1H-13C INEPT MAS NMR correlation experiments with 1H-1H mediated magnetization exchange to probe organization in lipid biomembranes

Two-dimensional 1H-13C INEPT MAS NMR experiments utilizing a 1H-1H magnetization exchange mixing period are presented for characterization of lipid systems. The introduction of the exchange period allows for structural information to be obtained via 1H-1H dipolar couplings but with 13C chemical shift resolution. It is shown that utilizing a RFDR recoupling sequence with short mixing times in place of the more standard NOE cross-relaxation for magnetization exchange during the mixing period allowed for the identification and separation of close 1H-1H dipolar contacts versus longer-range inter-molecular 1H-1H dipolar cross-relaxation. These 2D INEPT experiments were used to address both intra- and inter-molecular contacts in lipid and lipid/cholesterol mixtures.