Abstract
We have used deuterium nuclear magnetic resonance (2H 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 (from 25 to 70°C) and cholesterol concentration (0 - 40%). The constructed phase diagram exhibits both solid-ordered (so) + liquid-ordered (lo) and liquid-disordered (ld) + lo phase coexistence regions with a clear three-phase line at 37.5°C between 8 and approx. 18 mol% cholesterol. The ld + 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 (M1), at a given temperature. The so + lo region was defined using spectral subtraction. Analogous experiments were done using 1-palmitoyl,2-palmitoyl(D31)-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 39.5°C and extends from approx. 8 to 20 mol% sterol. The PSM/cholesterol membrane is significantly more ordered than the DPPC/cholesterol membrane in the liquid crystalline phase. For example, at 60°C and 20 mol% cholesterol, the average order parameter of PSM is 34% higher than for DPPC. This should be compared to the difference between the cholesterol-free membranes at 60°C, where the average order of PSM is 23% more than that of DPPC. However the PSM/chol interaction is more effective than that of DPPC/chol at removing conformational fluctuations in the palmitoyl chain.
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