of the environment. Therefore, the released peptides would be able to bind their complementary domain and interfere with the six-helix bundle formation necessary for viral fusion, and thus for the infection of the target cell. Our results provide new insight into molecular interactions between HA-derived peptides and cell membranes, which may contribute to the development of new influenza virus inhibitors.

Quercetin induces lipid domain-dependent permeability

Quercetin is a polyphenolic molecule with a broad spectrum of biological activities derived from its antioxidant property. Its mechanism of action has been explained by its binding and/or interference with enzymes, receptors, transporters and signal transduction systems. Since these important mechanisms generally occur in membrane environments, within and through lipid bilayers, investigating the biophysical properties related to the diversity of lipid compositions of cell membranes may be the key to understanding the role of cell membrane in these processes. In this work, we explored the interaction of quercetin with model membranes of different lipid compositions to access the importance of lipid phases and bilayer homogeneity to the action of quercetin and contribute to the understanding of quercetin multiple activities. Analysis of the influence of quercetin on the morphology and permeability of GUVs, the rigidity of LUVs and affinity to these vesicles showed that quercetin strongly partitions to the more homogeneous environments, but significantly permeates and modifies the more heterogeneous where liquid-disordered, liquid-ordered and solid phases coexist. Our findings support the condensing effect of quercetin, which is observed through a significant rigidifying of bilayers containing 40% cholesterol, but much less evidenced when it is reduced to 20% or in its absence. Nevertheless, the presence of sphingomyelin in the ternary system led to a more heterogeneous bilayer with the formation of micrometric and probably also nanometric domains, which coalesce in the presence of quercetin. This observation together with increased permeability points to an insertion effect.

The interaction of a β2 adrenoceptor agonist drug with biomimetic cell membrane models: The case of terbutaline sulphate

Terbutaline sulphate (TS) is a selective short-acting β₂ adrenoceptor agonist used for asthma treatment. The pharmacological activity of TS depends on its binding to the transmembrane protein, β₂ adrenoceptor. Thus, the interactions of this drug with biological membranes are expected, affecting its pharmacological activity. Using in vitro models to study the interaction of TS with biological membranes can provide important information about the activity of the drug. Here, liposomes with different lipid compositions were used as biomimetic models of cell membranes to evaluate the effect of composition, complexity, and physical state of membranes on TS-membrane interactions. For that, liposomes containing dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and liposomes containing DMPC and cholesterol (CHOL) were prepared. For the study of TS-membrane interactions, the TS lipophilicity was evaluated in terms of i) partition coefficient; ii) the preferential location of the drug within the membrane; iii) and the effect of TS on the membrane fluidity. The obtained data suggest that TS has an affinity for the lipid membrane, partitioning from the aqueous to the lipid phase. The affinity was dependent on the liposomes' compositions, showing a greater affinity for DMPC membranes than for DMPC:CHOL model. Dynamic light scattering (DLS) results revealed that this is due to the rigidizing effect caused by CHOL molecules. These findings provide valuable insights in the understanding of the complex interaction of TS with biomembrane models as well as the relevance of lipid compositions and membrane structure in such interactions, which may be related to its pharmacological activity and side effects.

Fluoroquinolone-Transition Metal Complexes: A Strategy to Overcome Bacterial Resistance

Fluoroquinolones (FQs) are antibiotics widely used in the clinical practice due to their large spectrum of action against Gram-negative and some Gram-positive bacteria. Nevertheless, the misuse and overuse of these antibiotics has triggered the development of bacterial resistance mechanisms. One of the strategies to circumvent this problem is the complexation of FQs with transition metal ions, known as metalloantibiotics, which can promote different activity and enhanced pharmacological behaviour. Here, we discuss the stability of FQ metalloantibiotics and their possible translocation pathways. The main goal of the present review is to frame the present knowledge on the conjunction of biophysical and biological tools that can help to unravel the antibacterial action of FQ metalloantibiotics. An additional goal is to shed light on the studies that must be accomplished to ensure stability and viability of such metalloantibiotics. Potentiometric, spectroscopic, microscopic, microbiological, and computational techniques are surveyed. Stability and partition constants, interaction with membrane porins and elucidation of their role in the influx, determination of the antimicrobial activity against multidrug-resistant (MDR) clinical isolates, elucidation of the mechanism of action, and toxicity assays are described for FQ metalloantibiotics.

