Amphipathic Character Research Articles

Conformational solution studies of the anti‐microbial temporin A retro‐analogues by using NMR spectroscopy

Temporin A (TA) is a small, basic and highly hydrophobic peptide, isolated from the skin of the European red frog, Rana temporaria. The TA (FLPLIGRVLSGIL-NH2) displays a broad spectrum of anti-microbial activity against Gram-positive bacteria and fungi Candida albicans. In this study we investigate the solution structure of two TA retro-analogues, (6-1)(7-13)-TA (GILPLFRVLSGIL-NH2) and retro-TA (LIGSLVRGILPLF-NH2) by using nuclear magnetic resonance (NMR). The 3D solution structure of the analogues was established by using inter-proton distances and vicinal coupling constants in the Simulated Annealing (SA) calculations (XPLOR program). The NMR conformational studies show the existence of the helical structure in the middle part of the (6-1)(7-13)-TA peptide and an unordered structure of the retro-TA analogue under the D3-TFE/H2O (3:7, v/v) conditions. Our investigations have shown that the hydrophobic cluster at N-terminus with the Pro amino acid residue in position 3 or 4, the helical structure and the amphipathic character of the peptide are responsible for the anti-microbial activity of the TA analogues.

Electron Paramagnetic Resonance Spectroscopy of Site-directed Spin Labels Reveals the Structural Heterogeneity in the N-terminal Domain of ApoA-I in Solution

Apolipoprotein A-I (apoA-I) is the major protein constituent of high density lipoprotein (HDL) and plays a central role in phospholipid and cholesterol metabolism. This 243-residue long protein is remarkably flexible and assumes numerous lipid-dependent conformations. Consequently, definitive structural determination of lipid-free apoA-I in solution has been difficult. Using electron paramagnetic spectroscopy of site-directed spin labels in the N-terminal domain of apoA-I (residues 1-98) we have mapped a mixture of secondary structural elements, the composition of which is consistent with findings from other in-solution methods. Based on side chain mobility and their accessibility to polar and non-polar spin relaxers, the precise location of secondary elements for amino acids 14-98 was determined for both lipid-free and lipid-bound apoA-I. Based on intermolecular dipolar coupling at positions 26, 44, and 64, these secondary structural elements were arranged into a tertiary fold to generate a structural model for lipid-free apoA-I in solution.

Structure and Mechanism of Action of the Antimicrobial Peptide Piscidin

Piscidin, an antibacterial peptide isolated from the mast cells of striped bass, has potent antimicrobial activity against a broad spectrum of pathogens in vitro. We investigated the mechanism of action of this 22-residue cationic peptide by carrying out structural studies and electrophysiological experiments in lipid bilayers. Circular dichroism experiments showed that piscidin was unstructured in water but had a high alpha-helix content in dodecylphosphocholine (DPC) micelles. 1H NMR data in water and TFE confirmed these results and demonstrated that the segment of residues 8-17 adopted an alpha-helical structure in a micellar environment. This molecule has a marked amphipathic character, due to well-defined hydrophobic and hydrophilic sectors. This structure is similar to those determined for other cationic peptides involved in permeabilization of the bacterial membrane. Multichannel experiments with piscidin incorporated into azolectin planar bilayers gave reproducible I-V curves at various peptide concentrations and unambiguously showed that this peptide permeabilized the membrane. This pore forming activity was confirmed by single-channel experiments, with well-defined ion channels obtained at different voltages. The characteristics of the ion channels (voltage dependence, only one or two states of conductance) clearly suggest that piscidin is more likely to permeabilize the membrane by toroidal pore formation rather than via the "barrel-stave" mechanism.

