NEW ASPECTS OF THE INTERACTION OF CHOLESTEROL WITH DIPALMITOYLPHOSPHATIDYLCHOLINE BILAYERS AS REVEALED BY HIGH-SENSITIVITY DIFFERENTIAL SCANNING CALORIMETRY

We have investigated the effects of cholesterol on the thermotropic ph ase behavior of annealed and unannealed aqueous dispersions of dipalmi toylphosphatidylcholine (DPPC) using high-sensitivity differential sca nning calorimetry (DSC), concentrating particularly on the cholesterol concentration range from 0 to 20 mol%. We find that the incorporation of cholesterol into low-temperature annealed DPPC bilayers decreases the enthalpy of the subtransition without affecting the transition tem perature, such that the subtransition is abolished by 20 mol% choleste rol. Similarly, the incorporation of cholesterol progressively decreas es the temperature and enthalpy of the pretransition and abolishes it entirely at cholesterol concentrations above 5 mol%. The incorporation of increasing quantities of cholesterol also alters the main or chain -melting phase transition. At cholesterol concentrations of 2 to 20 mo l% cholesterol, the DSC endotherm arising from the main transition con sists of superimposed sharp and broad components, the former due to th e melting of cholesterol-poor and the latter to the melting of the cho lesterol-rich DPPC domains. The temperature and cooperativity of the s harp component decreases slightly with increasing cholesterol concentr ation whereas the enthalpy decreases markedly, becoming zero at 20-25 mol% cholesterol. In contrast, the temperature and enthalpy of the bro ad component increases, and the cooperativity decreases markedly over this same range of cholesterol concentrations. An apparent increase in cooperativity of the overall DPPC endotherm at 7 mol% cholesterol is shown to arise because of a convergence in the transition temperatures of the sharp and broad components of the DSC endotherms. Some of our experimental findings, particularly the absence of any evidence for th e existence of a triple point near 7.5 mol% cholesterol, do not accord with a recently proposed DPPC/cholesterol phase diagram derived from DSC and H-2-NMR data (see Vist, M.R. and Davis, J.H. (1990) Biochemist ry 29, 451-464). In addition, we examined the effect of cholesterol on phosphatidylcholines (PCs) of different chain lengths and confirm tha t a eutectic point does not exist for any of these PC/cholesterol mixt ures. We then propose a new, more complete DPPC/cholesterol phase diag ram based on our high-sensitivity DSC data as well as some recent spec troscopic data on PC/cholesterol mixtures and explore some of its biol ogical implications.