C-13-NMR DETERMINATION OF THE SOLUBILITY AND MOLECULAR-DYNAMICS OF CHOLESTERYL OLEATE IN DIMYRISTOYLPHOSPHATIDYLCHOLINE VESICLES

Using C-13-NMR measurements of T-1, T-2 and the nuclear Overhauser enh ancement factor at 50.29, 90.55 and 150.87 MHz, we have measured the d ynamics of 1.5 mol% cholesteryl oleate in dimyristoylphosphatidylcholi ne (DMPC) small unilamellar vesicles (SUVs) at 37 degrees C. Using the model-free approach of Lipari and Szabo [J. Am. Chem. Sec. 104: 4546- 4559, 1982], we have found that the motion of the rigid steroid ring c an be described by an equal contribution from two effective motions wi th correlation times of 15 and 0.38 ns. The motions of the C-26 and C- 27 carbon atoms of cholesteryl oleate were found to have an effective correlation time of 7 +/- 2 ps and a value for the square of the gener alised order parameter of 0.03 +/- 0.01. The corresponding values for the C-25 carbon atom were 14 +/- 9 ps and 0.17 +/- 0.05, showing slowe r motion and greater order for this carbon atom, which is nearer to th e rigid steroid ring. This establishes that the motion of the steroid ring of cholesteryl oleate in DMPC vesicles is significantly different to that previously measured for cholesterol in DMPC vesicles. On the other hand, the effective motion of the portions of the corresponding alkyl chains about the steroid ring is the same. On preparing vesicles , when the ester was at the limit of miscibility with the DMPC, the so lubility of cholesteryl oleate in DMPC vesicles at 55 degrees C was fo und to be 2.8 +/- 0.3 mol%. Using aqueous Mn2+ as a paramagnetic relax ation enhancement agent, comparison of the appropriate T-1 values indi cated that the carbonyl group of the cholesteryl ester was the same di stance from the aqueous phase as the C-4 carbon of the steroid ring. C opyright (C) 1996 Elsevier Science Ireland Ltd