Conformational study of new amphipathic α‐helical peptide models of apoA‐I as potential atheroprotective agents

Aiming at contributing to the development of potential atheroprotective agents, we report on the concept and design of two peptide models, which mimic the amphipathic helices of apoA-I and incorporate Met into their sequences to validate its role as oxidant scavenger: Ac-ESK(Palm)KELSKSW(10)SEM(13)LKEK(Palm)SKS-NH(2) (model 1 [W(10), M(13)]) and Ac-ESK(Palm)KELSKSM(10)SEW(13)LKEK(Palm)SKS-NH(2) (model 2 [M(10), W(13)]). Hydrophobic residues of both models cover about the half of the surface, while the positively and negatively charged residues constitute two separate clusters on the hydrophilic face. Palmitoyl groups were introduced into the Lys-N(epsilon)H(2) groups at positions 3 and 17 to contribute to the amphipathic character of the peptides and stabilize the nonpolar face of the helix. Conformational study by the combined application of 2D-NMR and molecular dynamics simulations, CD, FTIR, and fluorescence spectroscopy revealed that model 1 adopts helical conformation and Met is well exposed to the microenvironment. Model 2 that derives from model 1 by exchanging W(10) (model 1) with M(10) and M(13) (model 1) with W(13) also displays helical characteristics, while Met is rather shielded. Oxidation experiments indicated that model 1 exhibits a 2-fold more potent antioxidant activity towards LDL oxidation, compared to model 2, confirming the role of Met, when is devoid of steric hindrances, as oxidant scavenger for the protection of LDL.

Production in Escherichia coli of a recombinant C-terminal truncated precursor of surfactant protein B (rproSP-B Δc). Structure and interaction with lipid interfaces

SP-B, a protein absolutely required to maintain the lungs open after birth, is synthesized in the pneumocytes as a precursor containing C-terminal and N-terminal domains flanking the mature sequence. These flanking-domains are cleaved to produce mature SP-B, coupled with its assembly into pulmonary surfactant lipid–protein complexes. In the present work we have optimized over-expression in Escherichia coli and purification of rproSP-B ΔC, a recombinant form of human proSP-B lacking the C-terminal flanking peptide, which is still competent to restore SP-B function in vivo. rProSP-B ΔC has been solubilized, purified and refolded from bacterial inclusion bodies in amounts of about 4 mg per L of culture. Electrophoretic mobility, immunoreactivity, N-terminal sequencing and peptide fingerprinting all confirmed that the purified protein had the expected mass and sequence. Once refolded, the protein was soluble in aqueous buffers. Circular dichroism and fluorescence emission spectra of bacterial rproSP-B ΔC indicated that the protein is properly folded, showing around 32% α-helix and a mainly hydrophobic environment of its tryptophan residues. Presence of zwitterionic or anionic phospholipids vesicles caused changes in the fluorescence emission properties of rproSP-B ΔC that were indicative of lipid–protein interaction. The association of this SP-B precursor with membranes suggests an intrinsic amphipathic character of the protein, which spontaneously adsorbs at air–liquid interfaces either in the absence or in the presence of phospholipids. The analysis of the structure and properties of recombinant proSP-B ΔC in surfactant-relevant environments will open new perspectives on the investigation of the mechanisms of lipid and protein assembly in surfactant complexes.

Membrane-Bound Dimer Structure of a β-Hairpin Antimicrobial Peptide from Rotational-Echo Double-Resonance Solid-State NMR

The intermolecular packing of a beta-hairpin antimicrobial peptide, PG-1, in lipid bilayers is determined using solid-state NMR distance measurements. Previous spin counting experiments showed that PG-1 associates as dimers in POPC bilayers; however, the detailed dimer structure was unknown. We have now measured several intermolecular 13C-19F, 1H-13C, and 15N-13C distances in site-specifically labeled PG-1 to constrain the structure of the intermolecular interface. The distances are measured using the rotational-echo double-resonance (REDOR) technique under magic-angle spinning. The results indicate that two PG-1 molecules align in a parallel fashion with the C-terminal strand of the hairpin forming the dimer interface. Six hydrogen bonds stabilize this interface, and the Phe12 side chain adopts the g- conformation in the membrane as in solution. The parallel packing of the peptide in the lipid bilayer differs from the antiparallel dimer found in DPC micelles and may be stabilized by its strong amphipathic character, which should facilitate its insertion into the amphipathic lipid bilayer. This study demonstrates the utility of the REDOR NMR technique for the elucidation of the oligomeric structure of membrane proteins.

