Method of oriented circular dichroism

Circular dichroism of relativistically-moving chiral molecules

Understanding the impact of the relativistic motion of a chiral molecule on its optical response is a prime challenge for fundamental science, but it also has a direct practical relevance in our search for extraterrestrial life. To contribute to these significant developments, we describe a multi-scale computational framework that combines quantum chemistry calculations and full-wave optical simulations to predict the chiral optical response from molecules moving at relativistic speeds. Specifically, the effect of a relativistic motion on the transmission circular dichroism (TCD) of three life-essential biomolecules, namely, B-DNA, chlorophyll a, and chlorophyll b, is investigated. Inspired by previous experiments to detect interstellar chiral molecules, we assume that the molecules move between a stationary observer and a light source, and we study the rotationally averaged TCD as a function of the speed of the molecule.We find that the TCD spectrum that contains the signatures of the molecules shifts with increasing speed to shorter wavelengths, with the effects already being visible for moderate velocities.

Theory predicts 2D chiral polaritons based on achiral Fabry-Pérot cavities using apparent circular dichroism

Realizing polariton states with high levels of chirality offers exciting prospects for quantum information, sensing, and lasing applications. Such chirality must emanate from either the involved optical resonators or the quantum emitters. Here, we theoretically demonstrate a rare opportunity for realizing polaritons with so-called 2D chirality by strong coupling of the optical modes of (high finesse) achiral Fabry-Pérot cavities with samples exhibiting "apparent circular dichroism" (ACD). ACD is a phenomenon resulting from an interference between linear birefringence and dichroic interactions. By introducing a quantum electrodynamical theory of ACD, we identify the design rules based on which 2D chiral polaritons can be produced, and their chirality can be optimized.

The Functionality of Membrane-Inserting Proteins and Peptides: Curvature Sensing, Generation, and Pore Formation

Proteins and peptides with hydrophobic and amphiphilic segments are responsible for many biological functions. The sensing and generation of membrane curvature are the functions of several protein domains or motifs. While some specific membrane proteins play an essential role in controlling the curvature of distinct intracellular membranes, others participate in various cellular processes such as clathrin-mediated endocytosis, where several proteins sort themselves at the neck of the membrane bud. A few membrane-inserting proteins form nanopores that permeate selective ions and water to cross the membrane. In addition, many natural and synthetic small peptides and protein toxins disrupt the membrane by inducing nonspecific pores in the membrane. The pore formation causes cell death through the uncontrolled exchange between interior and exterior cellular contents. In this article, we discuss the insertion depth and orientation of protein/peptide helices, and their role as a sensor and inducer of membrane curvature as well as a pore former in the membrane. We anticipate that this extensive review will assist biophysicists to gain insight into curvature sensing, generation, and pore formation by membrane insertion.

Conformational analysis of membrane-proximal segments of GDAP1 in a lipidic environment using synchrotron radiation suggests a mode of assembly at the mitochondrial outer membrane

The mitochondrial outer membrane creates a diffusion barrier between the cytosol and the mitochondrial intermembrane space, allowing the exchange of metabolic products, important for efficient mitochondrial function in neurons. The ganglioside-induced differentiation-associated protein 1 (GDAP1) is a mitochondrial outer membrane protein with a critical role in mitochondrial dynamics and metabolic balance in neurons. Missense mutations in the GDAP1 gene are linked to the most common human peripheral neuropathy, Charcot-Marie-Tooth disease (CMT). GDAP1 is a distant member of the glutathione-S-transferase (GST) superfamily, with unknown enzymatic properties or functions at the molecular level. The structure of the cytosol-facing GST-like domain has been described, but there is no consensus on how the protein interacts with the mitochondrial outer membrane. Here, we describe a model for GDAP1 assembly on the membrane using peptides vicinal to the GDAP1 transmembrane domain. We used oriented circular dichroism spectroscopy (OCD) with synchrotron radiation to study the secondary structure and orientation of GDAP1 segments at the outer and inner surfaces of the outer mitochondrial membrane. These experiments were complemented by small-angle X-ray scattering, providing the first experimental structural models for full-length human GDAP1. The results indicate that GDAP1 is bound into the membrane via a single transmembrane helix, flanked by two peripheral helices interacting with the outer and inner leaflets of the mitochondrial outer membrane in different orientations. Impairment of these interactions could be a mechanism for CMT in the case of missense mutations affecting these segments instead of the GST-like domain.

Promoting the activity of a receptor tyrosine phosphatase with a novel pH-responsive transmembrane agonist inhibits cancer-associated phenotypes

Cell signaling by receptor protein tyrosine kinases (RTKs) is tightly controlled by the counterbalancing actions of receptor protein tyrosine phosphatases (RPTPs). Due to their role in attenuating the signal-initiating potency of RTKs, RPTPs have long been viewed as therapeutic targets. However, the development of activators of RPTPs has remained limited. We previously reported that the homodimerization of a representative member of the RPTP family (protein tyrosine phosphatase receptor J or PTPRJ) is regulated by specific transmembrane (TM) residues. Disrupting this interaction by single point mutations promotes PTPRJ access to its RTK substrates (e.g., EGFR and FLT3), reduces RTK's phosphorylation and downstream signaling, and ultimately antagonizes RTK-driven cell phenotypes. Here, we designed and tested a series of first-in-class pH-responsive TM peptide agonists of PTPRJ that are soluble in aqueous solution but insert as a helical TM domain in lipid membranes when the pH is lowered to match that of the acidic microenvironment of tumors. The most promising peptide reduced EGFR's phosphorylation and inhibited cancer cell EGFR-driven migration and proliferation, similar to the PTPRJ's TM point mutations. Developing tumor-selective and TM-targeting peptide binders of critical RPTPs could afford a potentially transformative approach to studying RPTP's selectivity mechanism without requiring less specific inhibitors and represent a novel class of therapeutics against RTK-driven cancers.

Temperature-Dependent Re-alignment of the Short Multifunctional Peptide BP100 in Membranes Revealed by Solid-State NMR Spectroscopy and Molecular Dynamics Simulations

BP100 is a cationic undecamer peptide with antimicrobial and cell-penetrating activities. The orientation of this amphiphilic α-helix in lipid bilayers was examined under numerous conditions using solid-state ¹⁹ F, ¹⁵ N and ² H NMR. At high temperatures in saturated phosphatidylcholine lipids, BP100 lies flat on the membrane surface, as expected. Upon lowering the temperature towards the lipid phase transition, the helix is found to flip into an upright transmembrane orientation. In thin bilayers, this inserted state was stable at low peptide concentration, but thicker membranes required higher peptide concentrations. In the presence of lysolipids, the inserted state prevailed even at high temperature. Molecular dynamics simulations suggest that BP100 monomer insertion can be stabilized by snorkeling lysine side chains. These results demonstrate that even a very short helix like BP100 can span (and thereby penetrate through) a cellular membrane under suitable conditions.

