C-13-NMR DETERMINATION OF THE MOLECULAR-DYNAMICS OF CHOLESTEROL IN DIMYRISTOYLPHOSPHATIDYLCHOLINE VESICLES

Using C-13-NMR measurements of T1, T2 and the nuclear Overhauser enhancement factor at 50.32, 90.56 and 150.87 MHz, we have measured the dynamics of cholesterol in dimyristoylphosphatidylcholine (DMPC) vesicles from 28 to 50-degrees-C. Using the model-free approach of Lipari and Szabo, we have found that at 37-degrees-C the motion of the rigid steroid ring can be described by an equal contribution from two effective motions with correlation times of 63 and 0.85 ns. The C26 and C27 Carbon atoms of cholesterol were found to have an effective correlation time of 8 +/- 2 ps and a value for the square of the generalised order parameter of 0.03 +/- 0.01. The corresponding values for the C25 carbon atom were 17 +/- 4 ps and 0.09 +/- 0.02, showing slower motion and greater order for this carbon atom, which is nearer to the rigid steroid ring. Apart from the effect of vesicle size on T2, no concentration dependence of the dynamics of cholesterol was detected over the cholesterol concentration range 2-30 mol%. The order parameters and correlation times from the present C-13-NMR experiments are shown to be compatible with those from H-2-NMR experiments. This establishes the validity of the present approach, which we are currently extending to low concentrations of cholesteryl oleate in DMPC vesicles.