Gramicidin A--phospholipid model systems

Properties of electrode-supported lipid cubic mesophase films with embedded gramicidin A: structure and ion-transport studies

The lipid cubic phase (LCP) is a nanomaterial composed of water channels surrounded by lipid bilayers. LCPs are stable at room temperature and are biocompatible. These features make the lipid cubic phases similar to biological membranes, and hence, are favorable for embedding membrane proteins. We show that the monoolein cubic phase deposited on the electrode forms a 3D lipid bilayer film convenient for electrochemical investigations of membrane proteins. In this research, we studied the effect of embedding an ionophoric peptide, gramicidin A (gA), on the structure and properties of the LCP film. The phase identity and structural parameters of the gramicidin-doped phase were characterized by small-angle X-ray scattering (SAXS). The potassium ion transport through the film were studied by electroanalytical methods: alternating current voltammetry (ACV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). Increased values for the current of the gramicidin-doped cubic phase compared to the empty cubic phase and changes of the EIS parameters confirmed that the peptide remained in the film in its active dimeric form. Our results show that the LCP can be considered a suitable 3D biomimetic film for the investigation of ion channels and other transporting membrane proteins, and for their application in electrochemical sensors.

Methods and Approaches for the Solid-Phase Synthesis of Peptide Alcohols

Many methods have been developed for attaching an alcohol functionality to a solid support. However, not all of these methods are used to obtain peptide alcohols. In this Minireview, we will discuss several of the most important methods and approaches for the synthesis of peptide alcohols and the attachment of hydroxy groups to a solid support for the synthesis of cyclic peptides. Some of the methods include the use of functionalized Wang resin and the attachment of an alcohol to an enol ether resin. We also discuss the use of the chlorotrityl resin, one of the most common linkers used to obtain peptide alcohols. In addition, we outline the recently developed resins with the Rink, Ramage and Sieber handles. The majority of these methods have been used to synthesize many important drugs, such as octreotide and the antibiotic peptaibols.

Antimicrobial Peptide Structures: From Model Membranes to Live Cells

The rise in antibiotic resistance has led to a renewed interest in antimicrobial peptides (AMPs) that target membranes. The mode of action of AMPs involves the disruption of the lipid bilayer and leads to growth inhibition and death of the bacteria. However, details at the molecular level of how these peptides kill bacteria and the reasons for the observed differences in selectivity remain unclear. Structural information is crucial for defining the molecular mechanism by which these peptides recognize, self-assemble and interact with a particular lipid membrane. Solid-state NMR is a non-invasive technique that allows the study of the structural details of lipid-peptide and peptide-peptide interactions. Following on from studies of antibiotic and lytic peptides, gramicidin A and melittin, respectively, we investigated maculatin 1.1, an AMP from the skin of Australian tree frogs that acts against Gram-positive bacteria. By using perdeuterated phospholipids and specifically labelled peptides, ² H, ³¹ P and {³¹ P}¹⁵ N REDOR solid-state NMR experiments have been used to localize, maculatin 1.1 in neutral and anionic model membranes. However, the structure, location and activity depend on the composition of the model membrane and current advances in solid-state NMR spectroscopy now allow structure determination of AMPs in live bacteria.

Synergistic interaction between silver nanoparticles and membrane-permeabilizing antimicrobial peptides

Silver nanoparticles, as well as antimicrobial peptides (AMPs), can be used to fight infectious diseases. Since AMPs are known to permeabilize bacterial membranes and might therefore help silver nanoparticles to access internal target sites, we investigated their combined activities and showed synergistic effects between polymyxin B and silver nanoparticles for gram-negative bacteria.

