Lipid chain-length dependence for incorporation of alamethicin in membranes: electron paramagnetic resonance studies on TOAC-spin labeled analogs

Recent advances and applications of peptide-agent conjugates for targeting tumor cells

BACKGROUND

Cancer, being a complex disease, presents a major challenge for the scientific and medical communities. Peptide therapeutics have played a significant role in different medical practices, including cancer treatment.

METHOD

This review provides an overview of the current situation and potential development prospects of anticancer peptides (ACPs), with a particular focus on peptide vaccines and peptide-drug conjugates for cancer treatment.

RESULTS

ACPs can be used directly as cytotoxic agents (molecularly targeted peptides) or can act as carriers (guiding missile) of chemotherapeutic agents and radionuclides by specifically targeting cancer cells. More than 60 natural and synthetic cationic peptides are approved in the USA and other major markets for the treatment of cancer and other diseases. Compared to traditional cancer treatments, peptides exhibit anticancer activity with high specificity and the ability to rapidly kill target cancer cells. ACP's target and kill cancer cells via different mechanisms, including membrane disruption, pore formation, induction of apoptosis, necrosis, autophagy, and regulation of the immune system. Modified peptides have been developed as carriers for drugs, vaccines, and peptide-drug conjugates, which have been evaluated in various phases of clinical trials for the treatment of different types of solid and leukemia cancer.

CONCLUSIONS

This review highlights the potential of ACPs as a promising therapeutic option for cancer treatment, particularly through the use of peptide vaccines and peptide-drug conjugates. Despite the limitations of peptides, such as poor metabolic stability and low bioavailability, modified peptides show promise in addressing these challenges. Various mechanism of action of anticancer peptides. Modes of action against cancer cells including: inducing apoptosis by cytochrome c release, direct cell membrane lysis (necrosis), inhibiting angiogenesis, inducing autophagy-mediated cell death and immune cell regulation.

Domain Size Regulation in Phospholipid Model Membranes Using Oil Molecules and Hybrid Lipids

The formation of domains in multicomponent lipid mixtures has been suggested to play a role in moderating signal transduction in cells. Understanding how domain size may be regulated by both hybrid lipid molecules and impurities is important for understanding real biological processes; at the same time, developing model systems where domain size can be regulated is crucial to enable systematic studies of domain formation kinetics and thermodynamics. Here, we perform a model study of the effects of oil molecules, which swell the bilayer, and line-active hybrid phospholipids using a thermally induced liquid-solid phase separation in planar, free-standing lipid bilayers consisting of DOPC and DPPC (1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, respectively). The experiments show that the kinetics of domain growth are significantly affected by the type and molecular structure of the oil (squalene, hexadecane, or decane), with the main contributing factors being the degree of swelling of the bilayer and the changes in line tension induced by the different oils, with smaller domains resulting from systems with smaller values of the line tension. POPC (1-palmitoyl-sn-2-oleoyl-glycero-3-phosphocholine), on the other hand, acts as a line-active hybrid lipid, reducing the domain size when added in small amounts and slowing down domain coarsening. Finally, we show that despite the regulation of domain size by both methods, the phase transition temperature is influenced by the presence of oil molecules but not significantly by the presence of hybrid lipids. Overall, our results show how to regulate domain size in binary membrane model systems, over a wide range of length scales, by incorporating oil molecules and hybrid lipids.

Anti-cancer peptides: their current trends in the development of peptide-based therapy and anti-tumor drugs

Human cancer remains a cause of high mortality throughout the world. The conventional methods and therapies currently employed for treatment are followed by moderate-to-severe side effects. They have not generated curative results due to the ineffectiveness of treatments. Besides, the associated high costs, technical requirements, and cytotoxicity further characterize their limitations. Due to relatively higher presidencies, bioactive peptides with anti-cancer attributes have recently become treatment choices within the therapeutic arsenal. The peptides act as potential anti-cancer agents explicitly targeting tumor cells while being less toxic to normal cells. The anti-cancer peptides are isolated from various natural sources, exhibit high selectivity and high penetration efficiency, and could be quickly restructured. The therapeutic benefits of compatible anti-cancer peptides have contributed to the significant expansion of cancer treatment; albeit, the mechanisms by which bioactive peptides inhibit the proliferation of tumor cells remain unclear. This review will provide a framework for assessing anti-cancer peptides' structural and functional aspects. It shall provide appropriate information on their mode of action to support and strengthen efforts to improve cancer prevention. The article will mention the therapeutic health benefits of anti-cancer peptides. Their importance in clinical studies is elaborated for reducing cancer incidences and developing sustainable treatment models.

Clinical Applications and Anticancer Effects of Antimicrobial Peptides: From Bench to Bedside

Cancer is a multifaceted global health issue and one of the leading causes of death worldwide. In recent years, medical science has achieved great advances in the diagnosis and treatment of cancer. Despite the numerous advantages of conventional cancer therapies, there are major drawbacks including severe side effects, toxicities, and drug resistance. Therefore, the urgency of developing new drugs with low cytotoxicity and treatment resistance is increasing. Antimicrobial peptides (AMPs) have attracted attention as a novel therapeutic strategy for the treatment of various cancers, targeting tumor cells with less toxicity to normal tissues. In this review, we present the structure, biological function, and underlying mechanisms of AMPs. The recent experimental studies and clinical trials on anticancer peptides in different cancer types as well as the challenges of their clinical application have also been discussed.