Protonectin peptides target lipids, act at the interface and selectively kill metastatic breast cancer cells while preserving morphological integrity

Despite the need for innovative compounds as antimicrobial and anticancer agents, natural sources of peptides remain underexplored. Protonectin (PTN), a cationic dodecapeptide of pharmacological interest, presents large hydrophobicity that is associated with the tendency to aggregate and supposedly influences bioactivity. A disaggregating role was assigned to PTN' N-terminal fragment (PTN_1-6), which enhances the bioactivity of PTN in a 1:1 mixture (PTN/PTN_1-6). Spectroscopic techniques and model membranes (phospholipid bilayers and SDS micelles) revealed that environment-dependent aggregation is reduced for PTN/PTN_1-6, but cytotoxicity of PTNs on MDA-MB-231 breast cancer showed the same CC₅₀ values around 16 µM and on MCF-10A epithelial breast cells 6 to 5-fold higher values, revealing a selective interaction. Since PTN_1-6 lacks activity on breast cells, its presence should differently affect PTN activity, suggesting that aggregation could modulate activity depending on the membrane characteristics. Indeed, increased partitioning and lytic activity of PTN/PTN_1-6 were found in model membranes independently of charge density, but affected by the curvature tendency. PTN and PTN/PTN_1-6 do not alter morphology and roughness of cancer cells, indicating a superficial interaction with membranes and consistent with results obtained in NMR experiments. Our results indicate that aggregation of PTNs depends on the membrane characteristics and modulates the activity of the peptides.

The effect of acidic pH on the adsorption and lytic activity of the peptides Polybia-MP1 and its histidine-containing analog in anionic lipid membrane: a biophysical study by molecular dynamics and spectroscopy

Antimicrobial peptides (AMPs) are part of the innate immune system of many species. AMPs are short sequences rich in charged and non-polar residues. They act on the lipid phase of the plasma membrane without requiring membrane receptors. Polybia-MP1 (MP1), extracted from a native wasp, is a broad-spectrum bactericide, an inhibitor of cancer cell proliferation being non-hemolytic and non-cytotoxic. MP1 mechanism of action and its adsorption mode is not yet completely known. Its adsorption to lipid bilayer and lytic activity is most likely dependent on the ionization state of its two acidic and three basic residues and consequently on the bulk pH. Here we investigated the effect of bulk acidic (pH 5.5) and neutral pH (7.4) solution on the adsorption, insertion, and lytic activity of MP1 and its analog H-MP1 to anionic (7POPC:3POPG) model membrane. H-MP1 is a synthetic analog of MP1 with lysines replaced by histidines. Bulk pH changes could modulate this peptide efficiency. The combination of different experimental techniques and molecular dynamics (MD) simulations showed that the adsorption, insertion, and lytic activity of H-MP1 are highly sensitive to bulk pH in opposition to MP1. The atomistic details, provided by MD simulations, showed peptides contact their N-termini to the bilayer before the insertion and then lay parallel to the bilayer. Their hydrophobic faces inserted into the acyl chain phase disturb the lipid-packing.

Modulatory Effects of Acidic pH and Membrane Potential on the Adsorption of pH-Sensitive Peptides to Anionic Lipid Membrane

Anionic lipid membrane electrostatic potential and solution pH can influence cationic peptide adsorption to these bilayers, especially those containing simultaneously acid and basic residues. Here, we investigate the effects of the pH solution on MP1 (IDWKKLLDAAKQIL-NH2) adsorption to anionic (7POPC:3POPG) lipid vesicles in comparison to its analog H-MP1, with histidines substituting lysines. We used the association of adsorption isotherms and constant pH molecular dynamic simulations (CpHMD) to explore the effects of membrane potential and pH on peptides' adsorption on this lipid membrane. We analyzed the fluorescence and zeta potential adsorption isotherms using the Gouy-Chapman theory. In CpHMD simulations for the peptides in solution and adsorbed on the lipid bilayer, we used the conformations obtained by conventional MD simulations at a μs timescale. Non-equilibrium Monte Carlo simulations provided the protonation states of acidic and basic residues. CpHMD showed average pKa shifts of two to three units, resulting in a higher net charge for the analog than for MP1, strongly modulating the peptide adsorption. The fractions of the protonation of acidic and basic residues and the peptides' net charges obtained from the analysis of the adsorption isotherms were in reasonable agreement with those from CpHMD. MP1 adsorption was almost insensitive to solution pH. H-MP1 was much more sensitive to partitioning, at acidic pH, with an affinity ten times higher than in neutral ones.

Comparing the interaction of the antibiotic levofloxacin with zwitterionic and anionic membranes: Calorimetry, fluorescence, and spin label studies

The present work compares the interaction of the antibiotic levofloxacin (LVX) with zwitterionic and anionic liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG), respectively. By using differential scanning calorimetry (DSC), and with spin labels incorporated into liposomes at two different depths of the bilayers, we investigated the changes induced on the membrane by increasing concentrations of LVX. Further information was obtained using intrinsic LVX fluorescence. Under the conditions used here, all techniques evinced that LVX has little affinity for DPPC zwitterionic membrane. Opposite to that, LVX exhibits a considerable affinity for anionic bilayers, with membrane partition constants K_p = (3.3 ± 0.5) × 10² and (4.5 ± 0.3) × 10², for gel and fluid DPPG membranes, respectively. On binding to DPPG, LVX seems to give rise to the coexistence of LVX -rich and -poor domains on DPPG membranes, as detected by DSC. At the highest LVX concentration used (20 mol%), DSC trace shows an increase in the cooperativity of DPPG gel-fluid transition, also detected by spin labels as an increase in the bilayer packing. Moreover, LVX does not induce pore formation in either DPPG or POPG vesicles. Considering the possible relevance of LVX-membrane interaction for the biological and toxicological action of the antibiotic, the findings discussed here certainly contribute to a better understanding of its action, and the planning of new drugs.

