¹³C-NMR determination of the molecular dynamics of cholesterol in dimyristoylphosphatidylcholine vesicles

Using ¹³C-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°C. Using the model-free approach of Lipari and Szabo, we have found that at 37°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 ¹³C-NMR experiments are shown to be compatible with those from ²H-NMR experiments. This establishes the validity of the present approach, which we are currently extending to low concentrations of cholesteryl oleate in DMPC vesicles.