A Novel Dual-Targeted α-Helical Peptide With Potent Antifungal Activity Against Fluconazole-Resistant Candida albicans Clinical Isolates

Due to compromised immune system, fungal infection incidences have markedly increased in the last few decades. Pathogenic fungi have developed resistance to the clinically available antifungal agents. Antifungal resistance poses a great challenge to clinical treatment and has stimulated the demand for novel antifungal agents. A promising alternative to the treatment of fungal diseases is the use of antimicrobial peptides (AMPs). However, the antifungal activities of AMPs have not been fully determined. Therefore, this study aimed at designing and screening α-helical peptides with potential antifungal activities. The effects of key physicochemical parameters on antifungal activities were also investigated. A series of lengthened and residue-substituted derivatives of the template peptide KV, a hexapeptide truncated from the α-helical region of porcine myeloid antimicrobial peptide-36, were designed and synthesized. Enhancement of hydrophobicity by introducing aromatic hydrophobic amino acids (tryptophan and phenylalanine) significantly increased the efficacies of the peptides against Candida albicans strains, including fluconazole-resistant isolates. Increased hydrophobicity also elevated the toxic properties of these peptides. RF3 with moderate hydrophobicity exhibited potent anticandidal activities (GM = 6.96 μM) and modest hemolytic activities (HC10 > 64 μM). Additionally, repeated exposure to a subinhibitory concentration of RF3 did not induce resistance development. The antifungal mechanisms of RF3 were due to membrane disruptions and induction of reactive oxygen species production. Such a dual-targeted mechanism was active against drug-resistant fungi. These results show the important role of hydrophobicity and provide new insights into designing and developing antifungal peptides. Meanwhile, the successful design of RF3 highlights the potential utility of AMPs in preventing the spread of drug-resistant fungal infections.

Anti-cancer peptides: classification, mechanism of action, reconstruction and modification

Anti-cancer peptides (ACPs) are a series of short peptides composed of 10-60 amino acids that can inhibit tumour cell proliferation or migration, or suppress the formation of tumour blood vessels, and are less likely to cause drug resistance. The aforementioned merits make ACPs the most promising anti-cancer candidate. However, ACPs may be degraded by proteases, or result in cytotoxicity in many cases. To overcome these drawbacks, a plethora of research has focused on reconstruction or modification of ACPs to improve their anti-cancer activity, while reducing their cytotoxicity. The modification of ACPs mainly includes main chain reconstruction and side chain modification. After summarizing the classification and mechanism of action of ACPs, this paper focuses on recent development and progress about their reconstruction and modification. The information collected here may provide some ideas for further research on ACPs, in particular their modification.

Interaction of Oil and Lipids in Freestanding Lipid Bilayer Membranes Studied with Label-Free High-Throughput Wide-Field Second-Harmonic Microscopy

The interaction of oils and lipids is relevant for membrane biochemistry since the cell uses bilayer membranes, lipid droplets, and oily substances in its metabolic cycle. In addition, a variety of model lipid membrane systems, such as freestanding horizontal membranes and droplet interface bilayers, are made using oil to facilitate membrane monolayer apposition. We characterize the behavior of excess oil inside horizontal freestanding lipid bilayers using different oils, focusing on hexadecane and squalene. Using a combination of second-harmonic (SH) and white-light imaging, we measure how oil redistributes within the membrane bilayer after formation. SH imaging shows that squalene forms a wider annulus compared with hexadecane, suggesting that there is a higher quantity of squalene remaining in the bilayer compared with hexadecane. Excess oil droplets that appear right after membrane formation are tracked with white-light microscopy. Hexadecane droplets move directionally to the edge of the membrane with diffusion constants similar to those of single lipids, whereas squalene oil droplets move randomly with lower diffusion speeds similar to lipid condensed domains and remain trapped in the center of the bilayer for ∼1-3 h. We discuss the observed differences in terms of different coupling mechanisms between the oil and lipid molecules induced by the different chemical structures of the oils.

The Role of Terminal Functionality in the Membrane and Antibacterial Activity of Peptaibol-Mimetic Aib Foldamers

Peptaibols are peptide antibiotics that typically feature an N-terminal acetyl cap, a C-terminal aminoalcohol, and a high proportion of α-aminoisobutyric acid (Aib) residues. To establish how each feature might affect the membrane-activity of peptaibols, biomimetic Aib foldamers with different lengths and terminal groups were synthesised. Vesicle assays showed that long foldamers (eleven Aib residues) with hydrophobic termini had the highest ionophoric activity. C-terminal acids or primary amides inhibited activity, while replacement of an N-terminal acetyl with an azide group made little difference. Crystallography showed that N₃ Aib₁₁ CH₂ OTIPS folded into a 3₁₀ helix 2.91 nm long, which is close to the bilayer hydrophobic width. Planar bilayer conductance assays showed discrete ion channels only for N-acetylated foldamers. However long foldamers with hydrophobic termini had the highest antibacterial activity, indicating that ionophoric activity in vesicles was a better indicator of antibacterial activity than the observation of discrete ion channels.

Spin-label Order Parameter Calibrations for Slow Motion

Calibrations are given to extract orientation order parameters from pseudo-powder electron paramagnetic resonance line shapes of ¹⁴N-nitroxide spin labels undergoing slow rotational diffusion. The nitroxide z-axis is assumed parallel to the long molecular axis. Stochastic-Liouville simulations of slow-motion 9.4-GHz spectra for molecular ordering with a Maier-Saupe orientation potential reveal a linear dependence of the splittings, [Formula: see text] and [Formula: see text], of the outer and inner peaks on order parameter [Formula: see text] that depends on the diffusion coefficient [Formula: see text] which characterizes fluctuations of the long molecular axis. This results in empirical expressions for order parameter and isotropic hyperfine coupling: [Formula: see text] and [Formula: see text], respectively. Values of the calibration constants [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] are given for different values of [Formula: see text] in fast and slow motional regimes. The calibrations are relatively insensitive to anisotropy of rotational diffusion [Formula: see text], and corrections are less significant for the isotropic hyperfine coupling than for the order parameter.

Simulations of Membrane-Disrupting Peptides I: Alamethicin Pore Stability and Spontaneous Insertion

An all-atom molecular dynamics simulation of the archetype barrel-stave alamethicin (alm) pore in a 1,2-dioleoyl-sn-glycero-3-phosphocholine bilayer at 313 K indicates that ∼7 μs is required for equilibration of a preformed 6-peptide pore; the pore remains stable for the duration of the remaining 7 μs of the trajectory, and the structure factors agree well with experiment. A 5 μs simulation of 10 surface-bound alm peptides shows significant peptide unfolding and some unbinding, but no insertion. Simulations at 363 and 413 K with a -0.2 V electric field yield peptide insertion in 1 μs. Insertion is initiated by the folding of residues 3-11 into an α-helix, and mediated by membrane water or by previously inserted peptides. The stability of five alm pore peptides at 413 K with a -0.2 V electric field demonstrates a significant preference for a transmembrane orientation. Hence, and in contrast to the cationic antimicrobial peptide described in the following article, alm shows a strong preference for the inserted over the surface-bound state.

