CHAIN-MELTING TRANSITION-TEMPERATURES OF PHOSPHOLIPIDS WITH ACYLATED OR ALKYLATED HEADGROUPS (N-ACYL PHOSPHATIDYLETHANOLAMINES AND O-ALKYL PHOSPHATIDIC ACIDS), OR WITH ALPHA-BRANCHED CHAINS

The biphasic dependence of the chain-melting transition temperature on chainlength, n(N), of the headgroup-attached chain of N-acyl phosphat idylethanolamines and of O-alkyl phosphatidic acids is interpreted in terms of a different linear dependence of the transition enthalpy and entropy on n(N) for long and short chains, respectively. A consistent expression of the form T-t(n(N)) = T-N [\n(N)- Delta n(N)\ + Delta h'( n(0)(N)-n(N))]/[\n(N)-Delta n(N)\ + Delta s'(n(o)'(N)-n(N))], where De lta n(N) is the critical chainlength at which the packing mode of the headgroup-attached chains changes, is found to apply systematically to the transition temperatures for both sets of lipids, over the range n (N)=2-18. This thermodynamic analysis demonstrates that the headgroup- attached chain is located in an environment that differs for long and short chains. Similar considerations apply also to phosphatidylcholine s with alpha-branched glycerol-attached chains. (C) 1998 Elsevier Scie nce B.V. All rights reserved.