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Self-assembling peptides offer a versatile set of tools for bottom-up construction of supramolecular biomaterials. Among these compounds, non-natural peptidic foldamers experience increased focus due to their structural variability and lower sensitivity to enzymatic degradation. However, very little is known about their membrane properties and complex oligomeric assemblies - key areas for biomedical and technological applications. Here we designed short, acyclic β3-peptide sequences with alternating amino acid stereoisomers to obtain non-helical molecules having hydrophilic charged residues on one side, and hydrophobic residues on the other side, with the N-terminus preventing formation of infinite fibrils. Our results indicate that these β-peptides form small oligomers both in water and in lipid bilayers and are stabilized by intermolecular hydrogen bonds. In the presence of model membranes, they either prefer the headgroup regions or they insert between the lipid chains. Molecular dynamics (MD) simulations suggest the formation of two-layered bundles with their side chains facing opposite directions when compared in water and in model membranes. Analysis of the MD calculations showed hydrogen bonds inside each layer, however, not between the layers, indicating a dynamic assembly. Moreover, the aqueous form of these oligomers can host fluorescent probes as well as a hydrophobic molecule similarly to e.g. lipid transfer proteins. For the tested, peptides the mixed chirality pattern resulted in similar assemblies despite sequential differences. Based on this, it is hoped that the presented molecular framework will inspire similar oligomers with diverse functionality.

The interfacial hydrolysis of phospholipids catalyzed by phospholipase A1 (PLA1) was studied via sum frequency generation (SFG) vibrational spectroscopy and fluorescence microscopy. Both monolayer and bilayer setups were used to confirm the hydrolysis mechanism. During the hydrolysis, lysophospholipids, one of the hydrolysis products, were desorbed from the interface into the solution, while the other products, fatty acids, self-organized and accumulated with PLA1 at the interface to form the PLA1-induced regions, which can serve as nonspecific binding domains for proteins and thus lead to human vascular diseases. This experimental study provides the essential information on revealing the interfacial biochemical process related to the metabolism of the lipids, which is one of the basic building blocks for cells.

Antimicrobial peptides (AMPs) kill bacteria by targeting their membranes through various mechanisms involving peptide assembly, often coupled with disorder‐to‐order structural transition. However, for several AMPs, similar conformational changes in cases in which small organic compounds of both endogenous and exogenous origin have induced folded peptide conformations have recently been reported. Thus, the function of AMPs and of natural host defence peptides can be significantly affected by the local complex molecular environment in vivo; nonetheless, this area is hardly explored. To address the relevance of such interactions with regard to structure and function, we have tested the effects of the therapeutic drug suramin on the membrane activity and antibacterial efficiency of CM15, a potent hybrid AMP. The results provided insight into a dynamic system in which peptide interaction with lipid bilayers is interfered with by the competitive binding of CM15 to suramin, resulting in an equilibrium dependent on peptide‐to‐drug ratio and vesicle surface charge. In vitro bacterial tests showed that when CM15⋅suramin complex formation dominates over membrane binding, antimicrobial activity is abolished. On the basis of this case study, it is proposed that small‐molecule secondary structure regulators can modify AMP function and that this should be considered and could potentially be exploited in future development of AMP‐based antimicrobial agents.

Besides the outstanding potential in biomedical applications, extracellular vesicles (EVs) are also promising candidates to expand our knowledge on interactions between vesicular surface proteins and small-molecules which exert biomembrane-related functions. Here we provide mechanistic details on interactions between membrane active peptides with antimicrobial effect (MAPs) and red blood cell derived EVs (REVs) and we demonstrate that they have the capacity to remove members of the protein corona from REVs even at lower than 5 μM concentrations. In case of REVs, the Soret-band arising from the membrane associated hemoglobins allowed to follow the detachment process by flow-Linear Dichroism (flow-LD). Further on, the significant change on the vesicle surfaces was confirmed by transmission electron microscopy (TEM). Since membrane active peptides, such as melittin have the affinity to disrupt vesicles, a combination of techniques, fluorescent antibody labeling, microfluidic resistive pulse sensing, and flow-LD were employed to distinguish between membrane destruction and surface protein detachment. The removal of protein corona members is a newly identified role for the investigated peptides, which indicates complexity of their in vivo function, but may also be exploited in synthetic and natural nanoparticle engineering. Furthermore, results also promote that EVs can be used as improved model systems for biophysical studies providing insight to areas with so far limited knowledge.