Membrane binding properties of the C-terminal segment of retinol dehydrogenase 8

Light absorption by rhodopsin leads to the release of all-trans retinal (ATRal) in the lipid phase of photoreceptor disc membranes. Retinol dehydrogenase 8 (RDH8) then reduces ATRal into all-trans retinol, which is the first step of the visual cycle. The membrane binding of RDH8 has been postulated to be mediated by one or more palmitoylated cysteines located in its C-terminus. Different peptide variants of the C-terminus of RDH8 were thus used to obtain information on the mechanism of membrane binding of this enzyme. Steady-state and time-resolved fluorescence measurements were performed using short and long C-terminal segments of bovine RDH8, comprising one or two tryptophan residues. The data demonstrate that the amphipathic alpha helical structure of the first portion of the C-terminus of RDH8 strongly contributes to its membrane binding, which is also favored by palmitoylation of at least one of the cysteines located in the last portion of the C-terminus.

Passive Diffusion of Ciprofloxacin and its Metalloantibiotic: A Computational and Experimental study

Fluoroquinolones (FQ) are antibiotics widely used in clinical practise, but the development of bacterial resistance to these drugs is currently a critical public health problem. In this context, ternary copper complexes of FQ (CuFQPhen) have been studied as a potential alternative. In this study, we compared the passive diffusion across the lipid bilayer of one of the most used FQ, ciprofloxacin (Cpx), and its ternary copper complex, CuCpxPhen, that has shown previous promising results regarding antibacterial activity and membrane partition. A combination of spectroscopic studies and molecular dynamics simulations were used and two different model membranes tested: one composed of anionic phospholipids, and the other composed of zwitterionic phospholipids. The obtained results showed a significantly higher membrane permeabilization activity, larger partition, and a more favourable free energy landscape for the permeation of CuCpxPhen across the membrane, when compared to Cpx. Furthermore, the computational results indicated a more favourable translocation of CuCpxPhen across the anionic membrane, when compared to the zwitterionic one, suggesting a higher specificity towards the former. These findings are important to decipher the influx mechanism of CuFQPhen in bacterial cells, which is crucial for the ultimate use of CuFQPhen complexes as an alternative to FQ to tackle multidrug-resistant bacteria.

Structural information and membrane binding of truncated RGS9-1 Anchor Protein and its C-terminal hydrophobic segment

Visual phototransduction takes place in photoreceptor cells. Light absorption by rhodopsin leads to the activation of transducin as a result of the exchange of its GDP for GTP. The GTP-bound ⍺-subunit of transducin then activates phosphodiesterase (PDE), which in turn hydrolyzes cGMP leading to photoreceptor hyperpolarization. Photoreceptors return to the dark state upon inactivation of these proteins. In particular, PDE is inactivated by the protein complex R9AP/RGS9-1/Gβ5. R9AP (RGS9-1 anchor protein) is responsible for the membrane anchoring of this protein complex to photoreceptor outer segment disk membranes most likely by the combined involvement of its C-terminal hydrophobic domain as well as other types of interactions. This study thus aimed to gather information on the structure and membrane binding of the C-terminal hydrophobic segment of R9AP as well as of truncated R9AP (without its C-terminal domain, R9AP∆TM). Circular dichroism and infrared spectroscopic measurements revealed that the secondary structure of R9AP∆TM mainly includes ⍺-helical structural elements. Moreover, intrinsic fluorescence measurements of native R9AP∆TM and individual mutants lacking one tryptophan demonstrated that W79 is more buried than W173 but that they are both located in a hydrophobic environment. This method also revealed that membrane binding of R9AP∆TM does not involve regions near its tryptophan residues, while infrared spectroscopy validated its binding to lipid vesicles. Additional fluorescence measurements showed that the C-terminal segment of R9AP is membrane embedded. Maximum insertion pressure and synergy data using Langmuir monolayers suggest that interactions with specific phospholipids could be involved in the membrane binding of R9AP∆TM.

How Insertion of a Single Tryptophan in the N-Terminus of a Cecropin A-Melittin Hybrid Peptide Changes Its Antimicrobial and Biophysical Profile

In the era of antibiotic resistance, there is an urgent need for efficient antibiotic therapies to fight bacterial infections. Cationic antimicrobial peptides (CAMP) are promising lead compounds given their membrane-targeted mechanism of action, and high affinity towards the anionic composition of bacterial membranes. We present a new CAMP, W-BP100, derived from the highly active BP100, holding an additional tryptophan at the N-terminus. W-BP100 showed a broader antibacterial activity, demonstrating a potent activity against Gram-positive strains. Revealing a high partition constant towards anionic over zwitterionic large unilamellar vesicles and inducing membrane saturation at a high peptide/lipid ratio, W-BP100 has a preferential location for hydrophobic environments. Contrary to BP100, almost no aggregation of anionic vesicles is observed around saturation conditions and at higher concentrations no aggregation is observed. With these results, it is possible to state that with the incorporation of a single tryptophan to the N-terminus, a highly active peptide was obtained due to the π-electron system of tryptophan, resulting in negatively charged clouds, that participate in cation-π interactions with lysine residues. Furthermore, we propose that W-BP100 action can be achieved by electrostatic interactions followed by peptide translocation.