Length matters: Functional flip of the short TatA transmembrane helix

The twin arginine translocase (Tat) exports folded proteins across bacterial membranes. The putative pore-forming or membrane-weakening component (TatA_d in B. subtilis) is anchored to the lipid bilayer via an unusually short transmembrane α-helix (TMH), with less than 16 residues. Its tilt angle in different membranes was analyzed under hydrophobic mismatch conditions, using synchrotron radiation circular dichroism and solid-state NMR. Positive mismatch (introduced either by reconstitution in short-chain lipids or by extending the hydrophobic TMH length) increased the helix tilt of the TMH as expected. Negative mismatch (introduced either by reconstitution in long-chain lipids or by shortening the TMH), on the other hand, led to protein aggregation. These data suggest that the TMH of TatA is just about long enough for stable membrane insertion. At the same time, its short length is a crucial factor for successful translocation, as demonstrated here in native membrane vesicles using an in vitro translocation assay. Furthermore, when reconstituted in model membranes with negative spontaneous curvature, the TMH was found to be aligned parallel to the membrane surface. This intrinsic ability of TatA to flip out of the membrane core thus seems to play a key role in its membrane-destabilizing effect during Tat-dependent translocation.

Small-Angle Neutron Scattering for Studying Lipid Bilayer Membranes

Small-angle neutron scattering (SANS) is a powerful tool for studying biological membranes and model lipid bilayer membranes. The length scales probed by SANS, being from 1 nm to over 100 nm, are well-matched to the relevant length scales of the bilayer, particularly when it is in the form of a vesicle. However, it is the ability of SANS to differentiate between isotopes of hydrogen as well as the availability of deuterium labeled lipids that truly enable SANS to reveal details of membranes that are not accessible with the use of other techniques, such as small-angle X-ray scattering. In this work, an overview of the use of SANS for studying unilamellar lipid bilayer vesicles is presented. The technique is briefly presented, and the power of selective deuteration and contrast variation methods is discussed. Approaches to modeling SANS data from unilamellar lipid bilayer vesicles are presented. Finally, recent examples are discussed. While the emphasis is on studies of unilamellar vesicles, examples of the use of SANS to study intact cells are also presented.

The Candida albicans virulence factor candidalysin polymerizes in solution to form membrane pores and damage epithelial cells

Candida albicans causes severe invasive candidiasis. C. albicans infection requires the virulence factor candidalysin (CL) which damages target cell membranes. However, the mechanism that CL uses to permeabilize membranes is unclear. We reveal that CL forms membrane pores using a unique mechanism. Unexpectedly, CL readily assembled into polymers in solution. We propose that the basic structural unit in polymer formation is a CL oligomer, which is sequentially added into a string configuration that can close into a loop. CL loops appear to spontaneously insert into the membrane to become pores. A CL mutation (G4W) inhibited the formation of polymers in solution and prevented pore formation in synthetic lipid systems. Epithelial cell studies showed that G4W CL failed to activate the danger response pathway, a hallmark of the pathogenic effect of CL. These results indicate that CL polymerization in solution is a necessary step for the damage of cellular membranes. Analysis of CL pores by atomic force microscopy revealed co-existence of simple depressions and more complex pores, which are likely formed by CL assembled in an alternate oligomer orientation. We propose that this structural rearrangement represents a maturation mechanism that stabilizes pore formation to achieve more robust cellular damage. To summarize, CL uses a previously unknown mechanism to damage membranes, whereby pre-assembly of CL loops in solution leads to formation of membrane pores. Our investigation not only unravels a new paradigm for the formation of membrane pores, but additionally identifies CL polymerization as a novel therapeutic target to treat candidiasis.

Ca2+ -dependent interactions between lipids and the tumor-targeting peptide pHLIP

Cancerous tissues undergo extensive changes to their cellular environments that differentiate them from healthy tissues. These changes include changes in extracellular pH and Ca2+ concentrations, and the exposure of phosphatidylserine (PS) to the extracellular environment, which can modulate the interaction of peptides and proteins with the plasma membrane. Deciphering the molecular mechanisms of such interactions is critical for advancing the knowledge-based design of cancer-targeting molecular tools, such as pH-low insertion peptide (pHLIP). Here, we explore the effects of PS, Ca2+ , and peptide protonation state on the interactions of pHLIP with lipid membranes. Cellular studies demonstrate that exposed PS on the plasma membrane promotes pHLIP targeting. The magnitude of this effect is dependent on extracellular Ca2+ concentration, indicating that divalent cations play an important role in pHLIP targeting in vivo. The targeting mechanism is further explored with a combination of fluorescence and circular dichroism experiments in model membranes and microsecond-timescale all-atom molecular dynamics simulations. Our results demonstrate that Ca2+ is engaged in coupling peptide-lipid interactions in the unprotonated transmembrane conformation of pHLIP. The simulations reveal that while the pH-induced insertion leads to a strong depletion of PS around pHLIP, the Ca2+ -induced insertion has the opposite effect. Thus, extracellular levels of Ca2+ are crucial to linking cellular changes in membrane lipid composition with the selective targeting and insertion of pHLIP. The characterized Ca2+ -dependent coupling between pHLIP sidechains and PS provides atomistic insights into the general mechanism for lipid-coupled regulation of protein-membrane insertion by divalent cations.

Theory of Apparent Circular Dichroism Reveals the Origin of Inverted and Noninverted Chiroptical Response under Sample Flipping

Circular dichroism (CD) finds widespread application as an optical probe for the structure of molecules and supramolecular assemblies. Its underlying chiral light-matter interactions effectively couple between photonic spin states and select quantum-mechanical degrees of freedom in a sample, implying an intricate connection with photon-to-matter quantum transduction. However, effective transduction implementations likely require interactions that are antisymmetric with respect to the direction of light propagation through the sample, yielding an inversion of the chiroptical response upon sample flipping, which is uncommon for CD. Recent experiments on organic thin films have demonstrated such chiroptical behavior, which was attributed to "apparent CD" resulting from an interference between the sample's linear birefringence and linear dichroism. However, a theory connecting the underlying optical selection rules to the microscopic electronic structure of the constituent molecules remains to be formulated. Here, we present such a theory based on a combination of Mueller calculus and a Lorentz oscillator model. The theory reaches good agreement with experimental CD spectra and allows for establishing the (supra)molecular design rules for maximizing or minimizing this chiroptical effect. It furthermore highlights that, in addition to antisymmetrically, it can manifest symmetrically such that no chiroptical response inversion occurs, which is a consequence of a helical stacking of molecules in the light propagation direction.

