RECOUPLING OF HETERONUCLEAR DIPOLAR INTERACTIONS IN SOLID-STATE MAGIC-ANGLE-SPINNING NMR BY SIMULTANEOUS FREQUENCY AND AMPLITUDE-MODULATION

A new scheme is proposed for solid-state nuclear magnetic resonance (N MR) which uses simultaneous frequency and amplitude modulation (SFAM) to restore heteronuclear dipolar interactions between two unlike spin- 1/2 nuclei such as C-13 and N-15 under fast magic angle spinning (MAS) . The carrier frequency of one of the spins is modulated cosinusoidall y while its RF amplitude is modulated as a sine wave. It is shown by e xperiments and simulations that the scheme allows one to reintroduce t he heteronuclear dipolar interactions even at very high spinning speed s which are often required for suppression of chemical shift anisotrop y (CSA) in high magnetic fields. Unlike the well-known REDOR and TEDOR experiments, the SFAM method is not sensitive to effects of finite pu lse lengths. The advantages of the SFAM technique are illustrated for doubly labeled glycine ((NH2CH2COOH)-N-15-C-13). (C) 1997 Elsevier Sci ence B.V.