Steric interactions of valines 1, 5, and 7 in [valine 5, D-alanine 8] gramicidin A channels

When the central valine residues 6, 7, and 8 of gramicidin A (gA) are shifted by one position, the resulting [Val(5), D-Ala(8)]gA forms right-handed channels with a single-channel conductance and average duration somewhat less than gA channels. The reduction in channel duration has been attributed to steric conflict between the side chains of Val(1) and Val(5) in opposing monomers (Koeppe, R. E. II, D. V. Greathouse, A. Jude, G. Saberwal, L. L. Providence, and O. S. Andersen. 1994. J. Biol. Chem. 269:12567-12576). To investigate the orientations and motions of valines in [Val(5), D-Ala(8)]gA, we have incorporated (2)H labels at Val 1, 5, or 7 and recorded (2)H-NMR spectra of oriented and nonoriented samples in hydrated dimyristoylphosphatidylcholine. Spectra of nonoriented samples at 4 degrees C reveal powder patterns that indicate rapid side chain "hopping" for Val(5), and an intermediate rate of hopping for Val(1) and Val(7) that is somewhat slower than in gA. Oriented samples of deuterated Val(1) and Val(7) show large changes in the methyl and C(beta)-(2)H quadrupolar splittings (Deltanu(q)) when Ala(5) in native gA is changed to Val(5). Three or more peaks for the Val(1) methyls with Deltanu(q) values that vary with the echo delay, together with an intermediate spectrum for nonoriented samples at 4 degrees C, suggest unusual side chain dynamics for Val(1) in [Val(5), D-Ala(8)]gA. These results are consistent with a steric conflict that has been introduced between the two opposing monomers. In contrast, the acylation of gA has little influence on the side chain dynamics of Val(1), regardless of the identity of residue 5.

Effect of n-alkanols on lipid bilayer hydration

The effect of a homologous series of aliphatic n-alkanols on the presence of water within the head group and acyl chain region of lipid bilayers was investigated using time-resolved fluorescence spectroscopy according to a previously published approach [Ho, C., Slater, S. J., & Stubbs, C. D. (1995) Biochemistry 34, 6188-6195]. Upon addition of n-alkanols to phosphatidylcholine bilayers the fluorescence lifetime of N-[5-(dimethylamino)naphthalene-1-sulfonyl]dipalmitoylphosphatidyleth anolamine (dansyl-PE) decreased, indicative of an increased water content within the head group region, the effect being a linear function of n-alkanol chain length (C1-C8), based on the total n-alkanol concentration. The fluorescence lifetimes of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3, 5-hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-phosphatidylcholine (DPH-PC) and N-[[4-(6-phenyl-1,3, 5-hexatrienyl)phenyl]propyl]trimethylammonium p-toluenesulfonate (TMAP-DPH), and the fluorescence intensity ratio of the latter in D2O compared to that in H2O, were used to probe the level of water in the acyl chain region. There was a decrease in the lifetime and an increase in the D2O/H2O fluorescence intensity ratio upon addition of short-chain n-alkanols (C1-C3), suggestive of increased water content. By contrast, long-chain n-alkanols (C4-C8) increased the lifetime and decreased the ratio, suggestive of decreased water content. Acyl chain order, determined from DPH-PC fluorescence anisotropy, was decreased by all n-alkanols, indicating that the effects were not probe-dependent. The effects of short- and long-chain n-alkanols on the fluorescence lifetime of the tryptophans of gramicidin, incorporated into phosphatidylcholine bilayers as a model membrane protein, were similar to those obtained with TMAP-DPH and DPH-PC; ethanol decreased and hexanol increased the lifetime. Thus the effect of n-alkanols and general anesthetics on changes in the amount of water that may be accommodated within the acyl chain region of the bilayer is not predictable on the basis of the magnitude of effects on head group region or acyl chain order/fluidity.

C-terminal amino groups facilitate membrane incorporation of gramicidin derivatives

Gramicidin derivatives with (positively-charged) C-terminal amino groups are found to incorporate readily and refold quickly when added to dioleoylphosphatidylcholine lipid vesicles from concentrated methanol solutions. Neutral and negatively-charged derivatives do not.

