New aspects of the interaction of cholesterol with dipalmitoylphosphatidylcholine bilayers as revealed by high-sensitivity differential scanning calorimetry

Abstract

We have investigated the effects of cholesterol on the thermotropic phase behavior of annealed and unannealed aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) using high-sensitivity differential scanning 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 temperature, such that the subtransition is abolished by 20 mol% cholesterol. Similarly, the incorporation of cholesterol progressively decreases 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 mol% cholesterol, the DSC endotherm arising from the main transition consists of superimposed sharp and broad components, the former due to the melting of cholesterol-poor and the latter to the melting of the cholesterol-rich DPPC domains. The temperature and cooperativity of the sharp component decreases slightly with increasing cholesterol concentration whereas the enthalpy decreases markedly, becoming zero at 20-25 mol% cholesterol. In contrast, the temperature and enthalpy of the broad 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 the 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 2H-NMR data (see Vist, M.R. and Davis, J.H. (1990) Biochemistry 29, 451-464). In addition, we examined the effect of cholesterol on phosphatidylcholines (PCs) of different chain lengths and confirm that a eutectic point does not exist for any of these PC/cholesterol mixtures. We then propose a new, more complete DPPC/cholesterol phase diagram based on our high-sensitivity DSC data as well as some recent spectroscopic data on PC/cholesterol mixtures and explore some of its biological implications.

Phase separation is induced by phenothiazine derivatives in phospholipid/sphingomyelin/cholesterol mixtures containing low levels of cholesterol and sphingomyelin

“…As a matter of fact also other authors, using low scanning rates have not recorded two components for cholesterol/DPPC mixtures [18]. According to the phase diagram proposed in [16] we may assume that…”

“…The existence of only one component in our calorimetric results presumably was a caused by the fact that in our samples apart from cholesterol also sphingomyelin was present. In our experiments we used scanning rate higher than used in [16], what also can be a reason for appearance of only one component in thermograms. As a matter of fact also other authors, using low scanning rates have not recorded two components for cholesterol/DPPC mixtures [18].…”

“…According to the thorough description of the influence of cholesterol on phase properties of DPPC presented by McMullen and McElhaney [16], in mixtures containing more than 5 mol% of this sterol the pretransition is completely abolished. Pretransition was absent also in the thermograms of DPPC/Chol/SM mixtures studied in the present work.…”

Application of the DSC and spectroscopy methods in the analysis of the protective effect of extracts from the blueberry fruit of the genus Vaccinium in relation to the lipid membrane

“…Changes of the main phase transition temperature (T m ) caused by the presence of additional compounds in the membrane are associated with changes in fluidity of the lipid bilayer and changes of the membrane structure. Pretransition temperature (T p ) changes provide evidence for changes rather in the polar part of the membrane that result in loosening its structure [37,41,42]. Calorimetric studies were carried out with the membrane model in the form of MLVs formed of DPPC modified with the extracts and polyphenolic compounds [22,[36][37][38][39].…”

Lipid and DNA interaction with the triorganotin dimethylaminophenylazobenzoates studied by DSC and spectroscopy methods

“…Incorporation of increasing levels of cholesterol broadens and eventually eliminates altogether the cooperative gel/liquid phase transition of the lipid bilayer. When the amount of cholesterol in the membrane formed from phosphatidylcholine ranges from 5 to 20 mol%, two phases with different cholesterol contents are formed [34][35][36][37][38]. In our experiments, two different concentrations of cholesterol were used: 5 mol% and 10 mol%, for which sharp main transition occurs (Figs.…”