Complementary molecular shapes and additivity of the packing parameter of lipids.
Physical dimensions of a membrane component influence its phase preference upon hydration. A dimensionless packing parameter, S, given by S = V/al, where V is the hydrocarbon volume, a is the area of the head group, and l is the critical length of the hydrocarbon chain, is useful in determining the phase preference of a lipid, and the value of S usually lies between 0.5 and 1 for bilayers. Here, the value of S is calculated for phosphatidylcholine (PC) and lysophosphatidylcholine (lysoPC) as a function of chain length, and it is shown that diacylPC having an S value of less than 0.74 does not form bilayers. For example, diacylPC, up to a chain length of eight carbon atoms, forms only micelles, whereas higher homologs with S greater than 0.74 form bilayers. It is also shown that when lipid molecules having complementary shapes associate, the value of S becomes additive. Using the additivity of S, a number of experimental results for lipid mixtures can be explained. For example, lysoPC and cholesterol form lamellar structures between 45 and approximately 80 mol% cholesterol, and the additive value of S for this region is between 0.74 and 1. Similarly, the additivity of S shows that the maximum amount of cholesterol that can be incorporated into PC bilayers is 50 mol%, in agreement with experimental studies.
Kumar
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Hormel Institute, University of Minnesota, Austin, MN 55912.
D002784:Cholesterol / Q000737:chemistry; D008051:Lipid Bilayers; D008244:Lysophosphatidylcholines / Q000737:chemistry*; D008567:Membranes, Artificial; D008823:Micelles; D008954:Models, Biological; D008968:Molecular Conformation; D010713:Phosphatidylcholines / Q000737:chemistry*; D010714:Phosphatidylethanolamines / Q000737:chemistry; D013329:Structure-Activity Relationship
D008051:Lipid Bilayers; D008244:Lysophosphatidylcholines; D008567:Membranes, Artificial; D008823:Micelles; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D002784:Cholesterol