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Current methods of determining the rotational diffusion tensors of proteins in solution byNMR spectroscopy exclusively utilize relaxation rate constants for backbone amide 15N spins.However, the distributions of orientations of N-H bond vectors are not isotropic in manyproteins, and correlations between bond vector orientations reduce the accuracy and precisionof rotational diffusion tensors extracted from 15N spin relaxation data. The inclusion of both13Cα and 15N spin relaxation rate constants increases the robustness of the diffusiontensor analysis because the orientations of the Cα-Hα bond vectors differ from theorientations of the N-H bond vectors. Theoretical and experimental results for calbindin D9k,granulocyte colony stimulating factor, and ubiquitin, three proteins with different distributionsof N-H and Cα-Hα bond vectors, are used to illustrate the advantages of thesimultaneous utilization of 13Cα and 15N relaxation data.