(482f) Calculation of Partition Coefficients of Chain Anchors in Liquid-Ordered and Liquid-Disordered Phases

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).