Solid-state NMR heteronuclear coherence transfer using phase and amplitude modulated rf irradiation at the Hartmann–Hahn sideband conditions

An improved variant of the popular double cross-polarization (DCP) experiment for heteronuclear dipolar recoupling in solid-state NMR spectroscopy under magic-angle-spinning is introduced. By simple phase and amplitude modulation of the rf irradiation at the Hartman-Hahn sideband conditions, the new pulse sequence, dubbed iDCP, enables broadband excitation with the high efficiency of γ-encoded coherence transfer. The efficiency and robustness of iDCP toward isotropic chemical shift variations and chemical shift anisotropies, in the order typically applying for the backbone atoms in uniformly 13C,15N-labeled proteins, is demonstrated numerically and experimentally by 15N to 13C coherence transfer for 15N-labeled N-Ac-l-valyl-l-leucine and 13C,15N-labeled-l-threonine.