R-1 rho relaxation outside of the fast-exchange limit

Chemical-exchange effects in NMR spectroscopy enable kinetic processes to be characterized at equilibrium in solution. Beginning with the Bloch-McConnell equations, new expressions are derived for the spin relaxation rate constant in the rotating frame, R-1rho for chemical exchange between two sites that have distinct magnetic environments and Larmor frequencies. The results are accurate provided that the spin relaxation decay is dominated by a single exponential damping constant and are applicable to a wider range of conditions than existing theoretical descriptions. The new expressions for R-1rho will be particularly useful in analyzing experimental data when exchange is not fast and site populations are unequal. (C) 2002 Elsevier Science.