Alamethicin Supramolecular Organization in Lipid Membranes from 19F Solid-State NMR

Alamethicins (ALMs) are antimicrobial peptides of fungal origin. Their sequences are rich in hydrophobic amino acids and strongly interact with lipid membranes, where they cause a well-defined increase in conductivity. Therefore, the peptides are thought to form transmembrane helical bundles in which the more hydrophilic residues line a water-filled pore. Whereas the peptide has been well characterized in terms of secondary structure, membrane topology, and interactions, much fewer data are available regarding the quaternary arrangement of the helices within lipid bilayers. A new, to our knowledge, fluorine-labeled ALM derivative was prepared and characterized when reconstituted into phospholipid bilayers. As a part of these studies, C¹⁹F₃-labeled compounds were characterized and calibrated for the first time, to our knowledge, for ¹⁹F solid-state NMR distance and oligomerization measurements by centerband-only detection of exchange (CODEX) experiments, which opens up a large range of potential labeling schemes. The ¹⁹F-¹⁹F CODEX solid-state NMR experiments performed with ALM in POPC lipid bilayers and at peptide/lipid ratios of 1:13 are in excellent agreement with molecular-dynamics calculations of dynamic pentameric assemblies. When the peptide/lipid ratio was lowered to 1:30, ALM was found in the dimeric form, indicating that the supramolecular organization is tuned by equilibria that can be shifted by changes in environmental conditions.

Length-Dependent Formation of Transmembrane Pores by 310-Helical α-Aminoisobutyric Acid Foldamers

The synthetic biology toolbox lacks extendable and conformationally controllable yet easy-to-synthesize building blocks that are long enough to span membranes. To meet this need, an iterative synthesis of α-aminoisobutyric acid (Aib) oligomers was used to create a library of homologous rigid-rod 3₁₀-helical foldamers, which have incrementally increasing lengths and functionalizable N- and C-termini. This library was used to probe the inter-relationship of foldamer length, self-association strength, and ionophoric ability, which is poorly understood. Although foldamer self-association in nonpolar chloroform increased with length, with a ∼14-fold increase in dimerization constant from Aib₆ to Aib₁₁, ionophoric activity in bilayers showed a stronger length dependence, with the observed rate constant for Aib₁₁ ∼70-fold greater than that of Aib₆. The strongest ionophoric activity was observed for foldamers with >10 Aib residues, which have end-to-end distances greater than the hydrophobic width of the bilayers used (∼2.8 nm); X-ray crystallography showed that Aib₁₁ is 2.93 nm long. These studies suggest that being long enough to span the membrane is more important for good ionophoric activity than strong self-association in the bilayer. Planar bilayer conductance measurements showed that Aib₁₁ and Aib₁₃, but not Aib₇, could form pores. This pore-forming behavior is strong evidence that Aib_m (m ≥ 10) building blocks can span bilayers.

The role of spontaneous lipid curvature in the interaction of interfacially active peptides with membranes

Research on antimicrobial peptides is in part driven by urgent medical needs such as the steady increase in pathogens being resistant to antibiotics. Despite the wealth of information compelling structure-function relationships are still scarce and thus the interfacial activity model has been proposed to bridge this gap. This model also applies to other interfacially active (membrane active) peptides such as cytolytic, cell penetrating or antitumor peptides. One parameter that is strongly linked to interfacial activity is the spontaneous lipid curvature, which is experimentally directly accessible. We discuss different parameters such as H-bonding, electrostatic repulsion, changes in monolayer surface area and lateral pressure that affect induction of membrane curvature, but also vice versa how membrane curvature triggers peptide response. In addition, the impact of membrane lipid composition on the formation of curved membrane structures and its relevance for diverse mode of action of interfacially active peptides and in turn biological activity are described. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Lipid Fluid-Gel Phase Transition Induced Alamethicin Orientational Change Probed by Sum Frequency Generation Vibrational Spectroscopy

Alamethicin has been extensively studied as an antimicrobial peptide (AMP) and is widely used as a simple model for ion channel proteins. It has been shown that the antimicrobial activity of AMPs is related to their cell membrane orientation, which may be influenced by the phase of the lipid molecules in the cell membrane. The "healthy" cell membranes contain fluid phase lipids, while gel phase lipids can be found in injured or aged cells or in some phase separated membrane regions. Thus, investigations on how the phase of the lipids influences the membrane orientation of AMPs are important to understand more details regarding the AMP's action on cell membranes. In this study, we determined the orientational changes of alamethicin molecules associated with planar substrate supported single lipid bilayers (serving as model cell membranes) with different phases (fluid or gel) as a function of peptide concentration using sum frequency generation (SFG) vibrational spectroscopy. The phase changes of the lipid bilayers were realized by varying the sample temperature. Our SFG results indicated that alamethicin lies down on the surface of fluid and gel phase 1,2-dimyristoyl(d54)-sn-glycero-3-phosphocholine (d-DMPC) lipid bilayers when the lipid bilayers are in contact with a peptide solution with a low concentration of 0.84 μM. However, at a medium peptide concentration of 10.80 μM, alamethicin inserts into the fluid phase lipid bilayer. Its orientation switches from a transmembrane to an in-plane (or lying down) orientation when the phase of the lipid bilayer changes from a fluid state to a gel state. At a high peptide concentration of 21.60 μM, alamethicin adopts a transmembrane orientation while associated with both fluid and gel phase lipid bilayers. We also studied the structural changes of the fluid and gel phase lipid bilayers upon their interactions with alamethicin molecules at different peptide concentrations.

