Effects of headgroup methylation and acyl chain length on the volume of melting of phosphatidylethanolamines.

The change in volume associated with the gel to liquid-crystalline phase transition for phosphatidylethanolamines of various chain lengths and headgroup methylation was determined by measuring the pressure dependence of the phase transition temperature and computing the volume change by using the Clausius-Clapyron equation. The volumes thus obtained were comparable to those previously obtained by using scanning dilatometry. The melting volume was larger for lipids with longer acyl chains, as found previously. The melting volume for a series of N-methylated dipalmitoylphosphatidylethanolamines (DPPEs) did not increase monotonically with increasing headgroup methylation. Instead, the melting volume increased in the order N,N-dimethyl-DPPE less than N-methyl-DPPE less than DPPE less than dipalmitoylphosphatidylcholine. This unanticipated result is hypothesized to result from the competing effects of headgroup methylation on molecular volume and hydrogen bonding on the volume of melting.