1,4- and 1,5-Disubstituted triazole amino acid monomers have gained increasing interest among peptidic foldamers, as they are easily prepared via Cu- and Ru-catalyzed click reactions, with the potential for side chain variation. While the latter is key to their applicability, the synthesis and structural properties of the chiral mono- or disubstituted triazole amino acids have only been partially addressed. We here present the synthesis of all eight possible chiral derivatives of a triazole monomer prepared via a ruthenium-catalyzed azide alkyne cycloaddition (RuAAC). To evaluate the conformational properties of the individual building units, a systematic quantum chemical study was performed on all monomers, indicating their capacity to form several low energy conformers. This feature may be used to effect structural diversity when the monomers are inserted into various peptide sequences. We envisage that these results will facilitate new applications for these artificial oligomeric compounds in diverse areas, ranging from pharmaceutics to biotechnology.

Macromolecular associates, such as membraneless organelles or lipid-protein assemblies, provide a hydrophobic environment, i.e., a liquid protein phase (LP), where folding preferences can be drastically altered. LP as well as the associated phase change from water (W) is an intriguing phenomenon related to numerous biological processes and also possesses potential in nanotechnological applications. However, the energetic effects of a hydrophobic yet water-containing environment on protein folding are poorly understood. Here, we focus on small β-sheets, the key motifs of proteins, undergoing structural changes in liquid–liquid phase separation (LLPS) and also model the mechanism of energy-coupled unfolding, e.g., in proteases, during W → LP transition. Due to the importance of the accurate description for hydrogen bonding patterns, the employed models were studied by using quantum mechanical calculations. The results demonstrate that unfolding is energetically less favored in LP by ~0.3–0.5 kcal·mol−1 per residue in which the difference further increased by the presence of explicit structural water molecules, where the folded state was preferred by ~1.2–2.3 kcal·mol−1 per residue relative to that in W. Energetics at the LP/W interfaces was also addressed by theoretical isodesmic reactions. While the models predict folded state preference in LP, the unfolding from LP to W renders the process highly favorable since the unfolded end state has &gt, 1 kcal·mol−1 per residue excess stabilization.

Body fluids flow all over the body and affect the biological processes at biointerfaces. To simulate such a case, sum frequency generation (SFG) vibrational spectroscopy and a self-designed microfluidic chip were combined together to investigate the interaction between a pH-responsive polymeric drug, poly(α-propylacrylic acid) (PPAAc), and the model cell membranes in different liquid environments. By examining the SFG spectra under the static and flowing conditions, the drug-membrane interaction was revealed comprehensively. The interfacial water layer was screened as the key factor affecting the drug-membrane interaction. The interfacial water layer can prevent the side propyl groups on PPAAc from inserting into the model cell membrane but would be disrupted by numerous ions in buffer solutions. Without flowing, at pH 6.6, the interaction between PPAAc and the model cell membrane was strongest; with flowing, at pH 5.8, the interaction was strongest. Flowing was proven to substantially affect the interaction between PPAAc and the model cell membranes, suggesting that the fluid environment was of key significance for biointerfaces. This work demonstrated that, by combining SFG and microfluidics, new information about the molecular-level interaction between macromolecules and the model cell membranes can be acquired, which cannot be obtained by collecting the normal static SFG spectra.