Dynamics and conformational studies of TOAC spin labeled analogues of Ctx(Ile(21))-Ha peptide from Hypsiboas albopunctatus

Antimicrobial peptides (AMPs) isolated from several organisms have been receiving much attention due to some specific features that allow them to interact with, bind to, and disrupt cell membranes. The aim of this paper was to study the interactions between a membrane mimetic and the cationic AMP Ctx(Ile(21))-Ha as well as analogues containing the paramagnetic amino acid 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) incorporated at residue positions n = 0, 2, and 13. Circular dichroism studies showed that the peptides, except for [TOAC(13)]Ctx(Ile(21))-Ha, are unstructured in aqueous solution but acquire different amounts of α-helical secondary structure in the presence of trifluorethanol and lysophosphocholine micelles. Fluorescence experiments indicated that all peptides were able to interact with LPC micelles. In addition, Ctx(Ile(21))-Ha and [TOAC(13)]Ctx(Ile(21))-Ha peptides presented similar water accessibility for the Trp residue located near the N-terminal sequence. Electron spin resonance experiments showed two spectral components for [TOAC(0)]Ctx(Ile(21))-Ha, which are most likely due to two membrane-bound peptide conformations. In contrast, TOAC(2) and TOAC(13) derivatives presented a single spectral component corresponding to a strong immobilization of the probe. Thus, our findings allowed the description of the peptide topology in the membrane mimetic, where the N-terminal region is in dynamic equilibrium between an ordered, membrane-bound conformation and a disordered, mobile conformation; position 2 is most likely situated in the lipid polar head group region, and residue 13 is fully inserted into the hydrophobic core of the membrane.

Dependence of Alamethicin Membrane Orientation on the Solution Concentration

Alamethicin has been extensively studied as an antimicrobial peptide and is widely used as a simple model for ion channel proteins. It has been shown that the antimicrobial activity of peptides is related to their membrane orientation. In this study, we determined the relationship between the solution concentration of alamethicin and its membrane orientation in lipid bilayers using sum frequency generation (SFG) vibrational spectroscopy. Our SFG results indicated that the alamethicin molecules more or less lay down on the surface of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers at a low peptide concentration of 0.84 μM; the α-helix segment tilts at about 88°, and 3₁₀-helix segment tilts at about 58° versus the surface normal. However, when the peptide concentration was increased to 15.6 μM, we observed that alamethicin molecules further inserted into the lipid bilayers: the α-helical component changes its orientation to make a 37° tilt from the lipid bilayer normal, and the 3₁₀-helical component tilts at about 50° versus the surface normal. This is in agreement with the barrel-stave mode for the alamethicin-cell membrane interaction as reported previously. Additionally, we have also studied membrane orientation of alamethicin as a function of peptide concentration with SFG. Our results showed that the membrane orientation of the alamethicin α-helical component changed substantially with the increase of the alamethicin concentration, while the membrane orientation of the 3₁₀-helical component remained more or less the same.

Short peptide constructs mimic agonist sites of AT(1)R and BK receptors

Extracellular peptide ligand binding sites, which bind the N-termini of angiotensin II (AngII) and bradykinin (BK) peptides, are located on the N-terminal and extracellular loop 3 regions of the AT(1)R and BKRB(1) or BKRB(2) G-protein-coupled receptors (GPCRs). Here we synthesized peptides P15 and P13 corresponding to these receptor fragments and showed that only constructs in which these peptides were linked by S-S bond, and cyclized by closing the gap between them, could bind agonists. The formation of construct-agonist complexes was revealed by electron paramagnetic resonance spectra and fluorescence measurements of spin labeled biologically active analogs of AngII and BK (Toac(1)-AngII and Toac(0)-BK), where Toac is the amino acid-type paramagnetic and fluorescence quencher 2, 2, 6, 6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid. The inactive derivatives Toac(3)-AngII and Toac(3)-BK were used as controls. The interactions characterized by a significant immobilization of Toac and quenching of fluorescence in complexes between agonists and cyclic constructs were specific for each system of peptide-receptor construct assayed since no crossed reactions or reaction with inactive peptides could be detected. Similarities among AT, BKR, and chemokine receptors were identified, thus resulting in a configuration for AT(1)R and BKRB cyclic constructs based on the structure of the CXCR(4), an α-chemokine GPCR-type receptor.

Observing a model ion channel gating action in model cell membranes in real time in situ: membrane potential change induced alamethicin orientation change

Ion channels play crucial roles in transport and regulatory functions of living cells. Understanding the gating mechanisms of these channels is important to understanding and treating diseases that have been linked to ion channels. One potential model peptide for studying the mechanism of ion channel gating is alamethicin, which adopts a split α/3(10)-helix structure and responds to changes in electric potential. In this study, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), has been applied to characterize interactions between alamethicin (a model for larger channel proteins) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers in the presence of an electric potential across the membrane. The membrane potential difference was controlled by changing the pH of the solution in contact with the bilayer and was measured using fluorescence spectroscopy. The orientation angle of alamethicin in POPC lipid bilayers was then determined at different pH values using polarized SFG amide I spectra. Assuming that all molecules adopt the same orientation (a δ distribution), at pH = 6.7 the α-helix at the N-terminus and the 3(10)-helix at the C-terminus tilt at about 72° (θ(1)) and 50° (θ(2)) versus the surface normal, respectively. When pH increases to 11.9, θ(1) and θ(2) decrease to 56.5° and 45°, respectively. The δ distribution assumption was verified using a combination of SFG and ATR-FTIR measurements, which showed a quite narrow distribution in the angle of θ(1) for both pH conditions. This indicates that all alamethicin molecules at the surface adopt a nearly identical orientation in POPC lipid bilayers. The localized pH change in proximity to the bilayer modulates the membrane potential and thus induces a decrease in both the tilt and the bend angles of the two helices in alamethicin. This is the first reported application of SFG to the study of model ion channel gating mechanisms in model cell membranes.

Hydrophobic mismatch of mobile transmembrane helices: Merging theory and experiments

Hydrophobic mismatch still represents a puzzle for transmembrane peptides, despite the apparent simplicity of this concept and its demonstrated validity in natural membranes. Using a wealth of available experimental ((2))H NMR data, we provide here a comprehensive explanation of the orientation and dynamics of model peptides in lipid bilayers, which shows how they can adapt to membranes of different thickness. The orientational adjustment of transmembrane α-helices can be understood as the result of a competition between the thermodynamically unfavorable lipid repacking associated with peptide tilting and the optimization of peptide/membrane hydrophobic coupling. In the positive mismatch regime (long-peptide/thin-membrane) the helices adapt mainly via changing their tilt angle, as expected from simple geometrical predictions. However, the adaptation mechanism varies with the peptide sequence in the flanking regions, suggesting additional effects that modulate hydrophobic coupling. These originate from re-adjustments of the peptide hydrophobic length and they depend on the hydrophobicity of the flanking region, the strength of interfacial anchoring, the structural flexibility of anchoring side-chains and the presence of alternative anchoring residues.

The spin label amino acid TOAC and its uses in studies of peptides: chemical, physicochemical, spectroscopic, and conformational aspects

We review work on the paramagnetic amino acid 2,2,6,6-tetramethyl-N-oxyl-4-amino-4-carboxylic acid, TOAC, and its applications in studies of peptides and peptide synthesis. TOAC was the first spin label probe incorporated in peptides by means of a peptide bond. In view of the rigid character of this cyclic molecule and its attachment to the peptide backbone via a peptide bond, TOAC incorporation has been very useful to analyze backbone dynamics and peptide secondary structure. Many of these studies were performed making use of EPR spectroscopy, but other physical techniques, such as X-ray crystallography, CD, fluorescence, NMR, and FT-IR, have been employed. The use of double-labeled synthetic peptides has allowed the investigation of their secondary structure. A large number of studies have focused on the interaction of peptides, both synthetic and biologically active, with membranes. In the latter case, work has been reported on ligands and fragments of GPCR, host defense peptides, phospholamban, and β-amyloid. EPR studies of macroscopically aligned samples have provided information on the orientation of peptides in membranes. More recent studies have focused on peptide–protein and peptide–nucleic acid interactions. Moreover, TOAC has been shown to be a valuable probe for paramagnetic relaxation enhancement NMR studies of the interaction of labeled peptides with proteins. The growth of the number of TOAC-related publications suggests that this unnatural amino acid will find increasing applications in the future.

Interactions of alamethicin with model cell membranes investigated using sum frequency generation vibrational spectroscopy in real time in situ

Structures of membrane-associated peptides and molecular interactions between peptides and cell membrane bilayers govern biological functions of these peptides. Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study such structures and interactions at the molecular level. In this research, SFG has been applied, supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), to characterize the interactions between alamethicin (a model for larger channel proteins) and different lipid bilayers in the absence of membrane potential. The orientation of alamethicin in lipid bilayers has been determined using SFG amide I spectra detected with different polarization combinations. It was found that alamethicin adopts a mixed alpha-helical and 3(10)-helical structure in fluid-phase lipid bilayers. The helix (mainly alpha-helix) at the N-terminus tilts at about 63 degrees versus the surface normal in a fluid-phase 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine-1,1,2,2-d4-N,N,N-trimethyl-d9 (d-DMPC)/1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer. The 3(10)-helix at the C-terminus (beyond the Pro14 residue) tilts at about 43 degrees versus the surface normal. This is the first time to apply SFG to study a 3(10)-helix experimentally. When interacting with a gel-phase lipid bilayer, alamethicin lies down on the gel-phase bilayer surface or aggregates or both, which does not have significant insertion into the lipid bilayer.

Tilt and rotation angles of a transmembrane model peptide as studied by fluorescence spectroscopy

In this study the membrane orientation of a tryptophan-flanked model peptide, WALP23, was determined by using peptides that were labeled at different positions along the sequence with the environmentally sensitive fluorescent label BADAN. The fluorescence properties, reflecting the local polarity, were used to determine the tilt and rotation angles of the peptide based on an ideal alpha-helix model. For WALP23 inserted in dioleoylphosphatidylcholine (DOPC), an estimated tilt angle of the helix with respect to the bilayer normal of 24 degrees +/- 5 degrees was obtained. When the peptides were inserted into bilayers with different acyl chain lengths or containing different concentrations of cholesterol, small changes in tilt angle were observed as response to hydrophobic mismatch, whereas the rotation angle appeared to be independent of lipid composition. In all cases, the tilt angles were significantly larger than those previously determined from (2)H NMR experiments, supporting recent suggestions that the relatively long timescale of (2)H NMR measurements may result in an underestimation of tilt angles due to partial motional averaging. It is concluded that although the fluorescence technique has a rather low resolution and limited accuracy, it can be used to resolve the discrepancies observed between previous (2)H NMR experiments and molecular-dynamics simulations.

Spin-Label EPR for Determining Polarity and Proticity in Biomolecular Assemblies: Transmembrane Profiles

Hyperfine couplings and g-values of nitroxyl spin labels are sensitive to polarity and hydrogen bonding in the environment probed. The dependences of these electronic paramagnetic resonance (EPR) properties on environmental dielectric permittivity and proticity are reviewed. Calibrations are given, in terms of the Block-Walker reaction field and local proton donor concentration, for the nitroxides that are commonly used in spin labeling of lipids and proteins. Applications to studies of the transverse polarity profiles in lipid bilayers, which constitute the permeability barrier of biological membranes, are reviewed. Emphasis is given to parallels with the permeation profiles of oxygen and nitric oxide that are determined from spin-label relaxation enhancements by using nonlinear continuous-wave EPR and saturation recovery EPR, and with permeation profiles of D(2)O that are determined by using (2)H electron spin echo envelope modulation spectroscopy.

Orientation and dynamics of transmembrane peptides: the power of simple models

In this review we discuss recent insights obtained from well-characterized model systems into the factors that determine the orientation and tilt angles of transmembrane peptides in lipid bilayers. We will compare tilt angles of synthetic peptides with those of natural peptides and proteins, and we will discuss how tilt can be modulated by hydrophobic mismatch between the thickness of the bilayer and the length of the membrane spanning part of the peptide or protein. In particular, we will focus on results obtained on tryptophan-flanked model peptides (WALP peptides) as a case study to illustrate possible consequences of hydrophobic mismatch in molecular detail and to highlight the importance of peptide dynamics for the experimental determination of tilt angles. We will conclude with discussing some future prospects and challenges concerning the use of simple peptide/lipid model systems as a tool to understand membrane structure and function.

In situ molecular level studies on membrane related peptides and proteins in real time using sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study the molecular structures of surfaces and interfaces in different chemical environments. This review summarizes recent SFG studies on hybrid bilayer membranes and substrate-supported lipid monolayers and bilayers, the interaction between peptides/proteins and lipid monolayers/bilayers, and bilayer perturbation induced by peptides/proteins. To demonstrate the ability of SFG to determine the orientations of various secondary structures, studies on the interactions between different peptides/proteins (melittin, G proteins, alamethicin, and tachyplesin I) and lipid bilayers are discussed. Molecular level details revealed by SFG in these studies show that SFG can provide a unique understanding on the interactions between a lipid monolayer/bilayer and peptides/proteins in real time, in situ and without any exogenous labeling.

Structure of self-aggregated alamethicin in ePC membranes detected by pulsed electron-electron double resonance and electron spin echo envelope modulation spectroscopies

PELDOR spectroscopy was exploited to study the self-assembled super-structure of the [Glu(OMe)(7,18,19)]alamethicin molecules in vesicular membranes at peptide to lipid molar ratios in the range of 1:70-1:200. The peptide molecules were site-specifically labeled with TOAC electron spins. From the magnetic dipole-dipole interaction between the nitroxides of the monolabeled constituents and the PELDOR decay patterns measured at 77 K, intermolecular-distance distribution functions were obtained and the number of aggregated molecules (n approximately 4) was estimated. The distance distribution functions exhibit a similar maximum at 2.3 nm. In contrast to Alm16, for Alm1 and Alm8 additional maxima were recorded at 3.2 and approximately 5.2 nm. From ESEEM experiments and based on the membrane polarity profiles, the penetration depths of the different spin-labeled positions into the membrane were qualitatively estimated. It was found that the water accessibility of the spin-labels follows the order TOAC-1 > TOAC-8 approximately TOAC-16. The geometric data obtained are discussed in terms of a penknife molecular model. At least two peptide chains are aligned parallel and eight ester groups of the polar Glu(OMe)(18,19) residues are suggested to stabilize the self-aggregate superstructure.

Reaction fields in the environment of fluorescent probes: polarity profiles in membranes

Fluorescent probes in biological systems are sensitive to environmental polarity by virtue of their response to the reaction field created by polarization of the dielectric medium. Classically, fluorophore solvatochromism is analyzed in terms of the Lippert equation and later variants, all of which rely upon the original reaction field of Onsager. A recent survey of the solvent dependence of EPR spin-label probes, which are responsive solely to the reaction field in the ground state without the complication of excited states, shows that the reaction field of Block and Walker performs best in describing the polarity dependence. In this model, the step-function transition to the bulk dielectric medium used by Onsager is replaced by a graded transition. Analysis of the Stokes shifts for representative fluorescent membrane probes, such as PRODAN, DANSYL, and anthroyl fatty acid, reveals that, of several different reaction fields (including that of Onsager), the Block-Walker model best describes the dependence on solvent dielectric constant and refractive index for the different probes simultaneously. This is after full allowance is made for all contributions involving polarizability of the fluorophore, a point that is frequently neglected or treated incorrectly in studies using biological fluorescent probes. By using the full range of polar and apolar solvents, it is then possible to establish a common reference for the polarity dependence of different fluorophores and to relate this also to the polarity dependence of biologically relevant spin-label EPR probes. An important application is calibration of the transmembrane polarity profile recorded by fluorescent probes in terms of the high-resolution profile obtained from site-specifically spin-labeled lipid chains.

Intramembrane water associated with TOAC spin-labeled alamethicin: electron spin-echo envelope modulation by D2O

Alamethicin is a 20-residue, hydrophobic, helical peptide, which forms voltage-sensitive ion channels in lipid membranes. The helicogenic, nitroxyl amino acid TOAC was substituted isosterically for Aib at residue positions 1, 8, or 16 in a F50/5 alamethicin analog to enable EPR studies. Electron spin-echo envelope modulation (ESEEM) spectroscopy was used to investigate the water exposure of TOAC-alamethicin introduced into membranes of saturated or unsaturated diacyl phosphatidylcholines that were dispersed in D2O. Echo-detected EPR spectra were used to assess the degree of assembly of the peptide in the membrane, via the instantaneous diffusion from intermolecular spin-spin interactions. The profile of residue exposure to water differs between membranes of saturated and unsaturated lipids. In monounsaturated dioleoyl phosphatidylcholine, D2O-ESEEM intensities decrease from TOAC(1) to TOAC(8) and TOAC(16) but not uniformly. This is consistent with a transmembrane orientation for the protoassembled state, in which TOAC(16) is located in the bilayer leaflet opposite to that of TOAC(1) and TOAC(8). Relative to the monomer in fluid bilayers, assembled alamethicin is disposed asymmetrically about the bilayer midplane. In saturated dimyristoyl phosphatidylcholine, the D2O-ESEEM intensity is greatest for TOAC(8), indicating a more superficial location for alamethicin, which correlates with the difference in orientation between gel- and fluid-phase membranes found by conventional EPR of TOAC-alamethicin in aligned phosphatidylcholine bilayers. Increasing alamethicin/lipid ratio in saturated phosphatidylcholine shifts the profile of water exposure toward that with unsaturated lipid, consistent with proposals of a critical concentration for switching between the two different membrane-associated states.

