Differential multiple quantum relaxation caused by chemical exchange outside the fast exchange limit.

Differential relaxation of multiple quantum coherences is a signature for chemical exchange processes in proteins. Previous analyses of experimental data have used theoretical descriptions applicable only in the limit of fast exchange. Theoretical expressions for differential relaxation rate constants that are accurate outside fast exchange are presented for two-spin-system subject to two-site chemical exchange. The theoretical expressions are validated using experimental results for (15)N-(1)H relaxation in basic pancreatic trypsin inhibitor. The new theoretical expression is valuable for identification and characterization of exchange processes in proteins using differential relaxation of multiple quantum coherences.