Motionally restricted tryptophan environments at the peptide-lipid interface of gramicidin channels

The tryptophans in the gramicidin channel play a crucial role in the organization and function of the channel. The localization and dynamics of these tryptophans have been studied using fluorescence spectroscopy, especially utilizing environment-induced effects on the rates of solvent relaxation around these residues in membranes. When incorporated into model membranes of dioleoyl-sn-glycero-3-phosphocholine (DOPC), the tryptophans in the gramicidin channel exhibit a red edge excitation shift (REES) of 6 nm. In addition, fluorescence polarization shows both excitation and emission wavelength dependence. Fluorescence lifetime analysis shows a biexponential decay, corresponding to a short- and a long-lifetime component. The mean lifetime was found to be dependent on both excitation and emission wavelengths. Analysis of time-resolved emission spectra (TRES) shows a heterogeneous environment for the tryptophans consistent with the lifetime information. Taken together, these observations point out the motional restriction experienced by the tryptophans in the gramicidin channel. This is consistent with other studies in which such restrictions are thought to be imposed due to hydrogen bonding between the indole rings of the tryptophans and the neighboring lipid carbonyls. The significance of such organization in terms of functioning of the channel is brought out by the fact that substitution, photodamage, or chemical modification of these tryptophans is known to give rise to channels with conformation and reduced conductivity.

Alpha-helical hydrophobic polypeptides form proton-selective channels in lipid bilayers

Proton translocation is important in membrane-mediated processes such as ATP-dependent proton pumps, ATP synthesis, bacteriorhodopsin, and cytochrome oxidase function. The fundamental mechanism, however, is poorly understood. To test the theoretical possibility that bundles of hydrophobic alpha-helices could provide a low energy pathway for ion translocation through the lipid bilayer, polyamino acids were incorporated into extruded liposomes and planar lipid membranes, and proton translocation was measured. Liposomes with incorporated long-chain poly-L-alanine or poly-L-leucine were found to have proton permeability coefficients 5 to 7 times greater than control liposomes, whereas short-chain polyamino acids had relatively little effect. Potassium permeability was not increased markedly by any of the polyamino acids tested. Analytical thin layer chromatography measurements of lipid content and a fluorescamine assay for amino acids showed that there were approximately 135 polyleucine or 65 polyalanine molecules associated with each liposome. Fourier transform infrared spectroscopy indicated that a major fraction of the long-chain hydrophobic peptides existed in an alpha-helical conformation. Single-channel recording in both 0.1 N HCl and 0.1 M KCl was also used to determine whether proton-conducting channels formed in planar lipid membranes (phosphatidylcholine/phosphatidylethanolamine, 1:1). Poly-L-leucine and poly-L-alanine in HCl caused a 10- to 30-fold increase in frequency of conductive events compared to that seen in KCl or by the other polyamino acids in either solution. This finding correlates well with the liposome observations in which these two polyamino acids caused the largest increase in membrane proton permeability but had little effect on potassium permeability. Poly-L-leucine was considerably more conductive than poly-L-alanine due primarily to larger event amplitudes and, to a lesser extent, a higher event frequency. Poly-L-leucine caused two populations of conductive events, one in the 0.1-0.5 pA range, and one in the 1.0-5.0 pA range, whereas nearly all events caused by poly-L-alanine were in the 0.1-0.5 pA range at an applied voltage of +60 mV. The channel-like activity appeared to switch between conductive and nonconductive states, with most open-times in the range of 50-200 ms. We conclude that hydrophobic polyamino acids produce proton-conducting defects in lipid bilayers that may be used to model functional proton channels in biological membranes.