EcDBS1R4, an Antimicrobial Peptide Effective against Escherichia coli with In Vitro Fusogenic Ability

Discovering antibiotic molecules able to hold the growing spread of antimicrobial resistance is one of the most urgent endeavors that public health must tackle. The case of Gram-negative bacterial pathogens is of special concern, as they are intrinsically resistant to many antibiotics, due to an outer membrane that constitutes an effective permeability barrier. Antimicrobial peptides (AMPs) have been pointed out as potential alternatives to conventional antibiotics, as their main mechanism of action is membrane disruption, arguably less prone to elicit resistance in pathogens. Here, we investigate the in vitro activity and selectivity of EcDBS1R4, a bioinspired AMP. To this purpose, we have used bacterial cells and model membrane systems mimicking both the inner and the outer membranes of Escherichia coli, and a variety of optical spectroscopic methodologies. EcDBS1R4 is effective against the Gram-negative E. coli, ineffective against the Gram-positive Staphylococcus aureus and noncytotoxic for human cells. EcDBS1R4 does not form stable pores in E. coli, as the peptide does not dissipate its membrane potential, suggesting an unusual mechanism of action. Interestingly, EcDBS1R4 promotes a hemi-fusion of vesicles mimicking the inner membrane of E. coli. This fusogenic ability of EcDBS1R4 requires the presence of phospholipids with a negative curvature and a negative charge. This finding suggests that EcDBS1R4 promotes a large lipid spatial reorganization able to reshape membrane curvature, with interesting biological implications herein discussed.

Effect of pH on the influenza fusion peptide properties unveiled by constant-pH molecular dynamics simulations combined with experiment

The influenza virus fusion process, whereby the virus fuses its envelope with the host endosome membrane to release the genetic material, takes place in the acidic late endosome environment. Acidification triggers a large conformational change in the fusion protein, hemagglutinin (HA), which enables the insertion of the N-terminal region of the HA2 subunit, known as the fusion peptide, into the membrane of the host endosome. However, the mechanism by which pH modulates the molecular properties of the fusion peptide remains unclear. To answer this question, we performed the first constant-pH molecular dynamics simulations of the influenza fusion peptide in a membrane, extending for 40 µs of aggregated time. The simulations were combined with spectroscopic data, which showed that the peptide is twofold more active in promoting lipid mixing of model membranes at pH 5 than at pH 7.4. The realistic treatment of protonation introduced by the constant-pH molecular dynamics simulations revealed that low pH stabilizes a vertical membrane-spanning conformation and leads to more frequent contacts between the fusion peptide and the lipid headgroups, which may explain the increase in activity. The study also revealed that the N-terminal region is determinant for the peptide’s effect on the membrane.

N-terminal acetylation of a mastoparan-like peptide enhances PE/PG segregation in model membranes

The synthetic peptides L1A and its acetylated analog (acL1A) display potent Gram-negative bactericidal activities without being hemolytic. We have gathered evidence that the N-terminal acetylation of L1A enhances the lytic activity in anionic vesicles with high capability to insert into and disturb lipid packing of model membranes. Here, the impact of L1A and acL1A was evaluated on a model membrane that mimics the cytoplasmic membrane of Gram-negative bacteria, which is rich in phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), using 3:1 mixture of POPE/DOPG and a variety of techniques. We followed peptide adsorption and penetration by zeta potential determination of large unilamellar vesicles, accessibility of tryptophan residue to acrylamide by quenching assays, and Gibbs isotherms. The secondary structure of the peptide on the membranes was assessed using circular dichroism. Peptide mixing ability with the lipids and phase segregation was assessed by the observation of Langmuir monolayers with fluorescence microscopy, as well as with differential scanning calorimetry thermograms of multilamellar vesicles. All in all, the results indicate that both peptides adsorb and penetrate POPE/DOPG membranes with similar affinities, decreasing the surface charge, and adopting alpha structures. Both peptides mix with DOPG and demix from POPE, and consequently, persist at the interface to larger surface pressures in the presence of PG than in pure PE monolayers. This selective degree of mixing of the peptides with PE and PG leads to peptide-induced segregation of PG from PE, being the less charged peptide, acL1A, able to segregate the lipids more efficiently.

