Calculating Partition Coefficients of Chain Anchors in Liquid-Ordered and Liquid-Disordered Phases

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, dipalmitoyl phosphatidylcholine, and dioleoylphosphatidylcholine. The partition coefficients are calculated as a function of chain length, degree of saturation, and temperature. Partitioning depends on the difference between the lipid environments of the coexisting phases in which the anchors are embedded. Consequently, the partition coefficient depends on the nature of the anchor, and on the relative compositions of the coexisting phases. We find that saturated anchors prefer the denser liquid-ordered phase, and that the fraction of anchors in the liquid-ordered phase increases with increasing degree of saturation of the anchors. The partition coefficient also depends upon the location of the double bonds. Anchors with double bonds closer to the middle of the chain have a greater effect on partitioning than those near the end. Doubling the number of saturated chains increases the partitioning into the liquid-ordered phase for tails that are nearly as long or longer than those comprising the bilayer. Partitioning of such chains increases with decreasing temperature, indicating that energy considerations dominate entropic ones. In contrast, partitioning of shorter chains increases with increasing temperature, indicating that entropic considerations dominate.

Formation and Domain Partitioning of H-ras Peptide Nanoclusters: Effects of Peptide Concentration and Lipid Composition

“…Aiming at filling this gap, recent theoretical studies have begun to shed light on the thermodynamic basis for domain-specific partitioning of individual lipid-modified peptides and hybrid lipids 29-37 . For instance, Uline et al 29 used a mean-field approach to calculate the L o /L d partition coefficients of several lipid chain anchors. They found that the chain length, degree of saturation and architecture of the anchor, as well as the composition of the membrane, modulate the domain preference of the lipid anchors.…”

Mode specific elastic constants for the gel, liquid-ordered, and liquid-disordered phases of DPPC/DOPC/cholesterol model lipid bilayers

“…It has been shown both experimentally and theoretically that curvature can control the lateral sorting of membrane components 35,36 . There is also evidence that the separation into different phases, which are characterized by different partition coefficients, 29,30,37–39 can also be controlled by lateral tension and curvature. To couple these two effects is an ongoing effort.…”

“…We have recently published work on calculating the partition coefficients of lipid chain anchors of various degrees of unsaturation and number of tails in lo-ld phase separated lipid bilayer mixtures 29 . Here we use a similar methodology to determine how the binding of chain anchors to bilayers with a degree of curvature.…”

“…A greater detailed description of the molecular theory used in this work can be found in Uline et al 29 .…”

“…The conditions for phase equilibrium are that the temperature, surface tension, and the chemical potential of each of the molecular species be the same in both phases. Note that the surface tension of the equilibrium symmetric planer bilayer system is zero 27–29 . This determines that J ll = 3.1 × 10 −3 k B T* ( T* = 315K) for our calculations.…”