Dilatometric studies of isobranched phosphatidylcholines.

Absolute apparent specific volumes have been obtained for phosphatidylcholine lipids with saturated, isobranched hydrocarbon chains with ni = 15 to 20 carbons, with an emphasis upon phase transition behavior, both equilibrium and kinetic. The temperature of the chain-melting transition extrapolates with increasing chain length to the melting temperature of polyethylene with a small odd/even alternation. There are also odd/even alternations in the volume of transition and in the hysteresis of the chain-melting transition, but with the odd and even reversed when compared with the larger odd/even alternation in the lower solid-solid transition that occurs in the longer chain ni lipids. A phenomenological picture is given for the coalescence of the two transitions for shorter ni lipids and this picture is used to sharpen the discussion of the kinetic mechanism of melting. A temperature-reversal experiment shows that the melting from the lowest temperature crystal or C phase to the fluid F phase does not proceed via the metastable gel G phase for 16i. The dilatometric results are combined with recent X-ray structural results for the C and G phases of 17i and 20i to deduce various structural information, including the hydration numbers and the volume of the headgroup, VH = 341 A3, which agrees very well with VH for straight-chain phosphatidylcholines. For the chain-melted F phase the assumption that the methylene volumes of the different ni lipids should be the same at the same temperature is used to obtain the volumes of the methylene and the methyl groups.