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 2I(x)S(x) and 2I(y)S(y), induced by the differential relaxation. The effect is illustrated with an application to (13)C,(15)N-labeled quail CRP2(LIM2), by studying (15)N-(1)H(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 (1)H(N) and (15)N exchange contributions to multiple-quantum relaxation observed at these sites illustrate the complex interplay between hydrogen bonding events and conformational reorientations in proteins.