Butyrylcholinesterase attenuates amyloid fibril formation in vitro

In Alzheimer's disease, both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) colocalize with brain fibrils of amyloid-beta (Abeta) peptides, and synaptic AChE-S facilitates fibril formation by association with insoluble Abeta fibrils. Here, we report that human BChE and BSP41, a synthetic peptide derived from the BChE C terminus, inversely associate with the soluble Abeta conformers and delay the onset and decrease the rate of Abeta fibril formation in vitro, at a 1:100 BChE/Abeta molar ratio and in a dose-dependent manner. The corresponding AChE synthetic peptide (ASP)40 peptide, derived from the homologous C terminus of synaptic human (h)AChE-S, failed to significantly affect Abeta fibril formation, attributing the role of enhancing this process to an AChE domain other than the C terminus. Circular dichroism and molecular modeling confirmed that both ASP40 and BChE synthetic peptide (BSP)41 are amphipathic alpha-helices. However, ASP40 shows symmetric amphipathicity, whereas BSP41 presented an aromatic tryptophan residue in the polar side of the C terminus. That this aromatic residue is causally involved in the attenuating effect of BChE was further supported by mutagenesis experiments in which (W8R) BSP41 showed suppressed capacity to attenuate fibril formation. In Alzheimer's disease, BChE may have thus acquired an inverse role to that of AChE by adopting imperfect amphipathic characteristics of its C terminus.

Characterization of the Residues in Helix 8 of the Human β1-Adrenergic Receptor That Are Involved in Coupling the Receptor to G Proteins

Several key amino acids within amphipathic helix 8 of the human beta1-adrenergic receptor (beta1-AR) were mutagenized to characterize their role in signaling by G protein-coupled receptors. Mutagenesis of phenylalanine at position 383 in the hydrophobic interface to histidine (F383H) prevented the biosynthesis of the receptor, indicating that the orientation of helix 8 is important for receptor biosynthesis. Mutagenesis of aspartic acid at position 382 in the hydrophilic interface to leucine (D382L) reduced the binding and uncoupled the receptor from G protein activation. Mutagenesis of the basic arginine residue at position 384 to glutamine (R384Q) or to glutamic acid (R384E) increased basal and agonist-stimulated adenylyl cyclase activities. R384Q and R384E displayed features associated with constitutively active receptors because inverse agonists markedly reduced their elevated basal adenylyl cyclase activities. Isoproterenol increased the phosphorylation and promoted the desensitization of the Gly389 or Arg389 allelic variants of the wild type beta1-AR but failed to produce these effects in R384Q and R384E, because these receptors were maximally phosphorylated and desensitized under basal conditions. In contrast to the membranous distribution of the wild type beta1-AR, R384Q and R384E were localized mostly within intracellular punctate structures. Inverse agonists restored the membranous distribution of R384Q and R384E, indicating that they recycled normally when their constitutive internalization was blocked by inverse agonists. These data combined with computer modeling of the putative three-dimensional organization of helix 8 indicated that the amphipathic character of helix 8 and side chain projections of Asp382 and Arg384 within the hydrophilic interface might serve as a tethering site for the G protein.

The Ectodomain of the Viral Receptor YueB Forms a Fiber That Triggers Ejection of Bacteriophage SPP1 DNA

The irreversible binding of bacteriophages to their receptor(s) in the host cell surface triggers release of the naked genome from the virion followed by transit of viral DNA to the host cell cytoplasm. We have purified, for the first time, a receptor from a Gram-positive bacterium that is active to trigger viral DNA ejection in vitro. This extracellular region ("ectodomain") of the Bacillus subtilis protein YueB (YueB780) was a 7 S elongated dimer forming a 36.5-nm-long fiber. YueB780 bound to the tail tip of bacteriophage SPP1. Although a stable receptor-phage interaction occurred between 0 and 37 degrees C, complete blocking of phage DNA release or partial ejection events were observed at temperatures below 15 degrees C. We also showed that the receptor was exposed to the B. subtilis surface. YueB differed structurally from phage receptors from Gram-negative bacteria. Its properties revealed a fiber spanning the full length of the 30-nm-thick peptidoglycan layer. The fiber is predicted to be anchored in the cell membrane through transmembrane segments. These features, highly suitable for a virus receptor in Gram-positive bacteria, are very likely shared by a large number of phage receptors.