Hydration at the membrane protein-lipid interface

Evidence has been found for the existence water at the protein-lipid hydrophobic interface of the membrane proteins, gramicidin and apocytochrome C, using two related fluorescence spectroscopic approaches. The first approach exploited the fact that the presence of water in the excited state solvent cage of a fluorophore increases the rate of decay. For 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5- hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-PC (DPH-PC), where the fluorophores are located in the hydrophobic core of the lipid bilayer, the introduction of gramicidin reduced the fluorescence lifetime, indicative of an increased presence of water in the bilayer. Since a high protein:lipid ratio was used, the fluorophores were forced to be adjacent to the protein hydrophobic surface, hence the presence of water in this region could be inferred. Cholesterol is known to reduce the water content of lipid bilayers and this effect was maintained at the protein-lipid interface with both gramicidin and apocytochrome C, again suggesting hydration in this region. The second approach was to use the fluorescence enhancement induced by exchanging deuterium oxide (D2O) for H2O. Both the fluorescence intensities of trimethylammonium-DPH, located in the lipid head group region, and of the gramicidin intrinsic tryptophans were greater in a D2O buffer compared with H2O, showing that the fluorophores were exposed to water in the bilayer at the protein-lipid interface. In the presence of cholesterol the fluorescence intensity ratio of D2O to H2O decreased, indicating a removal of water by the cholesterol, in keeping with the lifetime data. Altered hydration at the protein-lipid interface could affect conformation, thereby offering a new route by which membrane protein functioning may be modified.

Model ion channels: gramicidin and alamethicin

We have discussed in some detail a variety of experimental studies which were designed to elucidate the conformational and dynamic properties of gramicidin and alamethicin. Although the behavior of these peptides is by no means fully characterized, these studies have already permitted aspects of ion channel activity to be understood in molecular terms. Studies with gramicidin in a variety of organic solutions have revealed conformational heterogeneity of this peptide; at least five major isomers exist, several of which have been characterized in detail using NMR spectroscopy and X-ray crystallography. When added to lipid membranes gramicidin undergoes a further conformational conversion. Although the conformation of gramicidin in membranes is not as well characterized as the solution conformation(s) and an X-ray structure is not yet available, detailed data, particularly from solid-state NMR studies, continue to become available and a right-handed beta 6.3 helical conformation of the peptide backbone is now generally accepted. Two of these beta 6.3 helices joined at their N-termini are believed to form the conducting channel. The conformational behavior of the side-chains of gramicidin in the membrane-bound form is not well established and several NMR, CD, fluorescence and theoretical studies are now focussed on this. Although the side-chains do not directly contact the permeating ions, they can have distinct effects on conductance and selectivity by altering the electrostatic environment sensed by the ion. The dynamics of both side-chain and backbone conformations of gramicidin appear critical to a detailed understanding of the ion transport process in this channel. As the description of the membrane-bound conformation of gramicidin becomes more detailed, simulations of ion transport using computational methods are likely to improve and will further our understanding of the processes of ion transport. As well as internal motion of the backbone and side-chains, gramicidin undergoes rotational and translational motion in the plane of the membrane. These motions do not appear to be essential for the process of ion transport but can affect channel lifetime since lifetime is determined by the rate of association and dissociation of gramicidin monomers. Gramicidin-membrane interactions are also likely to be involved in the frequency of occurrence of channel subconductance states, the frequency of channel flickering and fundamentally in the stability of the membrane-bound gramicidin conformation. Alamethicin forms channels in membranes which are strongly voltage-dependent. The molecular origin of voltage-dependent conductances has been a fundamental problem in biophysics for many years.(ABSTRACT TRUNCATED AT 400 WORDS)

Gramicidin channel selectivity. Molecular mechanics calculations for formamidinium, guanidinium, and acetamidinium