Foldamers are non-natural oligomers that mimic the structural behaviour of natural peptides, proteins and nucleotides by folding into a well-defined 3D conformation in solution. Since their first description about two decades ago, numerous studies have been undertaken dealing with the design, synthesis, characterization and application of foldamers. They have huge application potential as antimicrobial, anticancer and anti-HIV agents and in materials science. Despite their importance, there is no publicly available web resource providing comprehensive information on these compounds. Here we describe FoldamerDB, an open-source, fully annotated and manually curated database of peptidic foldamers. FoldamerDB holds the information about the sequence, structure and biological activities of the foldamer entries. It contains the information on over 1319 species and 1018 activities, collected from more than 160 research papers. The web-interface is designed to be clutter-free, user-friendly and it is compatible with devices of different screen sizes. The interface allows the user to search the database, browse and filter the foldamers using multiple criteria. It also offers a detailed help page to assist new users. FoldamerDB is hoped to bridge the gap in the freely available web-based resources on foldamers and will be of interest to diverse groups of scientists from chemists to biologists. The database can be accessed at http://foldamerdb.ttk.hu/.

Dynamic increase of resistant bacterial infectious diseases continuously requires development of novel compounds against them. The molecular level understanding of the mechanism and interactions of natural host-defense peptides or antimicrobial peptides (AMPs) is an important step towards rational design and development of compounds inspired by their function. A particular set of these peptides have disordered structure, the ordering of which may modify their antimicrobial properties. Recent experiments demonstrate that such conformational transitions of AMPs could be mediated by the presence of small organic compounds, such as approved drug molecules. However, the molecular mechanisms underlying these structural changes are unclear. In this study, we apply molecular docking and molecular dynamics-based approaches to rigorously analyze the interactions between the drug suramin and the AMP CM15, a synthetic unstructured hybrid peptide. We characterize the energetic properties of putative CM15-suramin complexes revealing particular impacts of CM15 residues as well as the parts of suramin on these interactions. We find that α-helical content of the peptide is increased in the presence of suramin, which is in agreement with the experimental data. Kinetics analysis from canonical molecular dynamics and metadynamics simulations suggest that the effect of suramin does not promote the formation of α-helix but rather results from its ability to stabilize the α-helical population in the conformational pool of the peptide. Potentially, understanding the physico-chemical basis underlying the interactions between drug molecules and disordered AMPs will prove useful in strategies for antimicrobial compound development. Further on, the given computational protocol for the analysis of such flexible systems provide a basis for future theoretical investigation of similar biomolecular complexes.

A peculiar polygonal protein scaffolding that resembles to spectrin-based skeleton of red blood cells can be reconstructed on the outer surface of vesicle-like nanoerythrosomes. The approximately 130 nm sized nanoerythrosomes are produced from red blood cell ghosts by addition of phospholipids (dipalmitoylphosphatidylcholine, DPPC). The scaffolding, constructed from the structural proteins of the cell membrane skeleton, covers the whole object resulting an enhanced stiffness. The protein pattern of the scaffolding is thermosensitive, reversible transformable in the biologically relevant temperature range. When the lipid additive is changed from DPPC to lysophospholipid (LPC), the protein network/scaffolding ceases to exist. By the variation of lipid type and ratio, a tailoring of the nanoerythrosomes can be achieved. During the tailoring process nanoerythrosomes or micelles, in a wide size range from 200 to 30 nm, are produced.

The increasing interest in novel foldamer constructs demands an accurate computational treatment on an extensive timescale. However, it is still a challenge to derive a force field (FF) that can reproduce the experimentally known fold while also allowing the spontaneous exploration of other structures. Here, aiming at a realistic reproduction of backbone torsional barriers, the relevant proper dihedrals of acyclic β2‐, β3‐ and β2,3‐amino acids were added to the CHARMM FF and optimized using a novel, self‐consistent iterative procedure based on quantum chemical relaxed scans. The new FF was validated by molecular dynamics simulations on three acyclic peptides. While they resided most of the time in their preferred fold (>80 % in helices and >50 % in hairpin), they also visited other conformations. Owing to the CHARMM36m‐consistent parametrization, the proposed extension is suitable for exploring new foldamer structures and assemblies, and their interactions with diverse biomolecules.

