Rotational resonance NMR: separation of dipolar coupling and zero quantum relaxation

The solid state NMR technique of rotational resonance ( R 2 ) has been used extensively to measure distances approaching 5–6 A between 13 C nuclei in a variety of compounds including amyloidogenic peptides and membrane proteins. The accuracy of the distance information extracted from the time-dependent spin dynamics at R 2 is often limited by the accuracy with which the relevant zero-quantum lineshape parameters are estimated. Here we demonstrate that measurement of spinning frequency dependent magnetization exchange dynamics provides data from which both distance and zero-quantum relaxation parameters can be extracted independently. In addition to providing more accurate distance information, this technique allows examination of the zero-quantum lineshape, which can indicate the presence of correlated relaxation or chemical shift distributions between dipolar-coupled sites. With this approach we have separated the contribution of dipolar couplings and zero quantum relaxation to R 2 exchange curves. Thus, we have significantly improved the accuracy of the measurement of the intramolecular, internuclear distances between a pair of 13 C ’s in two model compounds ( N -acetyl- d , l -valine and glycylglycine·HCl) that lie in the distance range 4.6–4.7 A.