Preparative chromatographic purification and surfactant properties of individual soyasaponins from soy hypocotyls

The amphipathic character of soyasaponins and their consequent biological and technological properties are well-recognised. However, mainly due to the absence of purified compounds, no data are available on the amphiphilic surfactant properties of individual soyasaponins. In this study we developed a preparative method for the purification of the main soyasaponins species from soy germ. Reversed-phase chromatography (Source 15 RPC) gave a good resolution of the various soyasaponins, and was used for the purification of non- and fully-acetylated soyasaponin Ab, DDMP-conjugated and unconjugated soyasaponins Ba and Bb. For these compounds, the critical micelle concentration (CMC), the minimal attainable surface tension ( γ CMC) and the surface density ( Γ max) were determined using the Wilhelmy plate method. The order of CMC values was as follows: soyasaponin Bb < Ba < βg < αg < Ab < non-acetylated Ab, and the CMC was found to range from 0.56 and to 3.2 g/L. It was concluded that the number of sugar side chains, the presence of acetyl groups and the presence of a DDMP-group are the main factors influencing the CMC of soyasaponins.

Orexin‐A is composed of a highly conserved C‐terminal and a specific, hydrophilic N‐terminal region, revealing the structural basis of specific recognition by the orexin‐1 receptor

Orexins-A and B, also called hypocretins-1 and 2, respectively, are neuropeptides that regulate feeding and sleep-wakefulness by binding to two orphan G protein-coupled receptors named orexin-1 (OX(1)R) and orexin-2 (OX(2)R). The sequences and functions of orexins-A and B are similar to each other, but the high sequence homology (68%) is limited in their C-terminal half regions (residues 15-33). The sequence of the N-terminal half region of orexin-A (residues 1-14), containing two disulfide bonds, is very different from that of orexin-B. The structure of orexin-A was determined using two-dimensional homonuclear and (15)N and (13)C natural abundance heteronuclear NMR experiments. Orexin-A had a compact conformation in the N-terminal half region, which contained a short helix (III:Cys6-Gln9) and was fixed by the two disulfide bonds, and a helix-turn-helix conformation (I:Leu16-Ala23 and II:Asn25-Thr32) in the remaining C-terminal half region. The C-terminal half region had both hydrophobic and hydrophilic residues, which existed on separate surfaces to provide an amphipathic character in helices I and II. The nine residues on the hydrophobic surface are also well conserved in orexin-B, and it was reported that the substitution of each of them with alanine resulted in a significant drop in the functional potency at the receptors. Therefore, we suggest that they form the surface responsible for the main hydrophobic interaction with the receptors. On the other hand, the residues on the hydrophilic surface, together with the hydrophilic residues in the N-terminal half region that form a cluster, are known to make only small contributions to the binding to the receptors through similar alanine-scan experiments. However, since our structure of orexin-A showed that large conformational and electrostatical differences between orexins-A and B were rather concentrated in the N-terminal half regions, we suggest that the region of orexin-A is important for the preference for orexin-A of OX(1)R.

Heme Binding by the N-Terminal Fragment 1−44 of Human Growth Hormone

Fragment 1-44 of human growth hormone (hGH), prepared in vitro by limited proteolysis of the hormone with pepsin at low pH, encompasses in full the N-terminal helix of this four-helix bundle protein [Spolaore, B., Polverino de Laureto, P., Zambonin, M., and Fontana, A. (2004) Biochemistry 40, 9460-9468]. Here, we report the new and interesting observation that fragment 1-44 can bind heme. The binding property is specific for the N-terminal helix of hGH, since heme binding does not occur with fragment 45-191 or the entire protein. The spectral characteristics of Fe-protoporphyrin IX are those of a low-spin, hexacoordinated iron ligated by two imidazole rings of His residues or His and Met residues. Far-UV circular dichroism (CD) measurements revealed that fragment 1-44 acquires a helical secondary structure upon heme binding. Heme appears to be bound to the fragment in a stereospecific way, since an induced dichroic signal is observed in the Soret region of the CD spectrum. The heme-fragment complex occurs in a 1:1 molar ratio, as determined by spectrophotometric titration, as well as by electrospray-ionization mass spectrometric analysis of the complex. The fragment alone is much more susceptible to tryptic digestion than the heme complex, implying a more folded and rigid structure of this last species. It is proposed that the molecular features of fragment 1-44 determining its heme-binding property reside in the amphipathic character of the helix adopted by the fragment, as well as in the presence in its polypeptide chain of His18, His21, and Met14. These residues can act as specific ligands for the heme-iron, as observed with cytochromes.

