Differential multiple-quantum relaxation arising from cross-correlated time-modulation of isotropic chemical shifts.

In this paper it is demonstrated that cross-correlated time modulation of isotropic chemical shifts ('conformational exchange') leads to differential relaxation of double- and zero-quantum coherences, respectively. Quantitative information can be obtained from the time dependence of the interconversion between the two two-spin coherences 2IxSx and 2IySy, induced by the differential relaxation. The effect is illustrated with an application to 13C,15N-labeled quail CRP2(LIM2), by studying 15N-1H(N) multiple-quantum relaxation. Significant cross-correlated fluctuations of isotropic chemical shifts were observed for residues which are part of a disordered loop region connecting two beta-strands in CRP2(LIM2). Differential 1H(N) and 15N exchange contributions to multiple-quantum relaxation observed at these sites illustrate the complex interplay between hydrogen bonding events and conformational reorientations in proteins.