Empirical energy function calculations were used to evaluate the effects of minimization on the structure of a gramicidin A channel and to analyze the energies of interaction between three cations (guanidinium, acetamidinium, formamidinium) and the channel as a function of position along the channel axis. The energy minimized model of the gramicidin channel, which was based on the results of Venkatachalam and Urry (1983), has a constriction at the channel entrance. If the channel is not allowed to relax in the presence of the ions (rigid model), there is a large potential energy barrier for all three cations. The barrier varies with cation size and is due to high van der Waals and ion deformation energies. If the channel is minimized in the presence of the ions, the potential energy barrier to formamidinium entry is almost eliminated, but a residual barrier remains for guanidinium and acetamidinium. The residual barrier is primarily due, not to the expansion of the helix, but, to the disruption of hydrogen bonds between the terminal ethanoloamine and the next turn of the helix which occurs when the carbonyls of the outer turn of the helix librate inward toward the ion as it enters the channel. The residual potential energy barriers could be a possible explanation for the measured selectivity of gramicidin for formamidinium over guanidinium. The results of this full-atomic model address the applicability of the size-exclusion concept for the selectivity of the gramicidin channel.

Gramicidin conformational studies with mixed-chain unsaturated phospholipid bilayer systems

The transition of gramicidin from a nonchannel to a channel form was investigated using mixed-chain phosphatidylcholine lipid bilayers. Gramicidin and phospholipids were codispersed, after removal of the solvents chloroform/methanol or trifluoroethanol which resulted in nonchannel and channel conformations, respectively, as confirmed using circular dichroism (CD). The fluorescence emission maxima of the nonchannel form were shifted toward shorter wavelengths by heating at 60 degrees C (for 0-12 h), which converted it to a channel form, again as confirmed by CD. The channel form did not respond to heat treatment. Heat treatment also increased the fluorescence anisotropy of the nonchannel gramicidin tryptophans. The rate of transition from the nonchannel to channel conformation was found to be faster if phosphatidylethanolamine was present in combination with phosphatidylcholine compared to phosphatidylcholine alone. Also, gramicidin in bilayers of the polyunsaturated 1-palmitoyl-2-docosahexaenoyl-phosphatidylcholine converted more rapidly compared to 1-palmitoyl-2-oleoylphosphatidylcholine. Using the fluorescence anisotropy of the membrane lipid probe 1,6-diphenyl-1,3,5-hexatriene, it was also shown that the motional properties of the surrounding lipid acyl chains differed for the channel and nonchannel gramicidin conformations. The possibility that lipids tending to favor the hexagonal phase (HII) would enhance the rate of the nonchannel to channel transition was supported by 31P NMR which revealed the presence of some HII lipids in the channel preparations. The results of this study suggest that gramicidin may serve as a useful model for similar conformational transitions in other more complex membrane proteins.

Synthesis of acylated gramicidins and the influence of acylation on the interfacial properties and conformational behavior of gramicidin A

Five gramicidin A analogs were synthesized in which various acyl chains, differing in length and unsaturation, were covalently coupled to the C-terminal ethanolamine group. The analogs were characterized by various spectroscopic techniques and their molecular properties were investigated using monolayer techniques and circular dichroism. It is demonstrated that neither the interfacial properties nor the conformational behavior of gramicidin A at the air/water interface are seriously affected upon acylation. It is proposed that at the limiting area the gramicidin molecule is oriented with its C-terminus towards the subphase with the covalently coupled acylchain located parallel to the helical axis in between the protruding tryptophans. Circular dichroism experiments, in which gramicidin-containing vesicles were prepared from different organic solvents, indicate that the presence of a covalently coupled fatty acylchain tends to stabilize the beta 6.3 helical conformation. It is demonstrated that, like for gramicidin A, also for the acylgramicidins the single-stranded beta 6.3 helical conformation, or channel conformation, is the preferred conformation upon incorporation in bilayers.

Radiation inactivation of ion channels formed by gramicidin A. Protection by lipid double bonds and by alpha-tocopherol

The conductance induced by the channel-forming peptide gramicidin A in lipid membranes is reduced by many orders of magnitude on exposure of the membrane and its aqueous environment to ionizing radiation. This results from an interaction of free radicals of water radiolysis with the tryptophan residues of gramicidin A. The sensitivity of the ion channels towards irradiation is strongly reduced in the presence of either vitamin E or of highly unsaturated lipids. An increase of the D37 dose up to a factor of 50 was found. The phenomena are interpreted via a reduction of the effective concentration of free radicals (such as OH.) in the membrane by reaction with unsaturated fatty acid residues or with vitamin E.