The cell membrane is an ordered environment, which anisotropically affects the structure and interactions of all of its molecules. Monitoring membrane orientation at a local level is rather challenging but could reward crucial information on protein conformation and interactions in the lipid bilayer. We monitored local lipid ordering changes upon varying the cholesterol concentration using polarized light spectroscopy and pyrene as a membrane probe. Pyrene, with a shape intermediate between a disc and a rod, can detect microscopic orientation variations at the level of its size. The global membrane orientation was determined using curcumin, a probe with nonoverlapping absorption relative to that of pyrene. While the macroscopic orientation of a liquid-phase bilayer decreases with increasing cholesterol concentration, the local orientation is improved. Pyrene is found to be sensitive to the local effects induced by cholesterol and temperature on the bilayer. Disentangling local and global orientation effects in membranes could provide new insights into functionally significant interactions of membrane proteins.

This study is aimed to assess the binding interaction between the antiparasitic cationic drug imidocarb (IMD) and sulfated glycosaminoglycans (GAGs), the ubiquitious nonprotein macromolecules of living organisms. These complex, heterogeneous polyanions are the integral constituents of cell membranes and the extracellular matrix and display affinity toward basic compounds, the binding of which may affect their biological functions. Exciton-type circular dichroism (CD) spectroscopic features measured at low salt concentration verify the heparin and heparan sulfate binding of IMD, which occurs in a cooperative manner by association of several drug molecules to a disaccharide unit. Isothermal titration calorimetry (ITC) measurements reassured the heparin interaction, resulting in a Kd value in the low micromolar range. In contrast, when considering high molar excess of the heparin-binding sites, closer resembling in vivo conditions, an entirely different CD signature was induced, suggesting a shift from the o...

Human serum albumin (HSA) binding of anticancer plant alkaloid ellipticine has been studied in conjunction with assessing its association to serum α1-acid-glycoprotein (AAG). Taking advantage of the ability of chiral protein environments to induce optical activity, circular dichroism (CD) spectroscopy was employed to characterize the alkaloid-protein non-covalent adducts. CD competition experiments revealed the simultaneous accommodation of two ellipticine molecules within the large crevice of HSA located in subdomain IB that induces an exciton coupling signature in the CD spectrum. Note that subdomain IIA was also found to host an ellipticine molecule. Supporting molecular docking simulations were also performed, which showed a hydrophobicity-driven stabilization of two ellipticine molecules within subdomain IB in a left-handed helical orientation. Similar to HSA, a pair of alkaloid ligands are inserted into the β-barrel cavity of AAG producing a polyphasic CD curve. The ‘F1/S’ and ‘A’ genetic variants of the protein bind ellipticine with comparable affinity albeit the distinct topography of the pockets provokes large intensity differences in the CD signals.

Entropy calculations represent one of the most challenging steps in obtaining the binding free energy in biomolecular systems. A novel computationally effective approach (IE) was recently proposed to calculate the entropy based on the computation of protein–ligand interaction energy directly from molecular dynamics (MD) simulations. We present a study focused on the application of this method to flexible molecular systems and compare its performance with well-established normal mode (NM) and quasiharmonic (QH) entropy calculation approaches. Our results demonstrated that the IE method is intended for calculating entropy change for binding partners in fixed conformations, as by the original definition of IE, and is not applicable to the molecular complexes in which the interacting partners undergo significant conformational changes during the binding process.

Peptidic foldamers have recently emerged as a novel class of artificial oligomers with properties and structural diversity similar to that of natural peptides, but possessing additional interesting features granting them great potential for applications in fields from nanotechnology to pharmaceuticals. Among these, foldamers containing 1,4- and 1,5-substitued triazole amino acids are easily prepared via the Cu- and Ru-catalyzed click reactions and may offer increased side chain variation, but their structural capabilities have not yet been widely explored. We here describe a systematic analysis of the conformational space of the two most important basic units, the 1,4-substitued (4Tzl) and the 1,5-substitued (5Tzl) 1,2,3-triazole amino acids, using quantum chemical calculations and NMR spectroscopy. Possible conformations of the two triazoles were scanned and their potential minima were located using several theoretical approaches (B3LYP/6-311++G(2d,2p), ωB97X-D/6-311++G(2d,2p), M06-2X/6-311++G(2d,2p) and MP2/6-311++G(2d,2p)) in different solvents. BOC-protected versions of 4Tzl and 5Tzl were also prepared via one step transformations and analyzed by 2D NOESY NMR. Theoretical results show 9 conformers for 5Tzl derivatives with relative energies lying close to each other, which may lead to a great structural diversity. NMR analysis also indicates that conformers preferring turn, helix and zig-zag secondary structures may coexist in solution. In contrast, 4Tzl has a much lower number of conformers, only 4, and these lack strong intraresidual interactions. This is again supported by NMR suggesting the presence of both extended and bent conformers. The structural information provided on these building units could be employed in future design of triazole foldamers.

