Effects of alcohols on the phase transition temperatures of mixed-chain phosphatidylcholines

The biphasic effect of ethanol on the main phase transition temperature (Tm) of identical-chain phosphatidyl-cholines (PCs) in excess H2O is now well known. This biphasic effect can be attributed to the transformation of the lipid bilayer, induced by high concentrations of ethanol, from the partially interdigitated L beta, phase to the fully interdigitated L beta I phase at T < Tm. The basic packing unit of the L beta I phase has been identified recently as a binary mixture of PC/ethanol at the molar ratio of 1:2. The ethanol effect on mixed-chain PCs, however, is not known. We have thus in this study investigated the alcohol effects on the Tm of mixed-chain PCs with different delta C values, where delta C is the effective acyl chain length difference between the sn-1 and sn-2 acyl chains. Initially, molecular mechanics (MM) simulations are employed to calculate the steric energies associated with a homologous series of mixed-chain PCs packed in the partially and the fully interdigitated L beta I motifs. Based on the energetics, the preference of each mixed-chain PC for packing between these two different motifs can be estimated. Guided by MM results, high-resolution differential scanning calorimetry is subsequently employed to determine the Tm values for aqueous lipid dispersions prepared individually from a series of mixed-chain PCs (delta C = 0.5-6.5 C-C bond lengths) in the presence of various concentrations of ethanol. Results indicate that aqueous dispersions prepared from mixed-chain PCs with a delta C value of less than 4 exhibit a biphasic profile in the plot of Tm versus ethanol concentration. In contrast, highly asymmetric PCs (delta C > 4) do not exhibit such biphasic behavior. In the presence of a longer chain n-alcohol, however, aqueous dispersions of highly asymmetric C(12):C(20)PC (delta C = 6.5) do show such biphasic behavior against ethanol. Our results suggest that the delta C region in a highly asymmetric PC packed in the L beta I phase is most likely the binding site for n-alcohol.