Polymorphic phospholipid phase transitions as tools to understand peptide-lipid interactions

The effect of peptides on bilayer----non-bilayer phase transitions can be used as a tool to investigate the molecular aspects of peptide-lipid interactions. In this contribution the action on membranes of the peptide antibiotic gramicidin A and the bee venom component melittin are compared. Although the known structures and locations of these peptides upon membrane binding are very different, their actions on membranes show striking parallels. A general model is proposed that explains the seemingly complex peptide-lipid interactions by making use of simple concepts.

The divalent cation-binding sites of gramicidin A transmembrane ion-channel

The conductance of the gramicidin A single channels in glycerolmonooleate membranes is strongly reduced in the presence of Mn2+ cations. The nmr experiments were performed for N-terminal to N-terminal gramicidin A dimer formed by two right-handed single-stranded helixes incorporated into the sodium dodecyl sulfate micelles in the presence of Mn2+ ions. Dependence of the nonselective spin-lattice relaxation rates of the gramicidin A protons on Mn2+ concentration was analyzed to determine coordinates of the divalent cation binding sites. It is inferred that Mn2+ ions are bound at the channel mouths at distances of 6.4, 8.6, and 8.8 A (+/- 2 A) from the oxygen atoms of exposed carbonyl groups of D-Leu 12, 14, and 10, respectively. The bounded Mn2+ retains its hydrate shell, the size of which (approximately 6 A) exceeds the inner pore diameter (approximately 4 A). That makes the gramicidin A channel impermeable for divalent cations.

Photo-modulated ion channels based on covalently linked gramicidins

The covalent coupling of two gramicidin A monomers proved to be a useful tool for the rational design of ion channels with predictable electrophysiological properties (Stankovic, C.J., Heinemann, S.H., Delfino, J.M., Sigworth, F.J. and Schreiber, S.L. (1989) Science 244, 813-817; Stankovic, C.J., Heinemann, S.H. and Schreiber, S.L. (1990) J. Am. Chem. Soc. 112, 3702-3704). Herein we report on our first efforts to equip such channels with an artificial gating mechanism. Gramicidin monomers were covalently linked with 3,3'-azobis(benzeneacetic acid). Based on computer modeling of the beta-helix channel motif, this linker in its dark-adapted (trans) form does not allow for the formation of unimolecular ion channels, while the photo-activated (cis) form was expected to provide this possibility. The electrophysiological assays showed that (A) the trans-isomer does form characteristic ion channels, and (B) irradiation transforms these channels into a new distinct, flickering channel type in a reversible manner. The results are discussed in the framework of intermolecular gramicidin aggregates.

Interaction of phospholipids with proteins, peptides and amino acids. New advances 1987-1989

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins, peptides and amino acids. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. The effect of the interaction on the structure and biological activity of the interacting biomolecules is discussed.

Solid-state 13C-NMR studies of the effects of sodium ions on the gramicidin A ion channel

End-to-end helical dimers of gramicidin A form transmembrane pores in lipid bilayers, through which monovalent ions may pass. The groups within the peptide that interact with these ions have been studied by application of solid-state spectroscopic methods to a series of gramicidin A analogues synthesized with 13C in selected peptide carbonyl groups. The resonances of D-Leu10, D-Leu12 and D-Leu14 analogues were perturbed in the presence of 0.16 M sodium ions, whereas the resonances of the carbonyls of Gly2, Ala3, D-Leu4 and Val7, which are closer to the formylated N-terminal end of the peptide, were unaffected. The observed changes in chemical shift anisotropy are indicative of a change in orientation of the abovementioned leucine carbonyls.

Effect of proteins on fluorophore lifetime heterogeneity in lipid bilayers

The effect of three different membrane proteins on the fluorescence lifetime heterogeneity of 1,6-diphenyl-1,3,5-hexatriene (DPH) in phospholipid vesicle systems was investigated. For large unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) at 37 degrees C, the fluorescence decay was essentially monoexponential (8.6 and 8.2 ns, respectively) except for a minor component typical of DPH. For gramicidin D reconstituted into DMPC vesicles at a protein/lipid molar ratio of 1/7, the most appropriate analysis of the data was found to be in the form of a bimodal Lorentzian distribution. Centers of the major lifetime components were almost identical with those recovered for vesicles without proteins, while broad distributional widths of some 4.0 ns were recovered. Variation of the protein/lipid molar ratio in sonicated POPC vesicles revealed an abrupt increase in distributional width at ratios approximating 1/15-1/20, which leveled off at about 2.5 ns. For bacteriorhodopsin in DMPC vesicles and cytochrome b5 in POPC, the most appropriate analysis of the data was again found to be in the form of a bimodal Lorentzian also with broad distributional widths in the major component. Lifetime centers were decreased for these proteins due to fluorescence energy transfer to the retinal of the bacteriorhodopsin and heme of the cytochrome b5. Fluorescence energy transfer is distance dependent, and since a range of donor-acceptor distances would be expected in a membrane, lifetime distributions should therefore be recovered independently of other effects for proteins possessing acceptor chromophores.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cholesterol or gramicidin on slow and fast motions of phospholipids in oriented bilayers

Nuclear spin-lattice relaxation both in the rotating frame and in the laboratory frame is used to investigate the slow and fast molecular motions of phospholipids in oriented bilayers in the liquid crystalline phase. The bilayers are prepared from a perdeuterated phospholipid labeled with a pair of 19F atoms at the 7 position of the 2-sn acyl chain. Phospholipid-cholesterol or phospholipid-gramicidin interactions are characterized by measuring the relaxation rates as a function of the bilayer orientation, the locking field, and the temperature. Our studies show that cholesterol or gramicidin can specifically enhance the relaxation due to slow motions in phospholipid bilayers with correlation times tau s longer than 10(-8) sec. The perturbations of the geometry of the slow motions induced by cholesterol are qualitatively different from those induced by gramicidin. In contrast, the presence of cholesterol or gramicidin slightly suppresses the fast motions with correlation times tau f = 10(-9) to 10(-10) sec without significantly affecting their geometry. Weak locking-field and temperature dependences are observed for both pure lipid bilayers and bilayers containing either cholesterol or gramicidin, suggesting that the motions of phospholipid acyl chains may have dispersed correlation times.

Slow motions in oriented phospholipid bilayers and effects of cholesterol or gramicidin. A 19F-NMR T1 rho study

In an extension of our earlier work (Peng, Z.-y., V. Simplaceanu, I. J. Lowe, and C. Ho. 1988. Biophys. J. 54:81-95), the rotating-frame nuclear spin-lattice relaxation (T1 rho) technique has been used to investigate the slow molecular motions (10(-4) - 10(-6) s) in lipid bilayers prepared from protonated or perdeuterated 19F-labeled phospholipids in the absence and presence of cholesterol or gramicidin as membrane-interacting molecules. Complications caused by the 19F-1H cross-polarization observed previously can be removed by the substitution of 2H for 1H in the acyl chains. Only a weak dependence of the T-1(1 rho) on the locking field strength is found for a phospholipid molecule with perdeuterated acyl chains, indicating that there are no slow motions with a single, well-defined correlation time between 5 x 10(-6) and 4 x 10(-5) s. However, the orientation dependences of the T-1(1 rho) can be well fitted by motional models with either one slow motion having an unspecified geometry or with a superposition of two specific types of slow motions. Cholesterol and gramicidin show distinct effects in altering either the geometry or the weighting of slow motions in phospholipid bilayers, as reflected by changes in the orientation dependence. These two additives also exhibit quite different label-position specificities. A qualitative understanding of the induced effects of cholesterol and gramicidin on the dynamics of phospholipid bilayers will be discussed.