Singular value decomposition analysis of the secondary structure features contributing to the circular dichroism spectra of model proteins

Amyloid fibril formation occurs in restricted environment, such as the interface between intercellular fluids and bio-membranes. Conformational interconversion from α-helix to β-structure does not progress in fluids; however, it can occur after sedimentary aggregation during amyloid fibril formation induced by heat treatment of hen egg white lysozyme (HEWL). Secondary structures of various proteins and denatured proteins titrated with 2,2,2-trifluoroethanol (TFE) were examined using their CD spectra. Gaussian peak/trough and singular value decompositions (SVD) showed that the spectral pattern of the α-helix comprised a sharp trough at wavelength 207 nm and a broad trough at 220 nm. Conversely, we distinguished two patterns for β-sheet-a spread barrel type, corresponding to ConA, and a tightly weaved type, corresponding to the soybean trypsin inhibitor. Herein, we confirmed that the spectral/conformational interconversion of the heat-treated HEWL was not observed in the dissolved fluid.

Recent Advances and Challenges in Nanodelivery Systems for Antimicrobial Peptides (AMPs)

Antimicrobial peptides (AMPs) can be used as alternative therapeutic agents to traditional antibiotics. These peptides have abundant natural template sources and can be isolated from animals, plants, and microorganisms. They are amphiphilic and mostly net positively charged, and they have a broad-spectrum inhibitory effect on bacteria, fungi, and viruses. AMPs possess significant rapid killing effects and do not interact with specific receptors on bacterial surfaces. As a result, drug resistance is rarely observed with treatments. AMPs, however, have some operational problems, such as a susceptibility to enzymatic (protease) degradation, toxicity in vivo, and unclear pharmacokinetics. However, nanodelivery systems loaded with AMPs provide a safe mechanism of packaging such peptides before they exert their antimicrobial actions, facilitate targeted delivery to the sites of infection, and control the release rate of peptides and reduce their toxic side effects. However, nanodelivery systems using AMPs are at an early stage of development and are still in the laboratory phase of development. There are also some challenges in incorporating AMPs into nanodelivery systems. Herein, an insight into the nanotechnology challenges in delivering AMPs, current advances, and remaining technological challenges are discussed in depth.

A photoswitchable helical peptide with light-controllable interface/transmembrane topology in lipidic membranes

The spontaneous insertion of helical transmembrane (TM) polypeptides into lipid bilayers is driven by three sequential equilibria: solution-to-membrane interface (MI) partition, unstructured-to-helical folding, and MI-to-TM helix insertion. A bottleneck for understanding these three steps is the lack of experimental approaches to perturb membrane-bound hydrophobic polypeptides out of equilibrium rapidly and reversibly. Here, we report on a 24-residues-long hydrophobic α-helical polypeptide, covalently coupled to an azobenzene photoswitch (KCALP-azo), which displays a light-controllable TM/MI equilibrium in hydrated lipid bilayers. FTIR spectroscopy reveals that trans KCALP-azo folds as a TM α-helix (TM topology). After trans-to-cis photoisomerization of the azobenzene moiety with UV light (reversed with blue light), the helical structure of KCALP-azo is maintained, but its helix tilt increased from 32 ± 5° to 79 ± 8°, indication of a reversible TM-to-MI transition. Further analysis indicates that this transition is incomplete, with cis KCALP-azo existing in a ∼90% TM and ∼10% MI mixture.

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We present an α-helical transmembrane peptide modified with a molecular photoswitch

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The peptide exhibits reversible photocontrol of its membrane topology

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A fraction moves to the membrane interface with UV and inserts back with blue light

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This system will be useful to address the molecular mechanism for membrane insertion

Treating anisotropic artefacts in circular dichroism spectroscopy enables investigation of lyotropic liquid crystalline polyaspartate solutions

Circular dichroism (CD) spectroscopy is commonly used for investigation of the secondary structure of biomolecular compounds as well as polymers in isotropic solution. In anisotropic solution, the usage of the apparent CD is prone to misinterpretations due to artefacts from contributions of e.g. linear dichroism (LD). Herein, a method for the complete cancelation of anisotropic artefacts in the apparent CD is developed and its validity proven. The approach is further used for investigation of the conformation and the lyotropic liquid crystalline (LLC) structure of a copolyaspartate. For this system, a temperature-dependent change of the polymer's helical conformation (helix reversal) is known. Furthermore, a rotation of the aligned polymer helices inside a magnetic field (helix realignment) is independently present, occurring at a lower temperature compared to the helix reversal. In the current study, the helix reversal is confirmed and found to be accompanied by a change of the LLC structure. A cholesteric structure is detected and revealed to change its sense (cholesteric reversal) at the temperature at which the helix realigns in the magnetic field. The determination of the cholesteric sense is enabled by measuring the induced CD of an achiral dye, dissolved in the anisotropic polymer solution. Investigation of the anisotropic polymer solution is, thus, only made possible by cancellation of the aforementioned anisotropic artefacts. This allows the observation of changes of the liquid crystal structure from right-handed cholesteric, through left-handed cholesteric, to nematic with increasing temperature.

pH-triggered pore-forming peptides with strong composition-dependent membrane selectivity

Peptides that self-assemble into nanometer-sized pores in lipid bilayers could have utility in a variety of biotechnological and clinical applications if we can understand their physical chemical properties and learn to control their membrane selectivity. To empower such control, we have used synthetic molecular evolution to identify the pH-dependent delivery peptides, a family of peptides that assemble into macromolecule-sized pores in membranes at low peptide concentration but only at pH < ∼6. Further advancements will also require better selectivity for specific membranes. Here, we determine the effect of anionic headgroups and bilayer thickness on the mechanism of action of the pH-dependent delivery peptides by measuring binding, secondary structure, and macromolecular poration. The peptide pHD15 partitions and folds equally well into zwitterionic and anionic membranes but is less potent at pore formation in phosphatidylserine-containing membranes. The peptide also binds and folds similarly in membranes of various thicknesses, but its ability to release macromolecules changes dramatically. It causes potent macromolecular poration in vesicles made from phosphatidylcholine with 14 carbon acyl chains, but macromolecular poration decreases sharply with increasing bilayer thickness and does not occur at any peptide concentration in fluid bilayers made from phosphatidylcholine lipids with 20-carbon acyl chains. The effects of headgroup and bilayer thickness on macromolecular poration cannot be accounted for by the amount of peptide bound but instead reflect an inherent selectivity of the peptide for inserting into the membrane-spanning pore state. Molecular dynamics simulations suggest that the effect of thickness is due to hydrophobic match/mismatch between the membrane-spanning peptide and the bilayer hydrocarbon. This remarkable degree of selectivity based on headgroup and especially bilayer thickness is unusual and suggests ways that pore-forming peptides with exquisite selectivity for specific membranes can be designed or evolved.

PIP2 promotes conformation-specific dimerization of the EphA2 membrane region

The impact of the EphA2 receptor on cancer malignancy hinges on the two different ways it can be activated. EphA2 induces antioncogenic signaling after ligand binding, but ligand-independent activation of EphA2 is pro-oncogenic. It is believed that the transmembrane (TM) domain of EphA2 adopts two alternate conformations in the ligand-dependent and the ligand-independent states. However, it is poorly understood how the difference in TM helical crossing angles found in the two conformations impacts the activity and regulation of EphA2. We devised a method that uses hydrophobic matching to stabilize two conformations of a peptide comprising the EphA2 TM domain and a portion of the intracellular juxtamembrane (JM) segment. The two conformations exhibit different TM crossing angles, resembling the ligand-dependent and ligand-independent states. We developed a single-molecule technique using styrene maleic acid lipid particles to measure dimerization in membranes. We observed that the signaling lipid PIP2 promotes TM dimerization, but only in the small crossing angle state, which we propose corresponds to the ligand-independent conformation. In this state the two TMs are almost parallel, and the positively charged JM segments are expected to be close to each other, causing electrostatic repulsion. The mechanism PIP2 uses to promote dimerization might involve alleviating this repulsion due to its high density of negative charges. Our data reveal a conformational coupling between the TM and JM regions and suggest that PIP2 might directly exert a regulatory effect on EphA2 activation in cells that is specific to the ligand-independent conformation of the receptor.

Structural and functional characterization of the pore-forming domain of pinholin S2168

Pinholin S2168 triggers the lytic cycle of bacteriophage φ21 in infected Escherichia coli Activated transmembrane dimers oligomerize into small holes and uncouple the proton gradient. Transmembrane domain 1 (TMD1) regulates this activity, while TMD2 is postulated to form the actual "pinholes." Focusing on the TMD2 fragment, we used synchrotron radiation-based circular dichroism to confirm its α-helical conformation and transmembrane alignment. Solid-state 15N-NMR in oriented DMPC bilayers yielded a helix tilt angle of τ = 14°, a high order parameter (Smol = 0.9), and revealed the azimuthal angle. The resulting rotational orientation places an extended glycine zipper motif (G40xxxS44xxxG48) together with a patch of H-bonding residues (T51, T54, N55) sideways along TMD2, available for helix-helix interactions. Using fluorescence vesicle leakage assays, we demonstrate that TMD2 forms stable holes with an estimated diameter of 2 nm, as long as the glycine zipper motif remains intact. Based on our experimental data, we suggest structural models for the oligomeric pinhole (right-handed heptameric TMD2 bundle), for the active dimer (right-handed Gly-zipped TMD2/TMD2 dimer), and for the full-length pinholin protein before being triggered (Gly-zipped TMD2/TMD1-TMD1/TMD2 dimer in a line).

DAPTOMYCIN, its membrane-active mechanism vs. that of other antimicrobial peptides

Over 3000 membrane-active antimicrobial peptides (AMPs) have been discovered, but only three of them have been approved by the U.S. Food and Drug Administration (FDA) for therapeutic applications, i.e., gramicidin, daptomycin and colistin. Of the three approved AMPs, daptomycin is a last-line-of-defense antibiotic for treating Gram-positive infections. However its use has already created bacterial resistance. To search for its substitutes that might counter the resistance, we need to understand its molecular mechanism. The mode of action of daptomycin appears to be causing bacterial membrane depolarization through ion leakage. Daptomycin forms a unique complex with calcium ions and phosphatidylglycerol molecules in membrane at a specific stoichiometric ratio: Dap₂Ca₃PG₂. How does this complex promote ion conduction across the membrane? We hope that biophysics of peptide-membrane interaction can answer this question. This review summarizes the biophysical works that have been done on membrane-active AMPs to understand their mechanisms of action, including gramicidin, daptomycin, and underdeveloped pore-forming AMPs. The analysis suggests that daptomycin forms transient ionophores in the target membranes. We discuss questions that remain to be answered.

A peptide for transcellular cargo delivery: Structure-function relationship and mechanism of action

The rate of transport of small molecule drugs across biological barriers, such as the blood-brain barrier, is often a limiting factor in achieving a therapeutic dose. One proposed strategy to enhance delivery across endothelial or epithelial monolayers is conjugation to cell-penetrating peptides (CPPs); however, very little is known about the design of CPPs for efficient transcellular transport. Here, we report on transcellular transport of a CPP, designated the CL peptide, that increases the delivery of small-molecule cargoes across model epithelium approximately 10-fold. The CL peptide contains a helix-like motif and a polyarginine tail. We investigated the effect of cargo, helix-like motif sequence, polyarginine tail length, and peptide stereochemistry on cargo delivery. We showed that there is an optimal helix-like motif sequence (RLLRLLR) and polyarginine tail length (R₇) for cargo delivery. Furthermore, we demonstrated that the peptide-cargo conjugate is cleaved by cells in the epithelium at the site of a two-amino acid linker. The cleavage releases the cargo with the N-terminal linker amino acid from the peptide prior to transport out of the epithelium. These studies provide new insight into the sequence requirements for developing novel CPPs for transcellular delivery of cargo.

Mechanisms of a Small Membrane-Active Antimicrobial Peptide from Hyla punctata

Thousands of antimicrobial peptides have been observed and studied in the past decades; however, their membrane-active mechanisms are ambiguous due to their dynamic structure in the cell membrane. Here, we applied both molecular dynamics (MD) simulations and biophysical experiments to study the small membrane-active antimicrobial peptide Hylaseptin P1 (HSP1), which has significant selectivity towards anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (POPG) and bacterial model membranes. HSP1 does not bind and fold onto human red blood cell model membranes, and it only binds, but does not fold, in zwitterionic 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) membranes. This suggests that the lipid chemistry and membrane rigidity are key to prevent HSP1 binding onto membranes, and the lipid headgroup charge may further promote peptide folding in the membrane. Our experiment-validated MD simulations suggest a carpet-like model mechanism for HSP1 through peptide binding, folding, aggregation, and assembly. HSP1 is shorter than the membrane thickness; therefore, the folded peptides aggregate on the surface, cross the membrane, and the oligomeric structure is supported by several surface-bound peptides in both bilayer leaflets.

Divalent Cations and Lipid Composition Modulate Membrane Insertion and Cancer-Targeting Action of pHLIP

The pH-Low Insertion Peptide (pHLIP) has emerged as an important tool for targeting cancer cells; it has been assumed that its targeting mechanism depends solely on the mild acidic environment surrounding tumors. Here, we examine the role of Ca2+ and Mg2+ on pHLIP's insertion, cellular targeting, and drug delivery. We demonstrate that physiologically relevant concentrations of either cation can shift the protonation-dependent transition by up to several pH units toward basic pH and induce substantial protonation-independent transmembrane insertion of pHLIP at pH as high as 10. Consistent with these results, the ability of pHLIP to deliver the cytotoxic compound monomethyl-auristatin-F to HeLa cells is increased several fold in presence of Ca2+. Complementary measurements with model membranes confirmed this Ca2+/Mg2+-dependent membrane-insertion mechanism. The magnitude of this alternative Ca2+/Mg2+-dependent effect is also modulated by lipid composition-specifically by the presence of phosphatidylserine-providing new clues to pHLIP's unique tumor-targeting ability in vivo. These results exemplify the complex coupling between protonation of anionic residues and lipid-selective targeting by divalent cations, which is relevant to the general signaling on membrane interfaces.

Macroorganisation and flexibility of thylakoid membranes

The light reactions of photosynthesis are hosted and regulated by the chloroplast thylakoid membrane (TM) — the central structural component of the photosynthetic apparatus of plants and algae. The two-dimensional and three-dimensional arrangement of the lipid–protein assemblies, aka macroorganisation, and its dynamic responses to the fluctuating physiological environment, aka flexibility, are the subject of this review. An emphasis is given on the information obtainable by spectroscopic approaches, especially circular dichroism (CD). We briefly summarise the current knowledge of the composition and three-dimensional architecture of the granal TMs in plants and the supramolecular organisation of Photosystem II and light-harvesting complex II therein. We next acquaint the non-specialist reader with the fundamentals of CD spectroscopy, recent advances such as anisotropic CD, and applications for studying the structure and macroorganisation of photosynthetic complexes and membranes. Special attention is given to the structural and functional flexibility of light-harvesting complex II in vitro as revealed by CD and fluorescence spectroscopy. We give an account of the dynamic changes in membrane macroorganisation associated with the light-adaptation of the photosynthetic apparatus and the regulation of the excitation energy flow by state transitions and non-photochemical quenching.

The Effect of Phosphatidylserine on a pH-Responsive Peptide Is Defined by Its Noninserting End

The acidity-triggered rational membrane (ATRAM) peptide was designed to target acidic diseases such as cancer. An acidic extracellular medium, such as that found in aggressive tumors, drives the protonation of the glutamic acids in ATRAM, leading to the membrane translocation of its C-terminus and the formation of a transmembrane helix. Compared to healthy cells, cancerous cells often increase exposure of the negatively charged phosphatidylserine (PS) on the outer leaflet of the plasma membrane. Here we use a reconstituted vesicle system to explore how PS influences the interaction of ATRAM with membranes. To explore this, we used two new variants of ATRAM, termed K2-ATRAM and Y-ATRAM, with small modifications at the noninserting N-terminus. We observed that the effect of PS on the membrane insertion pK and lipid partitioning hinged on the sequence of the noninserting end. Our data additionally indicate that the effect of PS on the insertion pK does not merely depend on electrostatics, but it is multifactorial. Here we show how small sequence changes can impact the interaction of a peptide with membranes of mixed lipid composition. These data illustrate how model studies using neutral bilayers, which do not mimic the negative charge found in the plasma membrane of cancer cells, may fail to capture important aspects of the interaction of anticancer peptides with tumor cells. This information can guide the design of therapeutic peptides that target the acidic environments of different diseased tissues.

Bilayer Thickness and Curvature Influence Binding and Insertion of a pHLIP Peptide

The physical properties of lipid bilayers, such as curvature and fluidity, can affect the interactions of polypeptides with membranes, influencing biological events. Additionally, given the growing interest in peptide-based therapeutics, understanding the influence of membrane properties on membrane-associated peptides has potential utility. pH low insertion peptides (pHLIPs) are a family of water-soluble peptides that can insert across cell membranes in a pH-dependent manner, enabling the use of pH to follow peptide-lipid interactions. Here we study pHLIP interactions with liposomes varying in size and composition, to determine the influence of several key membrane physical properties. We find that pHLIP binding to bilayer surfaces at neutral pH is governed by the ease of access to the membrane's hydrophobic core, which can be facilitated by membrane curvature, thickness, and the cholesterol content of the membrane. After surface binding, if the pH is lowered, the kinetics of pHLIP folding to form a helix and subsequent insertion across the membrane depends on the fluidity and energetic dynamics of the membrane. We showed that pHLIP is capable of forming a helix across lipid bilayers of different thicknesses at low pH. However, the kinetics of the slow phase of insertion corresponding to the translocation of C-terminal end of the peptide across lipid bilayer, vary approximately twofold, and correlate with bilayer thickness and fluidity. Although these influences are not large, local curvature variations in membranes of different fluidity could selectively influence surface binding in mixed cell populations.

Membrane Bending Moduli of Coexisting Liquid Phases Containing Transmembrane Peptide

A number of highly curved membranes in vivo, such as epithelial cell microvilli, have the relatively high sphingolipid content associated with "raft-like" composition. Given the much lower bending energy measured for bilayers with "nonraft" low sphingomyelin and low cholesterol content, observing high curvature for presumably more rigid compositions seems counterintuitive. To understand this behavior, we measured membrane rigidity by fluctuation analysis of giant unilamellar vesicles. We found that including a transmembrane helical GWALP peptide increases the membrane bending modulus of the liquid-disordered (Ld) phase. We observed this increase at both low-cholesterol fraction and higher, more physiological cholesterol fraction. We find that simplified, commonly used Ld and liquid-ordered (Lo) phases are not representative of those that coexist. When Ld and Lo phases coexist, GWALP peptide favors the Ld phase with a partition coefficient of 3-10 depending on mixture composition. In model membranes at high cholesterol fractions, Ld phases with GWALP have greater bending moduli than the Lo phase that would coexist.

Mechanistic Landscape of Membrane-Permeabilizing Peptides

Membrane permeabilizing peptides (MPPs) are as ubiquitous as the lipid bilayer membranes they act upon. Produced by all forms of life, most membrane permeabilizing peptides are used offensively or defensively against the membranes of other organisms. Just as nature has found many uses for them, translational scientists have worked for decades to design or optimize membrane permeabilizing peptides for applications in the laboratory and in the clinic ranging from antibacterial and antiviral therapy and prophylaxis to anticancer therapeutics and drug delivery. Here, we review the field of membrane permeabilizing peptides. We discuss the diversity of their sources and structures, the systems and methods used to measure their activities, and the behaviors that are observed. We discuss the fact that "mechanism" is not a discrete or a static entity for an MPP but rather the result of a heterogeneous and dynamic ensemble of structural states that vary in response to many different experimental conditions. This has led to an almost complete lack of discrete three-dimensional active structures among the thousands of known MPPs and a lack of useful or predictive sequence-structure-function relationship rules. Ultimately, we discuss how it may be more useful to think of membrane permeabilizing peptides mechanisms as broad regions of a mechanistic landscape rather than discrete molecular processes.

Anisotropic Circular Dichroism of Light-Harvesting Complex II in Oriented Lipid Bilayers: Theory Meets Experiment

Anisotropic circular dichroism (ACD) spectroscopy of macroscopically aligned molecules reveals additional information about their excited states that is lost in the CD of randomly oriented solutions. ACD spectra of light-harvesting complex II (LHCII)-the main peripheral antenna of photosystem II in plants-in oriented lipid bilayers were recorded from the far-UV to the visible wavelength region. ACD spectra show a drastically enhanced magnitude and level of detail compared to the isotropic CD spectra, resolving a greater number of bands and weak optical transitions. Exciton calculations show that the spectral features in the chlorophyll Q _y region are well-reproduced by an existing Hamiltonian for LHCII, providing further evidence for the identity of energy sinks at chlorophylls a603 and a610 in the stromal layer and chlorophylls a604 and a613 in the luminal layer. We propose ACD spectroscopy to be a valuable tool linking the three-dimensional structure and the photophysical properties of pigment-protein complexes.

The Mechanisms of Action of Cationic Antimicrobial Peptides Refined by Novel Concepts from Biophysical Investigations

Even 30 years after the discovery of magainins, biophysical and structural investigations on how these peptides interact with membranes can still bear surprises and add new interesting detail to how these peptides exert their antimicrobial action. Early on, using oriented solid-state NMR spectroscopy, it was found that the amphipathic helices formed by magainins are active when being oriented parallel to the membrane surface. More recent investigations indicate that this in-planar alignment is also found when PGLa and magainin in combination exert synergistic pore-forming activities, where studies on the mechanism of synergistic interaction are ongoing. In a related manner, the investigation of dimeric antimicrobial peptide sequences has become an interesting topic of research which bears promise to refine our views how antimicrobial action occurs. The molecular shape concept has been introduced to explain the effects of lipids and peptides on membrane morphology, locally and globally, and in particular of cationic amphipathic helices that partition into the membrane interface. This concept has been extended in this review to include more recent ideas on soft membranes that can adapt to external stimuli including membrane-disruptive molecules. In this manner, the lipids can change their shape in the presence of low peptide concentrations, thereby maintaining the bilayer properties. At higher peptide concentrations, phase transitions occur which lead to the formation of pores and membrane lytic processes. In the context of the molecular shape concept, the properties of lipopeptides, including surfactins, are shortly presented, and comparisons with the hydrophobic alamethicin sequence are made.

Peptide-Dendron Hybrids that Adopt Sequence-Encoded β-Sheet Conformations

Rational design rules for programming hierarchical organization and function through mutations of monomers in sequence-defined polymers can accelerate the development of novel polymeric and supramolecular materials. Our strategy for designing peptide-dendron hybrids that adopt predictable secondary and quaternary structures in bulk is based on patterning the sites at which dendrons are conjugated to short peptides. To validate this approach, we have designed and characterized a series of β-sheet-forming peptide-dendron hybrids. Spectroscopic studies of the hybrids in films reveal that the peptide portion of the hybrids adopts the intended secondary structure.

A novel pH-dependent membrane peptide that binds to EphA2 and inhibits cell migration

Misregulation of the signaling axis formed by the receptor tyrosine kinase (RTK) EphA2 and its ligand, ephrinA1, causes aberrant cell-cell contacts that contribute to metastasis. Solid tumors are characterized by an acidic extracellular medium. We intend to take advantage of this tumor feature to design new molecules that specifically target tumors. We created a novel pH-dependent transmembrane peptide, TYPE7, by altering the sequence of the transmembrane domain of EphA2. TYPE7 is highly soluble and interacts with the surface of lipid membranes at neutral pH, while acidity triggers transmembrane insertion. TYPE7 binds to endogenous EphA2 and reduces Akt phosphorylation and cell migration as effectively as ephrinA1. Interestingly, we found large differences in juxtamembrane tyrosine phosphorylation and the extent of EphA2 clustering when comparing TYPE7 with activation by ephrinA1. This work shows that it is possible to design new pH-triggered membrane peptides to activate RTK and gain insights on its activation mechanism.

Membrane Active Peptides and Their Biophysical Characterization

In the last 20 years, an increasing number of studies have been reported on membrane active peptides. These peptides exert their biological activity by interacting with the cell membrane, either to disrupt it and lead to cell lysis or to translocate through it to deliver cargos into the cell and reach their target. Membrane active peptides are attractive alternatives to currently used pharmaceuticals and the number of antimicrobial peptides (AMPs) and peptides designed for drug and gene delivery in the drug pipeline is increasing. Here, we focus on two most prominent classes of membrane active peptides; AMPs and cell-penetrating peptides (CPPs). Antimicrobial peptides are a group of membrane active peptides that disrupt the membrane integrity or inhibit the cellular functions of bacteria, virus, and fungi. Cell penetrating peptides are another group of membrane active peptides that mainly function as cargo-carriers even though they may also show antimicrobial activity. Biophysical techniques shed light on peptide–membrane interactions at higher resolution due to the advances in optics, image processing, and computational resources. Structural investigation of membrane active peptides in the presence of the membrane provides important clues on the effect of the membrane environment on peptide conformations. Live imaging techniques allow examination of peptide action at a single cell or single molecule level. In addition to these experimental biophysical techniques, molecular dynamics simulations provide clues on the peptide–lipid interactions and dynamics of the cell entry process at atomic detail. In this review, we summarize the recent advances in experimental and computational investigation of membrane active peptides with particular emphasis on two amphipathic membrane active peptides, the AMP melittin and the CPP pVEC.

Optimal Hydrophobicity and Reorientation of Amphiphilic Peptides Translocating through Membrane

Cell-penetrating and some antimicrobial peptides can translocate across lipid bilayers without disrupting the membrane structure. However, the molecular properties required for efficient translocation are not fully understood. We employed the Metropolis Monte Carlo method together with coarse-grained models to systematically investigate free-energy landscapes associated with the translocation of secondary amphiphilic peptides. We studied α-helical peptides with different length, amphiphilicity, and distribution of hydrophobic content and found a common translocation path consisting of adsorption, tilting, and insertion. In the adsorbed state, the peptides are parallel to the membrane plane, whereas, in the inserted state, the peptides are perpendicular to the membrane. Our simulations demonstrate that, for all tested peptides, there is an optimal ratio of hydrophilic/hydrophobic content at which the peptides cross the membrane the easiest. Moreover, we show that the hydrophobicity of peptide termini has an important effect on the translocation barrier. These results provide general guidance to optimize peptides for use as carriers of molecular cargos or as therapeutics themselves.

Molecular Dynamics Simulations Are Redefining Our View of Peptides Interacting with Biological Membranes

Ever since the first molecular mechanics computer simulations of biological molecules became possible, there has been the dream to study all complex biological phenomena in silico, simply bypassing the enormous experimental challenges and their associated costs. For this, two inherent requirements need to be met: First, the time scales achievable in simulations must reach up to the millisecond range and even longer. Second, the computational model must accurately reproduce what is measured experimentally. Despite some recent successes, the general consensus in the field to date has been that neither of these conditions have yet been met and that the dream will be realized, if at all, only in the distant future. In this Account, we show that this view is wrong; instead, we are actually in the middle of the in silico molecular dynamics (MD) revolution, which is reshaping how we think about protein function. The example explored in this Account is a recent advance in the field of membrane-active peptides (MAPs). MD simulations have succeeded in accurately capturing the process of peptide binding, folding, and partitioning into lipid bilayers as well as revealing how channels form spontaneously from polypeptide fragments and conduct ionic and other cargo across membranes, all at atomic resolution. These game-changing advances have been made possible by a combination of steadily advancing computational power, more efficient algorithms and techniques, clever accelerated sampling schemes, and thorough experimental verifications. The great advantage of MD is the spatial and temporal resolution, directly providing a molecular movie of a protein undergoing folding and cycling through a functional process. This is especially important for proteins with transitory functional states, such as pore-forming MAPs. Recent successes are demonstrated here for the large class of antimicrobial peptides (AMPs). These short peptides are an essential part of the nonadaptive immune system for many organisms, ubiquitous in nature, and of particular interest to the pharmaceutical industry in the age of rising bacterial resistance to conventional antibiotic treatments. Unlike integral membrane proteins, AMPs are sufficiently small to allow converged sampling with the unbiased high-temperature sampling methodology outlined here and are relatively easy to handle experimentally. At the same time, AMPs exhibit a wealth of complex and poorly understood interactions with lipid bilayers, which allow not only tuning and validation of the simulation methodology but also advancement of our knowledge of protein-lipid interactions at a fundamental level. Space constraints limit our discussion to AMPs, but the MD methodologies outlined here can be applied to all phenomena involving peptides in membranes, including cell-penetrating peptides, signaling peptides, viral channel forming peptides, and fusion peptides, as well as ab initio membrane protein folding and assembly. For these systems, the promise of MD simulations to predict the structure of channels and to provide complete-atomic-detail trajectories of the mechanistic processes underlying their biological functions appears to rapidly become a reality. The current challenge is to design joint experimental and computational benchmarks to verify and tune MD force fields. With this, MD will finally fulfill its promise to become an inexpensive, powerful, and easy-to-use tool providing atomic-detail insights to researchers as part of their investigations into membrane biophysics and beyond.

Understanding membrane-active antimicrobial peptides

Bacterial membranes represent an attractive target for the design of new antibiotics to combat widespread bacterial resistance to traditional inhibitor-based antibiotics. Understanding how antimicrobial peptides (AMPs) and other membrane-active agents attack membranes could facilitate the design of new, effective antimicrobials. AMPs, which are small, gene-encoded host defense proteins, offer a promising basis for the study of membrane-active antimicrobial agents. These peptides are cationic and amphipathic, spontaneously binding to bacterial membranes and inducing transmembrane permeability to small molecules. Yet there are often confusions surrounding the details of the molecular mechanisms of AMPs. Following the doctrine of structure-function relationship, AMPs are often viewed as the molecular scaffolding of pores in membranes. Instead we believe that the full mechanism of AMPs is understandable if we consider the interactions of AMPs with the whole membrane domain, where interactions induce structural transformations of the entire membrane, rather than forming localized molecular structures. We believe that it is necessary to consider the entire soft matter peptide-membrane system as it evolves through several distinct states. Accordingly, we have developed experimental techniques to investigate the state and structure of the membrane as a function of the bound peptide to lipid ratio, exactly as AMPs in solution progressively bind to the membrane and induce structural changes to the entire system. The results from these studies suggest that global interactions of AMPs with the membrane domain are of fundamental importance to understanding the antimicrobial mechanisms of AMPs.

Computational studies of peptide-induced membrane pore formation

A variety of peptides induce pores in biological membranes; the most common ones are naturally produced antimicrobial peptides (AMPs), which are small, usually cationic, and defend diverse organisms against biological threats. Because it is not possible to observe these pores directly on a molecular scale, the structure of AMP-induced pores and the exact sequence of steps leading to their formation remain uncertain. Hence, these questions have been investigated via molecular modelling. In this article, we review computational studies of AMP pore formation using all-atom, coarse-grained, and implicit solvent models; evaluate the results obtained and suggest future research directions to further elucidate the pore formation mechanism of AMPs.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Hexagonally Ordered Arrays of α-Helical Bundles Formed from Peptide-Dendron Hybrids

Combining monodisperse building blocks that have distinct folding properties serves as a modular strategy for controlling structural complexity in hierarchically organized materials. We combine an α-helical bundle-forming peptide with self-assembling dendrons to better control the arrangement of functional groups within cylindrical nanostructures. Site-specific grafting of dendrons to amino acid residues on the exterior of the α-helical bundle yields monodisperse macromolecules with programmable folding and self-assembly properties. The resulting hybrid biomaterials form thermotropic columnar hexagonal mesophases in which the peptides adopt an α-helical conformation. Bundling of the α-helical peptides accompanies self-assembly of the peptide-dendron hybrids into cylindrical nanostructures. The bundle stoichiometry in the mesophase agrees well with the size found in solution for α-helical bundles of peptides with a similar amino acid sequence.

