Calculation of Partition Coefficients of Chain Anchors in Liquid-Ordered and Liquid-Disordered Phases in Model Lipid Bilayers

Abstract

We calculate partition coefficients of various chain anchors in liquid-ordered and liquid-disordered phases utilizing a theoretical model of a bilayer membrane containing cholesterol, dipalmitoylphophatidylcholine (DPPC), and dioleoylphosphatidylcholine (DOPC). The model qualitatively reproduces experimentally observed phase diagrams of this ternary system [R. Elliott, I. Szleifer, M. Schick, Phys. Rev. Letters, 96, 098101,2006]. The partition coefficients are calculated as a function of chain length, degree of saturation, and temperature. Perhaps our most important, model-independent, observation is that the partition coefficients must depend upon the relative compositions of the two liquid phases which coexist. For given phases in coexistence, we find that saturated anchors prefer the denser liquid-ordered phase and that their partition coefficients generally increase with the length of the anchor. Unsaturated chains and other bulky anchors prefer the less dense liquid-disordered phase. The fraction of anchors in the liquid-ordered phase decreases with decreasing degree of saturation of the anchors. For a given number of double bonds, the partition coefficient depends upon their location, with those near the chain ends causing a smaller decrease in the fraction of anchors in the liquid-ordered phase than double bonds closer to the middle of the anchor. The effect of doubling the number of chains in an anchor is to increase the partitioning into the liquid-ordered phase when the tails are nearly as long or longer than those comprising the bilayer, but is minimal when they are relatively short. A reduction of temperature also increases the partition coefficient of long chains, but again has little effect on shorter ones.