Synthesis and secondary structure in membranes of the Bcl‐2 anti‐apoptotic domain BH4

Solid phase synthesis of BH4, the 26 amino-acid domain (6RTGYDNREIVMKYIHYKLSQRGYEWD31) of the anti-apoptotic Bcl-2 protein has been accomplished using Fmoc chemistry. The use of peculiar cleavage conditions provided high yields after purification such that tens to hundreds of mg could be obtained. A 15N-labelled version of the peptide could also be synthesized for NMR studies in membranes. The peptide purity was not lower than 98% as controlled by UV and MALDI-TOF mass spectrometry. The secondary structure was determined in water, trifluoroethanol (TFE) and in lipid membrane using UV circular dichroism. The peptide shows dominant beta-sheeted structures in water that convert progressively into alpha-helical features upon addition of TFE or membrane. The amphipathic character of the helix suggests that the peptide might have a structure akin to those of antimicrobial peptides upon interaction with membranes.

Interaction of human stefin B in the prefibrillar oligomeric form with membranes

Protein aggregation is central to most neurodegenerative diseases, as shown by familial case studies and by animal models. A modified 'amyloid cascade' hypothesis for Alzheimer's disease states that prefibrillar oligomers, also called amyloid-beta-derived diffusible ligands or globular oligomers, are the responsible toxic agent. It has been proposed that these oligomeric species, as shown for amyloid-beta, beta2-microglobulin or prion fragments, exert toxicity by forming pores in membranes, initiating a cascade of detrimental events for the cell. Interaction of granular aggregates and globular oligomers of an amyloidogenic protein, human stefin B, with model lipid membranes and monolayers was studied. Prefibrillar oligomers/aggregates of stefin B are shown to cause concentration-dependent membrane leaking, in contrast to the homologous stefin A. Prefibrillar oligomers/aggregates of stefin B also increase the surface pressure at an air-water interface, i.e. they have amphipathic character and are surface seeking. In addition, they show stronger interaction with 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] monolayers than native stefin A or nonaggregated stefin B. Prefibrillar aggregates interact predominantly with acidic phospholipids, such as dioleoylphosphatidylglycerol or dipalmitoylphosphatidylserine, as shown by calcein release experiments and surface plasmon resonance. The same preparations are toxic to neuroblastoma cells, as determined by the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay, again in contrast to the homologue stefin A, which does not aggregate under any of the conditions studied. This study is aimed to contribute to the general model of cellular toxicity induced by prefibrillar oligomers of amyloidogenic proteins, not necessarily involved in pathology.

Antimicrobial and cytolytic properties of the frog skin peptide, kassinatuerin-1 and its l- and d-lysine-substituted derivatives

Kassinatuerin-1, a 21-amino-acid C-terminally α-amidated peptide first isolated from the skin of the African frog Kassina senegalensis, adopts an amphipathic α-helical conformation in a membrane-mimetic solvent (50% trifluoroethanol) and shows broad-spectrum antimicrobial activity. However, its therapeutic potential is limited by its relatively high cytolytic activity against mammalian cells. The antimicrobial and cytolytic properties of a peptide are determined by an interaction between cationicity, hydrophobicity, α-helicity and amphipathicity. Replacement of the C-terminal α-amide group in kassinatuerin-1 by carboxylic acid decreased both cationicity and α-helicity, resulting in an analog with decreased potency against Escherichia coli (4-fold) and Staphylococcus aureus (16-fold). Low cytolytic activities against human erythrocytes (LD50 > 400 μM) and L929 fibroblasts (LD50 = 105 μM) were also observed. Increasing cationicity, while maintaining amphipathic α-helical character, by progressively substituting Gly 7, Ser 18, and Asp 19 on the hydrophilic face of the α-helix with l-lysine, increased antimicrobial potency against S. aureus and Candida albicans (up to 4-fold) but also increased hemolytic and cytolytic activities. In contrast, analogs with d-lysine at positions 7, 18 and 19 retained activity against Gram-negative bacteria but displayed reduced hemolytic and cytolytic activities. For example, the carboxylic acid derivative of [ d-Lys 7, d-Lys 18, d-Lys 19]kassinatuerin-1 was active (minimum inhibitory concentration (MIC) = 6–12.5 μM) against a range of strongly antibiotic-resistant strains of E. coli but showed no detectable hemolytic activity at 400 μM and was 4-fold less cytolyic than kassinatuerin-1. However, the reduction in α-helicity produced by the d-amino acid substitutions resulted in analogs with reduced potencies against Gram-positive bacteria and against C. albicans.

