Intrinsic proteins and their effect upon lipid hydrocarbon chain order

We present evidence that at temperatures greater than their main transition temperature, phospholipid molecules that are trapped within clusters of intrinsic molecules such as polypeptides or proteins have the ends of their hydrocarbon chains more statically disordered than those of lipid molecules far from such intrinsic molecules. We have constructed a model in which the lipids are divided into three populations: (i) those that are not adjacent to any protein ("free" lipids), (ii) those that are adjacent to only one protein ("adjacent" lipids), and (iii) those that are "trapped" between two or three proteins. We applied this model to study deuterium nuclear magnetic resonance of dimyristoyl-3-sn-phosphatidylcholine (DMPC) bilayers containing gramicidin A' or cytochrome oxidase and found that while the methyl groups of adjacent lipids are slightly more statically ordered than those of free lipids, the methyl groups of trapped lipids are more statically disordered than those of free lipids. We propose a physical explanation for this and show that phosphorus-31 nuclear magnetic resonance data for DMPC-cytochrome oxidase bilayers can be understood as a consequence of changes in the polar region of only trapped lipids.