Effects of paramagnetic shift reagents on the 13C nuclear magnetic resonance spectra of egg phosphatidylcholine enriched with 13C in the N-methyl carbons.

Effects of paramagnetic shift reagents on the 13C NMR spectra obtained from single-walled vesicle dispersions of egg phosphatidylcholine enriched with 13C in the N-methyl carbons are investigated. Spectra obtained at 25.1 MHz show that, at Yb3+ to phospholipid molar ratios as low as 0.06, complete resolution of the N-methyl carbon resonances is obtained from molecules on the inner and outer faces of the vesicle bilayer. No precipitation of the vesicles is caused by Yb3+ at these concentrations nor is appreciable line broadening observed. Other paramagnetic shift reagents frequently used in proton NMR investigations of phosphatidylcholine vesicles do not give complete separation of the N-methyl 13C signals from the two bilayer surfaces. K3Fe(CN)b,Eu3+, and Pr3+ cause precipitation of the phosphatidylcholine vesicles at concentrations, which give only incomplete resolution of these signals. T1 measurements of the resonances separated by Yb3+ indicate that the choline groups on the inner bilayer surface are less mobile than are the same groups in the outer surface. Gated proton decoupling measurements, which show that the nuclear Overhauser effect is 2.8 +/- 0.1, indicate that the dominant mode of relaxation is dipolar interaction.