Chapter 3 - Measurement of Internuclear Distances in Biological Solids by Magic-Angle Spinning 13C NMR

Abstract

Elucidating the structure of macromolecules is of fundamental importance in a number of disciplines. X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy are two major techniques used for this purpose. The requirement for high-quality single crystals limits the general applicability of crystallography in biological systems. Liquid-state NMR is used to map the covalent structure of a wide variety of molecules, providing conformational information about proteins and other biological macromolecules. This chapter discusses the measurement of internuclear distances in biological solids using magic-angle spinning (MAS) 13C NMR. The accurate determination of internuclear distances can provide important structural information in a wide variety of solids. These distances are easily derived from the 1/r 3 distance dependence of the dipolar couplings between the spin pairs. Rapid MAS provides the line narrowing required to produce high-resolution solid-state NMR spectra and also averages these dipolar interactions to zero. This chapter reviews a number of techniques for recovering and measuring dipolar couplings under rapid MAS conditions. The popularity of the two most used techniques, REDOR (heteronuclear interactions) and R2 (homonuclear interactions), comes from their ease of implementation and proven ability to obtain accurate internuclear distances in complex molecular systems. The chapter concludes that development of solid-state NMR techniques for structure determination will remain an active area of research, and that NMR will continue to play a prominent role in the characterization of biological and nonbiological solids.