Membrane association of the bacterial riboregulator Hfq and functional perspectives

Hfq is a bacterial RNA binding protein that carries out several roles in genetic expression regulation, mainly at the post-transcriptional level. Previous studies have shown its importance in growth and virulence of bacteria. Here, we provide the direct observation of its ability to interact with membranes. This was established by co-sedimentation assay, cryo-transmission electron (cryo-TEM) and atomic force (AFM) microscopies. Furthermore, our results suggest a role for its C-terminus amyloidogenic domain in membrane disruption. Precisely, AFM images of lipid bilayers in contact with Hfq C-terminus fibrils show the emergence of holes with a size dependent on the time of interaction. Cryo-TEM observations also show that liposomes are in contact with clusters of fibrils, with occasional deformation of the vesicles and afterward the apparition of a multitude of tiny vesicles in the proximity of the fibrils, suggesting peptide-induced breakage of the liposomes. Finally, circular dichroism spectroscopy demonstrated a change in the secondary structure of Hfq C-terminus upon interaction with liposomes. Altogether, these results show an unexpected property of Hfq and suggest a possible new role for the protein, exporting sRNA outside of the bacterial cell.

Interaction of the Antimicrobial Peptide Aurein 1.2 and Charged Lipid Bilayer

Aurein 1.2 is a potent antimicrobial peptide secreted by frog Litoria aurea. As a short membrane-active peptide with only 13 amino acids in sequence, it has been found to be residing on the surface of lipid bilayer and permeabilizing bacterial membranes at high concentration. However, the detail at the molecular level is largely unknown. In this study, we investigated the action of Aurein 1.2 in charged lipid bilayers composed of DMPC/DMPG. Oriented Circular Dichroism results showed that the peptide was on the surface of lipid bilayer regardless of the charged lipid ratio. Only at a very high peptide-to-lipid ratio (~1/10), the peptide became perpendicular to the bilayer, however no pore was detected by neutron in-plane scattering. To further understand how it interacted with charged lipid bilayers, we employed Small Angle Neutron Scattering to probe lipid distribution across bilayer leaflets in lipid vesicles. The results showed that Aurein 1.2 interacted strongly with negatively charged DMPG, causing strong asymmetry in lipid bilayer. At high concentration, while the vesicles were intact, we found additional structure feature on the bilayer. Our study provides a glimpse into how Aurein 1.2 disturbs anionic lipid-containing membranes without pore formation.

Transmembrane helices containing a charged arginine are thermodynamically stable

Hydrophobic amino acids are abundant in transmembrane (TM) helices of membrane proteins. Charged residues are sparse, apparently due to the unfavorable energetic cost of partitioning charges into nonpolar phases. Nevertheless, conserved arginine residues within TM helices regulate vital functions, such as ion channel voltage gating and integrin receptor inactivation. The energetic cost of arginine in various positions along hydrophobic helices has been controversial. Potential of mean force (PMF) calculations from atomistic molecular dynamics simulations predict very large energetic penalties, while in vitro experiments with Sec61 translocons indicate much smaller penalties, even for arginine in the center of hydrophobic TM helices. Resolution of this conflict has proved difficult, because the in vitro assay utilizes the complex Sec61 translocon, while the PMF calculations rely on the choice of simulation system and reaction coordinate. Here we present the results of computational and experimental studies that permit direct comparison with the Sec61 translocon results. We find that the Sec61 translocon mediates less efficient membrane insertion of Arg-containing TM helices compared with our computational and experimental bilayer-insertion results. In the simulations, a combination of arginine snorkeling, bilayer deformation, and peptide tilting is sufficient to lower the penalty of Arg insertion to an extent such that a hydrophobic TM helix with a central Arg residue readily inserts into a model membrane. Less favorable insertion by the translocon may be due to the decreased fluidity of the endoplasmic reticulum (ER) membrane compared with pure palmitoyloleoyl-phosphocholine (POPC). Nevertheless, our results provide an explanation for the differences between PMF- and experiment-based penalties for Arg burial.

Insertion into lipid bilayer of truncated pHLIP®

The investigation of pH-dependent membrane-associated folding has both fundamental interest and practical applications for targeting of acidic tumors and specific delivery of therapeutic molecules across membrane of cancer cells. We and others investigated molecular mechanism and medical uses of class of water soluble membrane peptides, pH (Low) Insertion Peptides (pHLIP^® peptides). Here we employed optical spectroscopy methods to study interactions of the truncated pHLIP^® peptide (Short pHLIP^®) with lipid bilayer of membrane. Tryptophan fluorescence, CD and OCD data indicate on pH-triggered formation of transmembrane helical structure. Dual quenching and FRET assays demonstrated that Short pHLIP^® peptide spans lipid bilayer of membrane similar to Long pHLIP^® peptides. Truncated pHLIP^® peptides with multiple charged and protonatable residues in their sequences potentially can make these peptides to be less hydrophobic compared to Long pHLIP^® peptides, and might have utility in tumor imaging, and potentially, in pH-regulated cytoplasmic delivery of moderately hydrophobic drugs.

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Protonation of single Asp triggers pH-dependent insertion into membrane.

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Truncated pHLIP^® peptide adopts TM helical orientation at low pH.

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Truncated pHLIP^® peptide spans lipid bilayer similar to full-length long peptide.

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Potential applications in cytoplasmic delivery of small hydrophobic molecules.

Circular Dichroism of Electric-Field-Oriented CdSe/CdS Quantum Dots-in-Rods

Here we report anisotropy of intrinsic chiroptical response in CdSe/CdS quantum dot-in-rod systems. These nanostructures being oriented in an external electric field demonstrate dependence of circular dichroism signal on the orientation of the nanocrystals. The type of circular polarization in these nanostructures correlates with preferential direction of linear polarization, and the degree of circular polarization is the maximal for the first circular dichroism band corresponding to the absorption band edge. We also support our experimental data with a theoretical model. Using this model, we show a direct connection between theoretically derived morphological parameters of twisting in nanocrystals lattices and calculated from experimental data parameters of circular dichroism anisotropy.