Spin-state-selective excitation in gradient-selected heteronuclear cross-polarization NMR experiments.

Several heteronuclear coherence transfer mechanisms involved in proton-detected heteronuclear J-cross-polarization (HCP) NMR experiments have been theoretically derived and experimentally verified in isotropic solution. It is shown that in-phase and/or anti-phase heteronuclear coherence transfer can take place separately or simultaneously during the HCP process as a function of the relative phase between the HCP mixing sequence and the corresponding input magnetization. As the more important consequence, clean coherence-order and spin-state selective (S3) excitation with maximum sensitivity can be achieved from gradient-enhanced HCP experiments by proper co-addition/subtraction of in-phase and anti-phase magnetizations, offering an attractive alternative to widely used HSQC-type experiments.