The Physical Properties of Sphingomyelin/Cholesterol Membranes: a Deuterium NMR Study

We have used $^{2}$H NMR to study the effect of cholesterol on N-palmitoyl(D31)-D-erythro-sphingosylphosphorylcholine (PSM) membranes. NMR spectra were taken as a function of temperature and cholesterol concentration. The constructed phase diagram exhibits both solid-ordered (\textbf{so}) + liquid-ordered (\textbf{lo}) and liquid-disordered (\textbf{ld}) + \textbf{lo} phase coexistence regions with a clear three-phase line at 37$^{\circ}$C. The \textbf{ld} + \textbf{lo} region was characterized by examining the cholesterol dependence of the width of resolved peaks in the depaked spectra, as well as the average spectral width (M$_{1})$, at a given temperature. The \textbf{so} + \textbf{lo} region was defined using spectral subtraction. Analogous experiments were done using 1-palmitoyl,2-palmitoyl(D31)-\textit{sn}-glycero-3-phosphocholine (DPPC)/cholesterol membranes in order to carefully compare the data obtained using palmitoyl chains which have similar ``kinked'' conformations. The three-phase line in sn-2 perdeuterated DPPC/cholesterol is at 39$^{\circ}$C. The PSM/cholesterol membrane is significantly more ordered than the DPPC/cholesterol membrane in the liquid crystalline phase. This should be compared to the difference between the cholesterol-free membranes at, where the average order of PSM is more than that of DPPC.