The molecular composition of some lipid bilayer membranes in aqueous solution

Abstract

Summary. An analytical technique is described for direct determination of the molecular composition of lipid bilayer membranes in aqueous solution. Membranes formed from chemically pure, radioactively labeled components, were sampled by pipetting a mercury droplet through the bilayer-water interface. During this procedure, the membrane remains intact but decreases in area with a concomitant increase in the area of the surrounding bulk phase. It is shown that each mercury droplet is covered with a fragment of the bilayer membrane in the form of a closed vesicle. The chemical composition of the bilayer is determined from an analysis of the readioactivity on the mercury droplet.Bilayers generated from glyceryl monooleate in n-decane or n-hexadecane contain (4.7 -1-0.4) x 1014 molecules of monoglyceride per cm 2 and a minimum of (2.8; ___ 0.7) x 1014 molecules of solvent (n-hexadecane) per cm 2. It is estimated from these numbers that 37 vol % of the hydrocarbon core of the bilayer is occupied by solvent.The composition relationships between the bilayer and bulk membrane-forming solution were determined for mixtures of glyceryl monooleate (GMO) with cholesterol (Chol) or glyceryl monostearate (GMS). It was found that [GMS/GMO]bilayer-----[GMS/GMOlbulk, and [Chol/GMO]bilayer=0.5[Chol/GMO]bul k. While the molecular areas of glyceryl monooleate and glyceryl monostearate are unchanged in the mixed system, the average area for mixtures of cholesterol and glyceryl monooleate is decreased, suggesting a condensing effect of the sterol in the bilayer analogous to that observed in lipid monolayers.Extensive studies on phospholipid model systems in aqueous solution [16,19] and the recent use of physical techniques to probe the organization of natural membranes [11,18] strongly support the existence of a bimolecular lipid lamella as a mosaic element in many biological membranes [8]. Indeed, a number of important membrane-associated functions including facilitated diffusion [15], cation discrimination [21], and electrical excitability [12] have been observed in synthetic lipid bilayer membranes separating two aqueous phases either in unmodified systems or in membranes

Kinetics of the valinomycin-induced potassium ion leak from liposomes with potassium thiocyanate enclosed

“…Furthermore, the fluidity of the black lipid membrane may be significantly influenced by the presence of solvent, e.g. decane, in the film [17]. For the black lipid membrane system with membranes of phosphatidylinositol isolated from brain, and a mixture of soybean phospholipids, turnover constants of about 104 [11,13] and and 5 • 10 3 [15] ions/molecule valinomycin per s were obtained respectively at 25 '>C. On the other hand, we find for egg phosphatidylcholine liposomes at 30 ~'C a value of about 328 ions/molecule valinomycin per s (Table 1).…”

“…One of the drawbacks of the black lipid membrane system, especially in studying the effect of variation in the fatty acid composition on the valinomycin-mediated cation transport, is the presence of considerable amounts of solvent in the bilayer structure [17]. Another disadvantage is that such experiments require extended time periods, leading to the uncertainty of adsorption of valinomycin at the walls of the lest cells, as discussed in the accompanying paper [18].…”

Techniques in the Formation and Examination of “Black” Lipid Bilayer Membranes

“…If the amount of solvent estimated by Pagano et al (1972) is taken as an indication of the volume of trapped lenses, and a lens diameter of the order of 1 [Lm is assumed, the area reduction can be considerable. Thus a significant variation with time of the specific capacitance as the lenses coalesce and diffuse to the border would be expected.…”

Reconstitution of a Kv Channel into Lipid Membranes for Structural and Functional Studies

“…It is inevitable that there is residual decane left in the central bilayer portion of the membrane. Past estimation of the volume ratio of n-decane versus PC was 0.35-0.45 after the thinning of the bilayer 18,[20][21][22] . EM examination of the thinned bilayer showed that there were lenses of decane sandwiched between the two leaflets of a bilayer 22 .…”