Lipid Bilayers In Presence Research Articles

Glycopeptides Related to β-Endorphin Adopt Helical Amphipathic Conformations in the Presence of Lipid Bilayers

A series of glycosylated endorphin analogues designed to penetrate the blood-brain barrier (BBB) have been studied by circular dichroism and by 2D-NMR in the presence of water; TFE/water; SDS micelles; and in the presence of both neutral and anionic bicelles. In water, the glycopeptides showed only nascent helix behavior and random coil conformations. Chemical shift indices and nuclear Overhauser effects (NOE) confirmed helices in the presence of membrane mimics. NOE volumes provided distance constraints for molecular dynamics calculations used to provide detailed backbone conformations. In all cases, the glycopeptides were largely helical in the presence of membrane bilayer models (micelles or bicelles). Plasmon waveguide resonance (PWR) studies showed hen egg phosphatidyl choline (PC) bilayers produce amphipathic helices laying parallel to the membrane surface, with dissociation constants (K(D)) in the low nanomolar to micromolar concentration range. Two low-energy states are suggested for the glycosylated endorphin analogues, a flexible aqueous state and a restricted membrane bound state. Strong interactions between the glycopeptide amphipaths and membranes are crucial for penetration of the BBB via an endocytotic mechanism (transcytosis).

Blocking effects of the anaesthetic etomidate on human brain sodium channels

Sodium channels from human brain tissue were incorporated into voltage-clamped planar lipid bilayers in presence of batrachotoxin and exposed to increasing concentrations of the intravenous anaesthetic drug etomidate (0.03–1.02 mM). Etomidate interacted with the sodium-conducting pathway of the channel causing a concentration-dependent block of the time-averaged sodium conductance (computer fit of the concentration–response curve: half-maximal blocking concentration, EC 50, 0.19 mM; maximal block, block max, 38%). This block of sodium-conductance resulted from two distinct effects (I) major effect: reduction of the sodium-channel amplitude and (II) minor effect: reduction of the fractional channel open-time. These results were observed at concentrations above clinically-relevant serum concentrations (up to 0.01 mM), suggesting only a limited role for human brain sodium channels in the mechanism of action of etomidate during clinical anaesthesia.

Merocyanine interaction with phosphatidylcholine bilayers

Merocyanine (MC 540) is a fluorescent probe whose optical properties depend on the environmental polarity. In the presence of lipid bilayers, MC 540 binds to the membrane surface while simultaneously changing its fluorescence properties. Previous studies have shown that the fluorescence of merocyanine depends upon the lipid packing in the membrane. We measured the partitioning of MC 540 and its fluorescence properties in the presence of phosphatidylcholine membranes. We found that the fluorescence of MC 540 shows, as expected, a major change around the main phase transition of phosphatidylcholine membranes. However, instead of a step-like increase of fluorescence, the maximum at phase transition was observed. We were able to explain our data by combining two effects: dependence of MC 540 fluorescence on temperature and lipid fluidity. In addition, we established that the increase of the fluorescence intensity in the presence of lipid bilayers in the fluid state is due to the elevated partitioning of the probe into the lipid phase. The partition of MC 540 into the fluid membrane does not depends on the dye concentration in the aqueous phase. When lipid was in the gel phase the partitioning of the dye increased with its bulk concentration, whereas the fluorescence intensity remained unchanged. We conclude, therefore, that MC 540 forms nonfluorescent complexes when in the gel lipid membrane.

Ion channels formed by amphipathic helical peptides

Channel forming peptides (CFPs) are amphipathic peptides, of length ca. 20 residues, which adopt an alpha-helical conformation in the presence of lipid bilayers and form ion channels with electrophysiological properties comparable to those of ion channel proteins. We have modelled CFP channels as bundles of parallel trans-bilayer helices surrounding a central ion-permeable pore. Ion-channel interactions have been explored via accessible surface area calculations, and via evaluation of changes in van der Waals and electrostatic energies as a K+ ion is translated along the length of the pore. Two CFPs have been modelled: (a) zervamicin-A1-16, a synthetic apolar peptaibol related to alamethicin, and (b) delta-toxin from Staphylococcus aureus. Both of these CFPs have previously been shown to form ion channels in planar lipid bilayers, and have been shown to have predominantly helical conformations. Zervamicin-A1-16 channels were modelled as bundles of 4 to 8 parallel helices. Two related helix bundle geometries were explored. K(+)-channel interactions have been shown to involve exposed backbone carbonyl oxygen atoms. delta-Toxin channels were modelled as bundles of 6 parallel helices. Residues Q3, D11 and D18 generate favourable K(+)-channel interactions. Rotation of W15 about its C beta-C gamma bond has been shown to be capable of occluding the central pore, and is discussed as a possible model for sidechain conformational changes in relation to ion channel gating.

Coenzyme model reaction in lipid bilayers

Coenzyme model reactions, such as the H − (H + + 2e −) transfer from NADH models to triphenyl methane dyes, were investigated in the presence of lipid bilayers, for example, l-α-dimyristoyl phosphatidyl choline and egg yolk lecithin. In the temperature dependence of the acceleration effect by the lipid bilayer, discontinuous points were observed, corresponding to the phase transition point such as gel-liquid crystal ( T c) or the segregation point ( T s). The T c and T s values of the bilayers varied with the reactant as a result of the difference of perturbing effect on the structure of the bilayers. The pressure effect on the transition point was also studied. Transition points such as T c or T s became higher with increasing pressure, and dT c dP or dT s dP was different for various bilayers. In the gel phase of the membrane, stereospecific reduction of malachite green was observed by chiral nicotinamide: the difference in the catalytic effect on the reduction rate between ( R)- and ( S)-dihydronicotinamides was larger in the gel phase than that in the liquid crystal phase or in the phase separated state, which suggests that the gel-state molecule can recognize the molecular structure better than the liquid-crystal state molecule.

Permeability of lipid bilayers to methylmercuric chloride: Quantification by fluorescence quenching of a carbazole-labeled phospholipid

We investigated the permeabilities of lipid bilayers to the neurotoxin methylmercuric chloride (MMC). This mercurial is an efficient collisional quencher of the fluorescence of N-alkyl carbazole derivatives. Quenching of the fluorescence of β-(3-(9-carbazole)-propionyl-L-α-phosphatidylcholine (CPA-PC) in vesicles of dimyristoyl phosphatidylcholine and of dioleoyl phosphatidylcholine reveal rapid diffusion of MMC in the alkyl side chain regions of these bilayers. By a combination of (1) the lipid concentration dependence of the apparent quenching constants, (2) the solubility of MMC in concentrated lipid dispersions and (3) the 270 MHz proton magnetic resonance of methylmercury in the presence of lipid bilayers we conclude that the lipid-water partition coefficient of this mercurial is less than or equal to two. Using the fluorescence quenching and the partitioning data we estimate the diffusion coefficient of MMC in these bilayers to range from 0.13 to 0.31 × 10 −5 cm 2/sec, or 20–47% of its diffusion coefficient in ethanol. These data indicate that lipid bilayers do not pose a significant permeability barrier to the diffusional transport of MMC.