Biophysical study of phloretin with human serum albumin in liposomes using spectroscopic methods

The ability of drugs to diffuse through the lipid bilayer of cell membranes is important for their metabolism, distribution, and efficacy. In this study, the interaction between phloretin and human serum albumin (HSA) in an L-egg lecithin phosphatidylcholine (PC) liposome suspension was investigated by fluorescence and absorbance spectroscopy. The spectroscopic and fluorescence quenching experiments show that phloretin molecules penetrated into the lumen of the liposome. The partition coefficient of phloretin in the PC liposome suspensions was calculated from fluorescence quenching measurements. The results show that phloretin efficiently quenches the intrinsic fluorescence of HSA through a combination of dynamic and static quenching. The values of Gibbs free energy, and the enthalpy and entropic change in the binding process of phloretin with HSA in the PC liposome suspensions were negative, suggesting that the binding process of phloretin and HSA was spontaneous. Hydrogen bonding and van der Waals force interactions play an important role in the interaction between the two molecules. In addition, binding of phloretin to HSA in liposome suspensions was investigated by synchronous fluorescence spectroscopy.

The influence of myristoylation, liposome surface charge and nucleic acid interaction in the partition properties of HIV-1 Gag-N-terminal peptides to membranes

The group-specific antigen (GAG) polyprotein of HIV-1 is the main coordinator of the virus assembly process at the plasma membrane (PM) and is directed by its N-terminal matrix domain (MA). MA is myristoylated and possess a highly basic region (HBR) responsible for the interaction with the negative lipids of the PM, especially with PIP₂. In addition, MA binds RNA molecules proposed as a regulatory step of the assembly process. Here we study the interaction of a synthetic peptide (N-terminal 21 amino acids of MA) and liposomes of different compositions using a variety of biophysical techniques. Particularly, we use the fluorescence properties of the single tryptophan of the peptide to analyze its partition to membranes, where we harness for first time the analytical ability of spectral phasors method to study this interaction. We found that electrostatic interactions play an important role for peptide partition to membranes and myristoylation reduces the free energy of the process. Interestingly, we observe that while the presence of PIP₂ does not cause measurable changes on the peptide-membrane interaction, the interaction is favored by cholesterol. Additionally, we found that the partition process goes through a transition state involving peptide disaggregation and changes in the peptide secondary structure. On the other hand, we found that the presence of oligonucleotides competes with the interaction with lipids by increasing peptide solubility. In summary, we think that our results, in context of the current knowledge of the role of HIV-1 MA, contribute to a better molecular understanding of the membrane association process.

The Interaction of Temozolomide with Blood Components Suggests the Potential Use of Human Serum Albumin as a Biomimetic Carrier for the Drug

The interaction of temozolomide (TMZ) (the main chemotherapeutic agent for brain tumors) with blood components has not been studied at the molecular level to date, even though such information is essential in the design of dosage forms for optimal therapy. This work explores the binding of TMZ to human serum albumin (HSA) and alpha-1-acid glycoprotein (AGP), as well as to blood cell-mimicking membrane systems. Absorption and fluorescence experiments with model membranes indicate that TMZ does not penetrate into the lipid bilayer, but binds to the membrane surface with very low affinity. Fluorescence experiments performed with the plasma proteins suggest that in human plasma, most of the bound TMZ is attached to HSA rather than to AGP. This interaction is moderate and likely mediated by hydrogen-bonding and hydrophobic forces, which increase the hydrolytic stability of the drug. These experiments are supported by docking and molecular dynamics simulations, which reveal that TMZ is mainly inserted in the subdomain IIA of HSA, establishing π-stacking interactions with the tryptophan residue. Considering the overexpression of albumin receptors in tumor cells, our results propose that part of the administered TMZ may reach its target bound to plasma albumin and suggest that HSA-based nanocarriers are suitable candidates for designing biomimetic delivery systems that selectively transport TMZ to tumor cells.

Comparing activity, toxicity and model membrane interactions of Jelleine-I and Trp/Arg analogs: analysis of peptide aggregation

Increasing resistance in antibiotic and chemotherapeutic treatments has been pushing studies of design and evaluation of bioactive peptides. Designing relies on different approaches from minimalist sequences and endogenous peptides modifications to computational libraries. Evaluation relies on microbiological tests. Aiming a deeper understanding, we chose the octapeptide Jelleine-I (JI) for its selective and low toxicity profile, designed small modifications combining the substitutions of Phe by Trp and Lys/His by Arg and tested the antimicrobial and anticancer activity on melanoma cells. Biophysical methods identified environment-dependent modulation of aggregation, but critical aggregation concentrations of JI and analogs in buffer show that peptides start membrane interactions as monomers. The presence of model membranes increases or reduces the partial aggregation of peptides. Compared to JI, analog JIF2WR shows the lowest tendency to aggregation on bacterial model membranes. JI and analogs are lytic to model membranes. Their composition-dependent performance indicates preference for the higher charged anionic bilayers in line with their superior performance toward Staphylococcus aureus and Streptococcus pneumoniae. JIF2WR presented the higher partitioning, higher lytic activity and lower aggregated contents. Despite these increased membranolytic activities, JIF2WR exhibited comparable antimicrobial activity in relation to JI at the expenses of some loss in selectivity. We found that the substitution Phe/Trp (JIF2W) tends to decrease antimicrobial but to increase anticancer activity and aggregation on model membranes and the toxicity toward human cells. However, the concomitant substitution Lys/His by Arg (JIF2WR) modulates some of these tendencies, increasing both the antimicrobial and the anticancer activity while decreasing the aggregation tendency.