Solution Structure and Lipid Membrane Partitioning of VSTx1, an Inhibitor of the KvAP Potassium Channel,

VSTx1 is a voltage sensor toxin from the spider Grammostola spatulata that inhibits KvAP, an archeabacterial voltage-activated K(+) channel whose X-ray structure has been reported. Although the receptor for VSTx1 and the mechanism of inhibition are unknown, the sequence of the toxin is related to hanatoxin (HaTx) and SGTx, two toxins that inhibit eukaryotic voltage-activated K(+) channels by binding to voltage sensors. VSTx1 has been recently shown to interact equally well with lipid membranes that contain zwitterionic or acidic phospholipids, and it has been proposed that the toxin receptor is located within a region of the channel that is submerged in the membrane. As a first step toward understanding the inhibitory mechanism of VSTx1, we determined the three-dimensional solution structure of the toxin using NMR. Although the structure of VSTx1 is similar to HaTx and SGTx in terms of molecular fold and amphipathic character, the detailed positions of hydrophobic and surrounding charged residues in VSTx1 are very different than what is seen in the other toxins. The amphipathic character of VSTx1, notably the close apposition of basic and hydrophobic residues on one face of the toxin, raises the possibility that the toxin interacts with interfacial regions of the membrane. We reinvestigated the partitioning of VSTx1 into lipid membranes and find that VSTx1 partitioning requires negatively charged phospholipids. Intrinsic tryptophan fluorescence and acrylamide quenching experiments suggest that tryptophan residues on the hydrophobic surface of VSTx1 have a diminished exposure to water when the toxin interacts with membranes. The present results suggest that if membrane partitioning is involved in the mechanism by which VSTx1 inhibits voltage-activated K(+) channels, then binding of the toxin to the channel would likely occur at the interface between the polar headgroups and the hydrophobic phase of the membrane.

Synthesis and applications of silicone oil–soluble fluoroalkyl end‐capped cooligomers

AbstractFluoroalkyl end‐capped cooligomers containing polydimethylsiloxane and polyoxyethylene segments were prepared under very mild conditions by the cooligomerizations of fluoroalkanoyl peroxides with methacrylate monomers containing the corresponding segments and comonomers such as dimethylacrylamide and acryloylmorpholine. These obtained fluorinated cooligomers exhibited amphipathic characteristics and became soluble in water and common organic solvents. In particular interest, fluoroalkyl end‐capped cooligomers containing polyoxyethylene units were soluble not only in poly(methylphenylsiloxane) (silicone oil) but also in water, including common organic solvents except for hexane. Additionally, these fluorinated cooligomers were able to reduce the surface tension of water and m‐xylene quite effectively, to around 15 and 20 mN/m levels, respectively. In these fluorinated cooligomers, fluoroalkyl end‐capped acryloylmorpholine cooligomers containing polyoxyethylene segments were applicable as a novel emulsifying agent against water and silicone oil. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1467–1476, 2005

Amphipathic and Membrane‐Destabilizing Properties of the Cationic Acrylate Polymer Eudragit® E100

