An exchange-free measure of 15N transverse relaxation:: An NMR spectroscopy application to the study of a folding intermediate with pervasive chemical exchange

A series of experiments are presented that provide an exchange-free measure of dipole-dipole N-15 transverse relaxation, R-dd, that can then be substituted for N-15 R-1p or R-2 rates in the study of internal protein dynamics. The method is predicated on the measurement of a series of relaxation rates involving H-1-N-15 longitudinal order, anti-phase H-1 and N-15 single-quantum coherences, and H-1-N-15 multiple quantum coherences; the relaxation rates of all coherences are measured under conditions of spin-locking. Results from detailed simulations and experiments on a number of protein systems establish that Rdd values are independent of exchange and systematic errors from dipolar interactions with proximal protons are calculated to be less than 1-2%, on average, for applications to perdeuterated proteins. Simulations further indicate that the methodology is rather insensitive to the exact level of deuteration so long as proteins are reasonably highly deuterated (> 50%). The utility of the methodology is demonstrated with applications involving protein L, ubiquitin, and a stabilized folding intermediate of apocytochrome b(562) that shows large contributions to N-15 R-1p relaxation from chemical exchange.