Effect of dipole moment on amphiphile solubility and partition into liquid ordered and liquid disordered phases in lipid bilayers

Association of amphiphiles with biomembranes is important for their availability at specific locations in organisms and cells, being critical for their biological function. A prominent role is usually attributed to the hydrophobic effect, and to electrostatic interactions between charged amphiphiles and lipids. This work explores a closely related and complementary aspect, namely the contribution made by dipole moments to the strength of the interactions established. Two xanthene amphiphiles with opposite relative orientations of their dipole and amphiphilic moments have been selected (Rhodamine-C₁₄ and Carboxyfluorescein-C₁₄). The membranes studied have distinct lipid compositions, representing typical cell membrane pools, ranging from internal membranes to the outer and inner leaflet of the plasma membrane. A comprehensive study is reported, including the affinity of the amphiphiles for the different membranes, the stability of the amphiphiles as monomers and their tendency to form small clusters, as well as their transverse location in the membrane. The orientation of the amphiphile dipole moment, which determines whether its interaction with the membrane dipole potential is repulsive or attractive, is found to exert a large influence on the association of the amphiphile with ordered lipid membranes. These interactions are also responsible for the formation of small clusters or stabilization of amphiphile monomers in the membrane. The results obtained allow understanding the prevalence of protein lipidation at the N-terminal for efficient targeting to the plasma membrane, as well as the tendency of GPI-anchored proteins (usually lipidated at the C-terminal) to form small clusters in the membrane ordered domains.

Enfuvirtide-Protoporphyrin IX Dual-Loaded Liposomes: In Vitro Evidence of Synergy against HIV-1 Entry into Cells

We have developed a nanocarrier consisting of large unilamellar vesicles (LUVs) for combined delivery of two human immunodeficiency virus type 1 (HIV-1) entry inhibitors, enfuvirtide (ENF) and protoporphyrin IX (PPIX). The intrinsic lipophilicity of ENF and PPIX, a fusion inhibitor and an attachment inhibitor, respectively, leads to their spontaneous incorporation into the lipid bilayer of the LUVs nanocarrier. Both entry inhibitors partition significantly toward LUVs composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and a 9:1 mixture of POPC:1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DPPE-PEG₂₀₀₀), representative of conventional and immune-evasive drug delivery formulations, respectively. These colocalize in the core of lipid membranes. Dual-loaded nanocarriers are monodispersed and retain the size distribution, thermotropic behavior, and surface charge of the unloaded form. Combination of the two entry inhibitors in the nanocarrier resulted in improved synergy against HIV-1 entry compared to combination in free form, strongly when immune-evasive formulations are used. We propose that the improved action of the entry inhibitors when loaded into the nanocarriers results from their slow release at the site of viral entry. Overall, liposomes remain largely unexplored platforms for combination of viral entry inhibitors, with potential for improvement of current antiretroviral therapy drug safety and application. Our work calls for a reappraisal of the potential of entry inhibitor combinations and delivery for clinical use in antiretroviral therapy.

Revisiting the Interaction of Melittin with Phospholipid Bilayers: The Effects of Concentration and Ionic Strength

Melittin is an anti-microbial peptide (AMP) and one of the most studied membrane-disrupting peptides. There is, however, a lack of accurate measurements of the concentration-dependent kinetics and affinity of binding of melittin to phospholipid membranes. In this study, we used surface plasmon resonance spectroscopy to determine the concentration-dependent effect on the binding of melittin to 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) bilayers in vesicles. Three concentration ranges were considered, and when combined, covered two orders of magnitudes (0.04 µM to 8 µM), corresponding to concentrations relevant to the membrane-disrupting and anti-microbial activities of melittin. Binding kinetics data were analysed using a 1:1 Langmuir-binding model and a two-state reaction model. Using in-depth quantitative analysis, we characterised the effect of peptide concentration, the addition of NaCl at physiological ionic strength and the choice of kinetic binding model on the reliability of the calculated kinetics and affinity of binding parameters. The apparent binding affinity of melittin for POPC bilayers was observed to decrease with increasing peptide/lipid (P/L) ratio, primarily due to the marked decrease in the association rate. At all concentration ranges, the two-state reaction model provided a better fit to the data and, thus, a more reliable estimate of binding affinity. Addition of NaCl significantly reduced the signal response during the association phase; however, no substantial effect on the binding affinity of melittin to the POPC bilayers was observed. These findings based on POPC bilayers could have important implications for our understanding of the mechanism of action of melittin on more complex model cell membranes of higher physiological relevance.