The cationic acrylate polymer Eudragit E100 (E100) produces a biphasic effect on the stability of casein micelles disrupting their internal structure. These results suggested that this polymer could have some amphipathic character. Therefore, in this study the polymer was characterized with respect to its interaction with different amphipathic systems (bile-acid micelles, lipoproteins and liposomes), cell membranes (red blood cells) and virus membranes (Herpes simplex type 2 virus). As with caseins, a biphasic effect was observed with bile acids with a precipitation phase at low polymer/bile acid ratio and a solubilization phase when the polymer concentration was increased. Upon interaction with human plasma, an important reduction in cholesterol and triglycerides was observed upon remotion of E100 by a rise in pH to 8.5 and centrifugation. In agreement with this finding, an important reduction in plasma lipoproteins was observed upon its treatment with E100 and further remotion by pH rise and centrifugation. However, the amount of the major protein components of human plasma and the activity of several enzymes and antibodies were not affected by their treatment with E100. The membrane-destabilizing properties of E100 were confirmed by its lytic activity on liposomes and red blood cells and by an important antiviral effect of E100 on Herpes simplex virus type 2. Altogether, these results show that, despite its water solubility and cationic character, E100 displays a significative amphipathic and membrane-destabilizing character with potential biotechnological applications. [diagram in text].

A QSAR‐modeling perspective on cationic transfection lipids. 1. Predicting efficiency and understanding mechanisms

As gene therapy using viral vectors involves clinical risks, limited DNA-carrying capacity, and manufacturing problems, non-viral vectors, including cationic lipids, have been investigated. Unfortunately, these agents have significantly lower transfectional ability and, due to the complexity of the transfectional pathway, no general schemes exist for correlating cationic lipid chemistry with transfectional efficacy. Quantitative structure-activity relationship (QSAR) analyses were carried out on sets of routinely used, experimental, and unsuccessful cationic lipid vectors taken from the literature. This approach described the amphipathic character, basicity, headgroup size, lipophilicity and shape of cationic lipids using numerical parameters. Compounds were plotted onto various parameter diagrams, and correlations were sought between numerical parameters and transfectional efficiency. Transfectionally effective cationic lipids fell into restricted zones in various parameter spaces, indicating that amphipathic character, lipid shape and lipophilicity were generally significant factors, whilst basicity and headgroup size were only important for certain compounds. The data supported the general significance of membrane mixing followed by induction of membrane curvature, and the more limited role of osmotic shock, as mechanisms of membrane disruption. QSAR descriptions of effective lipids permitted detailed chemical guidelines for optimizing cationic lipid structure to be given. Limitations of the approach and models are discussed. QSAR modeling indicated that induction of membrane curvature and osmotic shock are important mechanisms for membrane disruption by cationic lipids. The models also allowed specification of chemically detailed guidelines for selection or design of optimal cationic lipids.

Design of potent, non-toxic antimicrobial agents based upon the structure of the frog skin peptide, pseudin-2

Pseudin-2, a naturally occurring 24 amino-acid-residue antimicrobial peptide first isolated from the skin of the South American paradoxical frog Pseudis paradoxa, has weak hemolytic and cytolytic activity but also relatively low potency against microorganisms. In a membrane-mimetic environment, the peptide exists in an amphipathic α-helical conformation. Analogs of the peptide with increased cationicity and α-helicity were chemically synthesized by progressively substituting neutral and acidic amino acid residues on the hydrophilic face of the α-helix by lysine. Analogs with up to three L-lysine substitutions showed increased potency against a range of gram-negative and gram-positive bacteria (up to 16-fold) whilst retaining low hemolytic activity. The analog [ d-Lys 3, d-Lys 10, d-Lys 14]pseudin-2 showed potent activity against gram-negative bacteria (minimum inhibitory concentration, MIC=5 μM against several antibiotic-resistant strains of Escherichia coli) but very low hemolytic activity (HC 50>500 μM) and cytolytic activity against L929 fibroblasts (LC 50=215 μM). Increasing the number of l-lysines to four and five did not enhance antimicrobial potency further but increased hemolytic activity towards human erythrocytes. Time-kill studies demonstrated that the analog [Lys 3, Lys 10, Lys 14, Lys 21]pseudin-2 at a concentration of 1×MIC was bacteriocidal against E. coli (99.9% cell death after 96 min) but was bacteriostatic against S. aureus. Increasing the hydrophobicity of pseudin-2, while maintaining the amphipathic character of the molecule, by substitution of neutral amino acids on the hydrophobic face of the α-helix by l-phenylalanine, had only minor effects on antimicrobial and hemolytic activities.