NMR RELAXATION AND PROTEIN MOBILITY

The increased availability of isotope-enriched proteins and the high sensitivity of proton-detected heteronuclear experiments have stimulated studies of protein mobility via N-15 and C-13 relaxation. Developments during the past few years include new pulse sequences that yield more reliable values for relaxation rates, and pulse sequences that enable the measurement of new types of relaxation parameters. Methods with which to map distribution functions of motional frequencies (spectral density functions) from combinations of relaxation parameters have been suggested. Extensive measurements of N-15 and C-13 relaxation parameters have been made for a number of proteins and interpreted on the basis of the so-called 'model-free' approach; often, active sites of proteins are found to be mobile. In a few cases, molecular dynamics simulations have been carried out to simulate relaxation parameters.