Cyclic Analogues of Horseshoe Crab Peptide Tachyplesin I with Anticancer and Cell Penetrating Properties

Tachyplesin-I (TI) is a host defense peptide from the horseshoe crab Tachypleus tridentatus that has outstanding potential as an anticancer therapeutic lead. Backbone cyclized TI (cTI) has similar anticancer properties to TI but has higher stability and lower hemolytic activity. We designed and synthesized cTI analogues to further improve anticancer potential and investigated structure-activity relationships based on peptide-membrane interactions, cellular uptake, and anticancer activity. The membrane-binding affinity and cytotoxic activity of cTI were found to be highly dependent on peptide hydrophobicity and charge. We describe two analogues with increased selectivity toward melanoma cells and one analogue with the ability to enter cells with high efficacy and low toxicity. Overall, the structure-activity relationship study shows that cTI can be developed as a membrane-active antimelanoma lead, or be employed as a cell penetrating peptide scaffold that can target and enter cells without damaging their integrity.

Chalcone derivatives targeting mitosis: synthesis, evaluation of antitumor activity and lipophilicity

This study describes the synthesis of a series of chalcones, including pyrazole and α,β-epoxide derivatives, and evaluation of their cell growth inhibitory activity in three human tumor cell lines, as well as their lipophilicity using liposomes as a biomimetic membrane model. Structure-activity and structure-lipophilicity relationships were established for the synthetized chalcones. From this work, nine chalcones (3, 5, 9, 11, 15-19) showing suitable drug-like lipophilicity with potent growth inhibitory activity were identified, being the growth inhibitory effect of compounds 15-17 associated with a pronounced antimitotic effect. Compounds 15-17 affected spindle assembly and, as a consequence, arrested cells at metaphase in NCI-H460 cells, culminating in cell death. Amongst the compounds tested, compound 15 exhibited the highest antimitotic activity as revealed by mitotic index calculation. Moreover, 15 was able to enhance chemosensitivity of tumor cells to low doses of paclitaxel in NCI-H460 cells. The results indicate that 15 exerts its antiproliferative activity by affecting microtubules and causing cell death subsequently to a mitotic arrest, and thus has the potential for antitumor activity.

Characterization of Tachyplesin Peptides and Their Cyclized Analogues to Improve Antimicrobial and Anticancer Properties

Tachyplesin I, II and III are host defense peptides from horseshoe crab species with antimicrobial and anticancer activities. They have an amphipathic β-hairpin structure, are highly positively-charged and differ by only one or two amino acid residues. In this study, we compared the structure and activity of the three tachyplesin peptides alongside their backbone cyclized analogues. We assessed the peptide structures using nuclear magnetic resonance (NMR) spectroscopy, then compared the activity against bacteria (both in the planktonic and biofilm forms) and a panel of cancerous cells. The importance of peptide-lipid interactions was examined using surface plasmon resonance and fluorescence spectroscopy methodologies. Our studies showed that tachyplesin peptides and their cyclic analogues were most potent against Gram-negative bacteria and melanoma cell lines, and showed a preference for binding to negatively-charged lipid membranes. Backbone cyclization did not improve potency, but improved peptide stability in human serum and reduced toxicity toward human red blood cells. Peptide-lipid binding affinity, orientation within the membrane, and ability to disrupt lipid bilayers differed between the cyclized peptide and the parent counterpart. We show that tachyplesin peptides and cyclized analogues have similarly potent antimicrobial and anticancer properties, but that backbone cyclization improves their stability and therapeutic potential.

The trip of a drug inside the body: From a lipid-based nanocarrier to a target cell

To date, enormous investigations have been conducted to enhance medicines' target-oriented delivery to improve their therapeutic index. In this regard, lipid-based carrier system might have been regarded as prime delivery systems that are very close to the naturally cell-derived vesicles used for biomolecular communication among cells from occasionally remote tissues. Upon examination of the literature, we found a chasm between groups of investigations in drug pharmaceutics and thought that maybe holistic research could provide better information with respect to drug delivery inside the body, especially when they are going to be injected directly into the bloodstream for systemic distribution. While a collection of older research in most cases dealt with the determination of drug partition coefficient between the aqueous and cell membrane compartments, the link has been overlooked in newer investigations that were mostly focused on drug formulation optimization and their association with particle biodistribution. This gap in the literature motivated us to present the current opinion paper, in which drug physicochemical properties like drug lipophilicity/hydrophilicity is considered as an important element in designing drug-carrying liposomes or micelles. How a hypothetical high throughput cell-embedded chromatographic technique might help to investigate a nanocarrier tissue distribution and to design 'multi-epitope grafted lipid-based drug carrier systems' are discussed. Whenever we would need support for our opinions, we have provided analogy from hydrophobic biomolecules like cholesterol, steroid hormones, and sex hormones and encouraged readers to consider our principle hypothesis: If these molecules could reach their targets far away from the site of production, then a large list of hydrophobic drugs could be delivered to their targets using the same principles.