Orientation and peptide-lipid interactions of alamethicin incorporated in phospholipid membranes: polarized infrared and spin-label EPR spectroscopy

Alamethicin is a 20-residue peptaibiotic that induces voltage-dependent ion channels in lipid membranes. The mode by which alamethicin inserts into membranes was investigated using measurements of peptide-lipid interactions by spin-label electron paramagnetic resonance (EPR) and of peptide orientation by polarized infrared (IR) spectroscopy. In fluid membranes, spin-labeled stearic acid shows no evidence of a specific motionally restricted population of lipid chains, such as that found at the intramembranous surface of integral membrane proteins or oligomeric assemblies of transmembrane alpha-helices. In agreement with recent results from TOAC-substituted alamethicin analogues, native alamethicin is predominantly monomeric in fluid lipid membranes and presents an intramembrane surface that integrates well with the lipid chains but is insufficiently extensive to induce specific motional restriction. Channel formation takes place by transient association of transmembrane monomers. In aligned fluid membranes, alamethicin exhibits a large tilt in short chain-length lipids that decreases first rapidly with increasing chain-length and then more gradually for the lipids with longer chains. This macroscopically low order contrasts with the high local order, relative to the local membrane normal, that is found by EPR for alamethicins spin-labeled with TOAC. The macroscopic behavior is consistent with predictions for the chain-length dependence of elastic bending fluctuations of the membrane surface, which was invoked recently to explain the spontaneous insertion of beta-barrel proteins in short-chain lipid membranes.

Protein-lipid interactions with Fusobacterium nucleatum major outer membrane protein FomA: spin-label EPR and polarized infrared spectroscopy

FomA, the major outer membrane protein of Fusobacterium nucleatum, was expressed and purified in Escherichia coli and reconstituted from detergent in bilayer membranes of phosphatidylcholines with chain lengths from C(12:0) to C(17:0). The conformation and orientation of membrane-incorporated FomA were determined from polarized, attenuated total reflection, infrared (IR) spectroscopy, and lipid-protein interactions with FomA were characterized by using electron paramagnetic resonance (EPR) spectroscopy of spin-labeled lipids. Approximately 190 residues of membranous FomA are estimated to be in a beta-sheet configuration from IR band fitting, which is consistent with a 14-strand transmembrane beta-barrel structure. IR dichroism of FomA indicates that the beta-strands are tilted by approximately 45 degrees relative to the sheet/barrel axis and that the order parameter of the latter displays a discontinuity corresponding to hydrophobic matching with fluid C(13:0) lipid chains. The stoichiometry ( N b = 23 lipids/monomer) of lipid-protein interaction from EPR demonstrates that FomA is not trimeric in membranes of diC(14:0) phosphatidylcholine and is consistent with a monomeric beta-barrel of 14-16 strands. The pronounced selectivity of interaction found with anionic spin-labeled lipids places basic residues of the protein in the vicinity of the polar-apolar membrane interfaces, consistent with current topology models. Comparison with similar data from the 8- to 22-stranded E. coli outer membrane proteins, OmpA, OmpG, and FhuA, supports the above conclusions.

Energetics of hydrophobic matching in lipid-protein interactions

Lipid chain length modulates the activity of transmembrane proteins by mismatch between the hydrophobic span of the protein and that of the lipid membrane. Relative binding affinities of lipids with different chain lengths are used to estimate the excess free energy of lipid-protein interaction that arises from hydrophobic mismatch. For a wide range of integral proteins and peptides, the energy cost is much less than the elastic penalty of fully stretching or compressing the lipid chains to achieve complete hydrophobic matching. The chain length dependences of the free energies of lipid association are described by a model that combines elastic chain extension with a free energy term that depends linearly on the extent of residual mismatch. The excess free energy densities involved lie in the region of 0.5-2.0 k(B)T x nm(-2). Values of this size could arise from exposure of hydrophobic groups to polar portions of the lipid or protein, but not directly to water, or alternatively from changes in tilt of the transmembrane helices that are energetically comparable to those activating mechanosensitive channels. The influence of hydrophobic mismatch on dimerization of transmembrane helices and their transfer between lipid vesicles, and on shifts in chain-melting transitions of lipid bilayers by incorporated proteins, is analyzed by using the same thermodynamic model. Segmental order parameters confirm that elastic lipid chain distortions are insufficient to compensate fully for the mismatch, but the dependence on chain length with tryptophan-anchored peptides requires that the free energy density of hydrophobic mismatch should increase with increasing extent of mismatch.

Incorporation of outer membrane protein OmpG in lipid membranes: protein-lipid interactions and beta-barrel orientation

OmpG is an intermediate size, monomeric, outer membrane protein from Escherichia coli, with n beta = 14 beta-strands. It has a large pore that is amenable to modification by protein engineering. The stoichiometry ( N b = 20) and selectivity ( K r = 0.7-1.2) of lipid-protein interaction with OmpG incorporated in dimyristoyl phosphatidylcholine bilayer membranes was determined with various 14-position spin-labeled lipids by using EPR spectroscopy. The limited selectivity for different lipid species is consistent with the disposition of charged residues in the protein. The conformation and orientation (beta-strand tilt and beta-barrel order parameters) of OmpG in disaturated phosphatidylcholines of odd and even chain lengths from C(12:0) to C(17:0) was determined from polarized infrared spectroscopy of the amide I and amide II bands. A discontinuity in the protein orientation (deduced from the beta-barrel order parameters) is observed at the point of hydrophobic matching of the protein with lipid chain length. Compared with smaller (OmpA; n beta = 8) and larger (FhuA; n beta = 22) monomeric E. coli outer membrane proteins, the stoichiometry of motionally restricted lipids increases linearly with the number of beta-strands, the tilt (beta approximately 44 degrees ) of the beta-strands is comparable for the three proteins, and the order parameter of the beta-barrel increases regularly with n beta. These systematic features of the integration of monomeric beta-barrel proteins in lipid membranes could be useful for characterizing outer membrane proteins of unknown structure.

