One of the most interesting models of biological membranes posits that they are not compositionally homogeneous, but have aggregates rich in cholesterol and saturated lipids that float, like rafts, in a sea of unsaturated lipids [1]. A disagreement centers on if the rafts and the sea are in fact domains of two coexisting phases denoted by liquid-order and liquid-disorder [2]. The liquid-order phase is rich in cholesterol and saturated lipids whose chains are relatively well ordered; the other, liquid-disorder phase, is rich in unsaturated lipids whose chains are not so well ordered. One reason for the interest in this picture is that an inhomogeneous membrane would provide a substrate within which different proteins would be enriched either in rafts or in the sea. Presumably saturated anchors prefer the environment of the raft, or liquid-order phase, while unsaturated and/or otherwise bulky anchors prefer the environment of the sea of unsaturated lipids, the liquid-disorder phase. In order to test this hypothesis on protein anchors, we use a theoretical model membrane that is able to produce coexisting liquid phases [3].
From our theoretical model-membrane ternary system of cholesterol, a lipid with two saturated tails of 16 carbons, such as dipalmitoylphophatidylcholine (DPPC), and a lipid with two mono-unsaturated tails of 18 carbons, such as dioleoylphosphatidylcholine (DOPC) [3], we report calculated partition coefficients for single chains of various lengths and saturation and a few cases of double-chained molecules. The results show that partition coefficients increase with chain length and degree of saturation.
References
[1] O. G. Mouritsen, Life ? As a Matter of Fat (Springer-Verlag, Berlin, 2005).
[2] T. McMullen and R. McElhaney, Biochim. Biophys. Acta 1234, 90 (1995).
[3] R. Elliott, M. Schick, and I. Szleifer, Phys. Rev. Letters. 96, 098101 (2006).