We have discovered a well-defined extended conformation of double-stranded DNA, which we call Σ-DNA, using laser-tweezers force-spectroscopy experiments. At a transition force corresponding to free energy change ΔG = 1·57 ± 0·12 kcal (mol base pair)−1 60 or 122 base-pair long synthetic GC-rich sequences, when pulled by the 3′−3′ strands, undergo a sharp transition to the 1·52 ± 0·04 times longer Σ-DNA. Intriguingly, the same degree of extension is also found in DNA complexes with recombinase proteins, such as bacterial RecA and eukaryotic Rad51. Despite vital importance to all biological organisms for survival, genome maintenance and evolution, the recombination reaction is not yet understood at atomic level. We here propose that the structural distortion represented by Σ-DNA, which is thus physically inherent to the nucleic acid, is related to how recombination proteins mediate recognition of sequence homology and execute strand exchange. Our hypothesis is that a homogeneously stretched DNA undergoes a ‘disproportionation’ into an inhomogeneous Σ-form consisting of triplets of locally B-like perpendicularly stacked bases. This structure may ensure improved fidelity of base-pair recognition and promote rejection in case of mismatch during homologous recombination reaction. Because a triplet is the length of a gene codon, we speculate that the structural physics of nucleic acids may have biased the evolution of recombinase proteins to exploit triplet base stacks and also the genetic code.

Structural conversion of one given protein sequence into different amyloid states, resulting in distinct phenotypes, is one of the most intriguing phenomena of protein biology. Despite great efforts the structural origin of prion diversity remains elusive, mainly because amyloids are insoluble yet noncrystalline and therefore not easily amenable to traditional structural-biology methods. We investigate two different phenotypic prion strains, weak and strong, of yeast translation termination factor Sup35 with respect to angular orientation of tyrosines using polarized light spectroscopy. By applying a combination of alignment methods the degree of fiber orientation can be assessed, which allows a relatively accurate determination of the aromatic ring angles. Surprisingly, the strains show identical average orientations of the tyrosines, which are evenly spread through the amyloid core. Small variations between the two strains are related to the local environment of a fraction of tyrosines outside the core, potentially reflecting differences in fibril packing.

Six water-soluble spiropyran derivatives have been characterized with respect to the thermal and photoinduced reactions over a broad pH-interval. A comprehensive kinetic model was formulated including the spiro- and the merocyanine isomers, the respective protonated forms, and the hydrolysis products. The experimental studies on the hydrolysis reaction mechanism were supplemented by calculations using quantum mechanical (QM) models employing density functional theory. The results show that (1) the substitution pattern dramatically influences the pKa-values of the protonated forms as well as the rates of the thermal isomerization reactions, (2) water is the nucleophile in the hydrolysis reaction around neutral pH, (3) the phenolate oxygen of the merocyanine form plays a key role in the hydrolysis reaction. Hence, the nonprotonated merocyanine isomer is susceptible to hydrolysis, whereas the corresponding protonated form is stable toward hydrolytic degradation.

Computer-designed artificial enzymes will require precise understanding of how conformation of active sites may control barrier heights of key transition states, including dependence on structure and dynamics at larger molecular scale. F o F 1 ATP synthase is interesting as a model system: a delicate molecular machine synthesizing or hydrolyzing ATP using a rotary motor. Isolated F 1 performs hydrolysis with a rate very sensitive to ATP concentration. Experimental and theoretical results show that, at low ATP concentrations, ATP is slowly hydrolyzed in the so-called tight binding site, whereas at higher concentrations, the binding of additional ATP molecules induces rotation of the central γ-subunit, thereby forcing the site to transform through subtle conformational changes into a loose binding site in which hydrolysis occurs faster. How the 1-Å-scale rearrangements are controlled is not yet fully understood. By a combination of theoretical approaches, we address how large macromolecular rearrangements may manipulate the active site and how the reaction rate changes with active site conformation. Simulations reveal that, in response to γ-subunit position, the active site conformation is fine-tuned mainly by small α-subunit changes. Quantum mechanics-based results confirm that the sub-Ångström gradual changes between tight and loose binding site structures dramatically alter the hydrolysis rate.

