Interdigitated structure of phospholipid-alcohol systems studied by x-ray diffraction

Abstract

In the interdigitated structure of phosphatidylcholine/alcohol systems, the one-dimensional electron density profile in the direction normal to the membrane surface is generated from the x-ray diffraction pattern. The membrane thickness for these systems is expressed by the sum of the hydrocarbon chain lengths of phosphatidylcholine and alcohol molecules. For this study, various sets of phosphatidylcholines and 1-alcohols were used; a phosphatidylcholine has a carbon number from 14 to 18 in a hydrocarbon chain, and an alcohol has a carbon number from 1 (methanol) to 4 (1-butanol). Based upon the results, we propose a model for the interdigitated structure in which 1) two alcohol molecules occupy a volume whose surface is surrounded interstitially by the headgroups of phosphatidylcholine molecules, and 2) the methyl ends of both hydrocarbon chains in alcohol and phosphatidylcholine molecules face each other at the bottom of the volume.

Mesoscopic Simulations of Biological Membranes

“…This distance of 0.1 nm is the length of one CH 2 -unit in the stretched chain of an alkane [84]. Furthermore, Adachi et al [62] show that two alcohol molecules can occupy a volume surrounded by the PC head groups of one layer. From this it follows that the number of alcohol molecules should be twice as high as the number of lipids in the bilayer independent of the length of the alcohol.…”

“…However, it is known experimentally that at low temperatures an interdigitated state, in which the terminal methyl groups of one monolayer interpenetrate the opposing layer, is also possible. This L βI phase does not spontaneously form in bilayers of symmetrical chain phospholipids, like dipalmitoylphosphatidylcholines (DPPC), [58] but can be induced by changes in the environment, like hydrostatic pressure or changes in the pH of the solution [59], or by incorporation, at the membrane interface of small amphiphilic molecules, like alcohols [60][61][62], or anesthetics [63]. Interdigitation can also be induced by changes in the lipid structure, for example by introducing an ester-linkage in the headgroup of the phospholipids [64,65].…”

“…The structure of the interdigitated phase as been studied in detail and based on the available experimental data Adachi et al [62] proposed the model shown in Fig. 12, in which the alcohol molecules fill the space and prevent the hydrophobic tails of the lipids to be exposed to water.…”

“…Adachi et al [62] proposed a model of the structure of the bilayer in which the terminal methyl group of the alcohol faces a terminal methyl group of a lipid chain (see Fig. 12).…”

“…12). The assumption is based on the experimental observation that the membrane thickness increases by about 0.08 nm per one methylene unit in either the alcohol molecules or the phospholipids [62,83]. This distance of 0.1 nm is the length of one CH 2 -unit in the stretched chain of an alkane [84].…”