Partial molecular volumes of lipids and cholesterol.

Volumetric measurements are reported for fully hydrated lipid/cholesterol bilayer mixtures using the neutral flotation method. Apparent specific volume data were obtained with the lipids DOPC, POPC and DMPC at T=30 degrees Celsius, DPPC at 50 degrees Celsius, and brain sphingomyelin (BSM) at 45 and 24 degrees Celsius for mole fractions of cholesterol x from 0 to 0.5. Unlike previous cholesterol mixture studies, we converted our raw data to partial molecular volume V(L) of the lipid and V(C) of the cholesterol. The partial molecular volumes were constant for POPC and DOPC as x was varied, but had sharp breaks for the other lipids at values of x(C) near 0.25+/-0.05. Results for x<x(C) clearly exhibit the condensation effect of cholesterol on DPPC, DMPC and BSM when measured at temperatures above their main transition temperatures T(M). The break points at x(C) are compared to phase diagrams in the literature. For x>x(C) the values of the partial molecular volumes of cholesterol clustered near 630+/-10A(3) in all the lipids when measured for T>T(M); we suggest that this is the most appropriate measure of the bare volume of cholesterol in lipid bilayers.