The ruthenium-catalyzed azide alkyne cycloaddition (RuAAC) affords 1,5-disubstituted 1,2,3-triazoles in one step and complements the more established copper-catalyzed reaction providing the 1,4-isomer. The RuAAC reaction has quickly found its way into the organic chemistry toolbox and found applications in many different areas, such as medicinal chemistry, polymer synthesis, organocatalysis, supramolecular chemistry, and the construction of electronic devices. This Review discusses the mechanism, scope, and applications of the RuAAC reaction, covering the literature from the last 10 years.

In recent years, multi-lipid bicellar systems have emerged as promising membrane models. The fast orientational diffusion and magnetic alignability made these systems very attractive for NMR investigations. However, their alignment was so far achieved with a strong magnetic field, which limited their use with other methods that require macroscopic orientation. Recently, it was shown that bicelles could be aligned also by shear flow in a Couette flow cell, making it applicable to structural and biophysical studies by polarized light spectroscopy. Considering the sensitivity of this lipid system to small variations in composition and physicochemical parameters, efficient use of such a flow-cell method with coupled techniques will critically depend on the detailed understanding of how the lipid systems behave under flow conditions. In the present study we have characterized the flow alignment behavior of the commonly used dimyristoyl phosphatidylcholine/dicaproyl phosphatidylcholine (DMPC/DHPC) bicelle system, for various temperatures, lipid compositions, and lipid concentrations. We conclude that at optimal flow conditions the selected bicellar systems can produce the most efficient flow alignment out of any lipid systems used so far. The highest degree of orientation of DMPC/DHPC samples is noticed in a narrow temperature interval, at a practical temperature around 25 C, most likely in the phase transition region characterized by maximum sample viscosity. The change of macroscopic orientation factor as function of the above conditions is now described in detail. The increase in macroscopic alignment observed for bicelles will most likely allow recording of higher resolution spectra on membrane systems, which provide deeper structural insight and analysis into properties of biomolecules interacting with solution phase lipid membranes. (C) 2013 Elsevier Ireland Ltd. All rights reserved.

The interactions between anionic or zwitterionic liposomes and a water-soluble, DNA-binding photochromic spiropyran are studied using UV/vis absorption and linear dichroism (LD) spectroscopy. The spectral characteristics as well as the kinetics of the thermal isomerization process in the absence and presence of the two different liposome types provide information about the environment and whether or not the spiropyran resides in the liposome membrane. By measuring LD on liposomes deformed and aligned by shear flow, further insight is obtained about interaction and binding geometry of the spiropyran at the lipid membranes. We show that the membrane interactions differ between the two types of liposomes used as well as the isomeric forms of the spiropyran photoswitch.

Catalysts are agents that by binding reactant molecules lower the energy barriers to chemical reaction. After reaction the catalyst is regenerated, its unbinding energy recruited from the environment, which is associated with an inevitable loss of energy. We show that combining several catalytic sites to become energetically and temporally phase-shifted relative to each other provides a possibility to sustain the overall reaction by internal 'energy recycling', bypassing the need for thermal activation, and in principle allowing the system to work adiabatically. Using an analytical model for superimposed, phase-shifted potentials of F-1-ATP synthase provides a description integrating main characteristics of this rotary enzyme complex.

Bicelles are excellent membrane-mimicking hosts for a dynamic and structural study of solutes with NMR, but the magnetic fields required for their alignment are hard to apply to optical conditions. Here we demonstrate that bicellar mixtures can be aligned by shear forces in a Couette flow cell, to provide orientation of membrane-bound retinoic acid, pyrene and cytochrome c (cyt c) protein, conveniently studied with linear dichroism spectroscopy.