Microsecond protein dynamics measured by C-13(alpha) rotating-frame spin relaxation
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NMR spin relaxation in the rotating frame (R-1p) is a unique method for atomic-resolution characterisation of conformational (chemical) exchange processes occuring on the microsecond timescale. We present a rotating frame C-13 relaxation dispersion experiment for measuring conformational dynamics in uniformly C-13-labeled proteins. The experiment was validated by using the E140Q mutant of the C-terminal fragment of calmodulin, which exhibits significant conformational exchange between two major conformations, as gauged from previous N-15 and H-1 relaxation studies. Consistent with previous work, the present C-13, R-1p experiment detects conformational-exchange dynamics throughout the protein. The average correlation time of <tau(ex)> = 25 +/- 8 mu s is in excellent agreement with those determined previously from H-1 and N-15 R-1p, relaxation data: <tau(ex)> = 19 +/- 7 and 21 +/- 3 mu s, respectively. The extracted chemical-shift differences between the exchanging states reveal significant fluctuations in dihedral angles within single regions of Ramachandran phi-psi space that were not identified from the H-1 and N-15 relaxation data. The present results underscore the advantage of using several types of nuclei to probe exchange dynamics in biomolecules.