Combining 25-Hydroxycholesterol with an HIV Fusion Inhibitor Peptide: Interaction with Biomembrane Model Systems and Human Blood Cells

The fusion between the viral and the target cell membrane is a crucial step in the life cycle of enveloped viruses. The blocking of this process is a well-known therapeutic approach that led to the development of the fusion inhibitor peptide enfuvirtide, clinically used against human immunodeficiency virus (HIV) type 1. Despite this significant advance on viral treatment, the appearance of resistance has limited its clinical use. Such a limitation has led to the development of other fusion inhibitor peptides, such as C34, that present the same structural domain as enfuvirtide (heptad repeat sequence) but have different functional domains (pocket-binding domain in the case of C34 and lipid-binding domain in the case of enfuvirtide). Recently, the antiviral properties of 25-hydroxycholesterol were demonstrated, which boosted the interest in this oxysterol. The combination of two distinct antiviral molecules, C34 and 25-hydroxycholesterol, may help to suppress the emergence of resistant viruses. In this work, we characterized the interaction of the C34-25-hydroxycholesterol conjugate with biomembrane model systems and human blood cells. Lipid vesicles and monolayers with defined lipid compositions were used as biomembrane model systems. The conjugate interacts preferentially with membranes rich in sphingomyelin (a lipid enriched in lipid rafts) and presents a poor partition to membranes composed solely of phosphatidylcholine and cholesterol. We hypothesize that cholesterol causes a repulsive effect that is overcome in the presence of sphingomyelin. Importantly, the peptide shows a preference for human peripheral blood mononuclear cells relative to erythrocytes, which shows its potential to target CD4⁺ cells. Antiviral activity results against different wild-type and drug-resistant HIV strains further demonstrated the potential of C34-HC as a good candidate for future studies.

Psd2 pea defensin shows a preference for mimetic membrane rafts enriched with glucosylceramide and ergosterol

Psd2 is a pea defensin with 47 amino acid residues that inhibits the growth of fungal species by an uncharacterized mechanism. In this work, Psd2 interactions with model membranes mimicking the lipid compositions of different organisms were evaluated. Protein-lipid overlay assays indicated that Psd2 recognizes Fusarium solani glucosylceramide (GlcCerF.solani) and ergosterol (Erg) in addition to phosphatidylcholine (POPC) and some phosphatidylinositol species, such as PtdIns (3)P, (5)P and (3,5)P2, suggesting that these lipids may play important roles as Psd2 targets. Assays using lipid vesicles were also performed to study the behaviour and dynamics that occur after peptide-membrane interactions. Surface plasmon resonance analysis showed that Psd2 has a higher affinity for pure POPC and POPC-based vesicles containing GlcCer and Erg at a 70:30 proportion than for vesicles containing cholesterol (Chol). Partition experiments by fluorescence spectroscopy showed a decrease in Trp42 quantum yield of Psd2 in the presence of GlcCerF.solani and Erg, individually or in simultaneously enriched membranes. The partition coefficient (Kp) obtained indicated a Psd2 partition preference for this vesicles, confirmed by quenching assays using acrylamide and 5/16-doxyl-stearic acid. Furthermore, we showed that the presence of C8C9 double bonds and a methyl group at position C9 of the sphingoid base backbone of GlcCer was relevant to Psd2 activity against Aspergillus nidulans. These results are consistent with the selectivity of Psd2 against fungi and its lack of toxicity in human erythrocytes. Psd2 represents a promising natural compound for the treatment of fungal infections.

Biophysical interaction of temozolomide and its active metabolite with biomembrane models: The relevance of drug-membrane interaction for Glioblastoma Multiforme therapy

Temozolomide (TMZ) is the first-line treatment for Glioblastoma Multiforme (GBM). After administration, TMZ is rapidly converted into its active metabolite (MTIC). However, its pharmacological activity is reduced due MTIC low bioavailability in the brain. Since drugs' permeability through biological barriers and tumor cell membranes affects its bioavailability, the ability of MTIC to interact with the biological membranes presents a major contribution on its pharmacological properties and activity. Biomembrane models mimic the physiological conditions, allowing to predict the drug's behavior at biological membranes and its effects on drug biodistribution profiles. In this work, lipid bilayer models using liposomes were applied for the drug-membrane interaction studies. The zwitterionic phospholipid, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and cholesterol were chosen for the composition of the model, since they represent the major components of the membranes of GBM cells and brain capillary endothelial cell. Thus, the molecular interactions between MTIC and these models were studied by the evaluation of the partition of the drug into the phospholipid's membrane, its location within the bilayer and its effect on the fluidity of the membrane. The attained results suggest that the composition of membranes affects drugs partition, showing that drug biodistribution depends not only on its physicochemical features, but also depends on the characteristics of the membrane such as the packing of the lipid molecules. Also, MTIC exhibited low affinity to biological membranes, explaining its low bioavailability on the target cells.