Polarity dependence of EPR parameters for TOAC and MTSSL spin labels: correlation with DOXYL spin labels for membrane studies

TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) is a nitroxyl amino acid that can be incorporated in the backbone of peptides. DOXYL (4,4-dimethyl-oxazolidine-1-oxyl) is a nitroxyl ring that can be attached rigidly at specific C-atom positions in the acyl chains of phospholipids. Spin-labelled phosphatidylcholines of the DOXYL type have been used previously to establish the transmembrane polarity profile in biological lipid bilayers [D. Marsh, Polarity and permeation profiles in lipid membranes, Proc. Natl. Acad. Sci. USA 87 (2001) 7777-7782]. Here, we determine the polarity dependence of the isotropic (14)N-hyperfine couplings, a(o)(N), and g-values, g(o), in a wide range of protic and aprotic media, for a TOAC-containing dipeptide (Fmoc-TOAC-Aib-OMe) and for a DOXYL-containing fatty acid (12-DOXYL-stearic acid). The correlation between datasets for TOAC and DOXYL nitroxides in the various solvents is used to establish the polarity profile for isotropic hyperfine couplings of TOAC in a transmembrane peptide. This calibration can be used to determine the location of TOAC at selected residue positions in a transmembrane or surface-active peptide. A similar calibration procedure is also applied to a(o)(N) and g(o) for the pyrroline methanethiosulphonate nitroxide (MTSSL) that is used in site-directed spin-labelling studies of membrane proteins.

Reaction fields and solvent dependence of the EPR parameters of nitroxides: the microenvironment of spin labels

The sensitivity of nitroxide spin-label EPR to the polarity of aprotic environments arises from the reaction field produced by polarisation of the surrounding dielectric by the nitroxide electric dipole moment. The performances of three different reaction fields that have been proposed as improvements on the original Onsager model are compared for representative spin-label nitroxides in a range of apolar and dipolar aprotic solvents. Explicit allowance is made for the polarisability of the nitroxide, which effectively renormalises the reaction field but has been neglected in previous analyses of nitroxide hyperfine couplings when using the improved reaction fields. It is found that the model of Block and Walker, which incorporates an exponential dependence of the dielectric permittivity on inverse radial distance from the nitroxide, gives the best description of the solvent dependence of the isotropic (14)N-hyperfine couplings. These results should be useful not only for calibration of environmental polarity using homogeneous solvents, but also for transferring polarity scales and polarity profiles (e.g., in membranes) between different nitroxide spin labels (e.g., of the TEMPO and DOXYL variety).

Backbone dynamics of alamethicin bound to lipid membranes: spin-echo electron paramagnetic resonance of TOAC-spin labels

Alamethicin F50/5 is a hydrophobic peptide that is devoid of charged residues and that induces voltage-dependent ion channels in lipid membranes. The peptide backbone is likely to be involved in the ion conduction pathway. Electron spin-echo spectroscopy of alamethicin F50/5 analogs in which a selected Aib residue (at position n = 1, 8, or 16) is replaced by the TOAC amino-acid spin label was used to study torsional dynamics of the peptide backbone in association with phosphatidylcholine bilayer membranes. Rapid librational motions of limited angular amplitude were observed at each of the three TOAC sites by recording echo-detected spectra as a function of echo delay time, 2tau. Simulation of the time-resolved spectra, combined with conventional EPR measurements of the librational amplitude, shows that torsional fluctuations of the peptide backbone take place on the subnanosecond to nanosecond timescale, with little temperature dependence. Associated fluctuations in polar fields from the peptide could facilitate ion permeation.

Lateral pressure profile, spontaneous curvature frustration, and the incorporation and conformation of proteins in membranes

Lipid-protein interactions are an important determinant of the stability and function of integral and transmembrane proteins. In addition to local interactions at the lipid-protein interface, global interactions such as the distribution of internal lateral pressure may also influence protein conformation. It is shown here that the effects of the membrane lateral pressure profile on the conformation or insertion of proteins in membranes are equivalent to the elastic response to the frustrated spontaneous curvature, c(o), of the component lipid monolayer leaflets. The chemical potential of the protein in the membrane is predicted to depend linearly on the spontaneous curvature of the lipid leaflets, just as does the contribution of the protein to the elastic bending energy of the lipid, and to be independent of the hydrophobic tension, gamma(phob), at the lipid-water interface. Analysis of the dependence of protein partitioning or conformational transitions on spontaneous curvature of the constituent lipids gives an experimental estimate for the cross-sectional intramembrane shape of the protein or its difference between conformations. Values in the region of 50-110 A(2) are estimated for the effective cross-sectional shape changes on the insertion and conductance transitions of alamethicin, and on the activation of CTP:phosphocholine cytidylyltransferase or rhodopsin in lipid membranes. Much larger values are estimated for the mechanosensitive channel, MscL. Values for the change in intramembrane shape may also be used, together with determinations of lipid relative association constants, to estimate contributions of direct lipid-protein interactions to the lateral pressure experienced by the protein. Changes in chemical potential approximately 12 kJ mol(-1) can be estimated for radial changes of 1 A in a protein of diameter 40 A.

Solvent dependence of the rotational diffusion of TOAC-spin-labeled alamethicin

Three derivatives of the hydrophobic, channel-forming peptaibiotic alamethicin (F50/5) have been synthesized, the original Aib residue at position 1, 8, or 16 being replaced with the spin-labeled amino acid TOAC (=2,2,6,6-tetramethylpiperidin-1-oxyl-4-amino-4-carboxylic acid). Electron-paramagnetic-resonance (EPR) spectroscopy was used to characterize the rotational diffusion of these compounds in five isotropic solvents of differing viscosity and polarity, including MeOH, EtOH, PrOH, i-PrOH, and hexanol (HxOH). In MeOH, the labeled alamethicins were found to rotate anisotropically as a monomer (axial ratio a/b=3). In aliphatic alcohols of increasing viscosity (and hydrophobicity), the rotational correlation times progressively increased. Even in HxOH, the (fivefold) increase in correlation time was no greater than the increase in viscosity. We conclude that TOAC-labeled alamethicins remain monomeric in these solvents of relatively high polarity.