Transmembrane channels based on tartaric acid-gramicidin A hybrids

The gramicidin A transmembrane channel is believed to consist of two head-to-head beta helices. Computer-generated models were used to formulate the structure of new single-chain channel molecules based on the gramicidin motif. The chemical synthesis of two tartaric acid-gramicidin A hybrids and single-channel analyses of their conducting properties are reported. These studies illustrate the rational design and synthesis of long-lived channels with tunable conductance properties and provide support for current molecular models of the natural (dimeric) gramicidin channel.

Structure and supramolecular architecture of membrane channel-forming peptides

Peptides gathering together to induce channels in lipid bilayers may be classified in several categories according to the spatial structures involved. For example, gramicidin A forms intramolecular tubes, alamethicin, bundles of helical rods with intermolecular pores, porins (being proteins, properly speaking) are rich in beta-sheets that may form barrels, whereas cyclic peptides might stack together resulting in the formation of pores. The chemical structure of these compounds is now well characterized. The transmembrane electrical signals that they transmit are also typical of the particular supramolecular configurations (or architecture). Investigations in this field are thus relevant to structure-function relationship studies due to the availability of natural or synthetic analogues allowing the measurement of the influence of physico-chemical parameters upon the energy profiles of the pores. Consequently, questions such as the existence and probabilities of conductance substrates, their voltage-dependence and their ion or molecular selectivity can be tackled. Today, the loosest aspect of these studies lies in the actual molecular conformations and architecture in the membranes of the peptide aggregates, the knowledge of which remains imprecise, even 'at rest' in the best-studied cases. This review attempts to point out still unresolved questions and to propose some plausible approaches concerning, for example: 1) the configurations of the molecular aggregates responsible for ion transfer; 2) the mechanisms for channel-opening and closing (gating); 3) the eventual cooperative phenomena between channels, via the bilayer or interfacial components. Possible applications of these structures will be tentatively outlined.

Dielectric spectroscopy of L-alpha-lysolecithin-packaged gramicidin A

The complex permittivities of L-alpha-lysolecithin in the absence and presence of the gramicidin A ion channel were measured over the temperature range 0-60 degrees C and over the frequency range 1-1000 MHz. One dielectric relaxation/loss has been observed. It is located at 103.3 MHz (1.54 ns) for a micellar 0.4 M L-alpha-lysolecithin solution at 20 degrees C, whereas it is shifted to 71.7 MHz (2.22 ns) for a lamellar L-alpha-lysolecithin-gramicidin A aqueous solution (0.4 M L-alpha-lysolecithin, 0.0308 M gramicidin A) at 20 degrees C. The dielectric relaxation decreases and the relaxation time increases when gramicidin A is incorporated into L-alpha-lysolecithin. These dielectric changes are related, in part, to the micellar-to-lamellar lipid phase transition induced by the incorporation of gramicidin A into lysolecithin. We suggest that the diffuse rotational motion of the polar head group of L-alpha-lysolecithin contributes to the dielectric relaxation/loss at around 100 MHz.

A model for gramicidin A'-phospholipid interactions in bilayers

A model is proposed for the effect of gramicidin A' on the order and structure of phospholipid dispersions. According to this model, the addition of gramicidin A' influences the surrounding lipids via two independent mechanisms. The first arises from a drop in surface pressure for those lipids substantially bounded by gramicidin A'. The second mechanism arises from the increase in the phospholipid headgroup spacing due to the small polar region of the polypeptide. The model provides an explanation for the currently available NMR, X-ray diffraction and Langmuir monolayer results. The model also suggests mechanisms for the ability of gramicidin A' to trigger a transition of the lipid from the lamellar to hexagonal II phase, the dependence of this transition on the lipid chain length and the formation of a lamellar phase with lysophosphatidylcholine.