AnuIyricu ChhicaAcra, 283 (1!993)1081-1101 Elsevier Science Publishers B.V., Amsterdam 1081 Nuclear magnetic resonance methods for measuring dipolar couplings in rotating solids Janet M. Grifftths and Robert G. Griffin Francis Bitter National Magnet Laboratory and Lkpartment of Chemistry, Massachusetts Institute of Technology, Cambridge, M-4 02139 (USA) (Received 4th June 1993; revised manuscript received 20th June 1993) The nuclear dipole-dipole coupling is a strong function of internuclear distance, and can therefore be used as a sensitive probe of local molecular structure. In most cases, however, the simultaneous presence of anisotropic chemical shielding and multiple dipolar interactions prevents isolation of the dipole coupling between individual spin pairs. Under these circumstances, it is customary to improve spectral resolution by employing magic angle spinning (hIAS) which preserves the isotropic part of the chemical shift interaction but partially or fully averages the desired dipolar interactions. This article reviews NMR methods designed to selectively restore and measure dipolar couplings between like (homonuclear) spins and unlike (heteronuclear) spins in rotating solids. Applications of these methods - rotational resonance (R’), rotational-echo double resonance (REDOR), transferred-echo double resonance (TEDOR), dipolar recovery at the magic angle (DRAMA), simple excitation for the dephasing of rotational-echo amplitudes (SEDRA), and &driven dipolar recoupling (RFDR) - to investigate the structure of biomolecules are presented. &ywo&: Nuclear magnetic resonance spectrometry; Dipolar coupling; Rotating solids; Solids; Review; Solid state NMR Nuclear magnetic resonance (NMR) spec- troscopy is a well-established method for deter- mining the local structure and dynamics of a variety of amorphous and polycrystalline materi- als. In particular, the strong distance dependence of the nuclear magnetic dipole-dipole interaction makes this a valuable probe of local geometry, spatial connectivity, and order between molecules. Recently, two-dimensional (2D) solution-phase NMR experiments such as nuclear Overhauser effect spectroscopy (NOESY) have utilized indi- rect cross-relaxation effects to measure multiple ‘H-‘H internuclear distances, and hence, to de- termine the molecular structure of soluble macro- molecules [1,2]. Analogous experiments in solids which utilize the direct through-space dipolar couplings are also possible, but are complicated by the simultaneous presence of the anisotropic chemical shift and dipolar interactions. Under these circumstances, it is customary to improve the resolution of solid state spectra by employing magic angle spinning @iAS) and to restore the dipolar couplings to the spectra using selective rotor-driven or &driven magnetization exchange, or rf-driven dephasing experiments. The focus of this article is the expanding repertoire of NMR methods designed to recover dipolar couplings while maintaining conditions of high-resolution by MAS. These techniques can be separated into two general categories; techniques that measure dipolar couplings between “like” Correrpon&~ce to: R.G. Griffin, Francis Bitter National Mag- net Laboratory and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139 (USA). 0003-2670/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved