Measurement of 15N relaxation in deuterated amide groups in proteins using direct nitrogen detection.

15N chemical shielding tensors contain useful structural information, and their knowledge is essential for accurate analysis of protein backbone dynamics. The anisotropic component (CSA) of 15N chemical shielding can be obtained from 15N relaxation measurements in solution. However, the predominant contribution to nitrogen relaxation from 15N-(1)H dipolar coupling in amide groups limits the sensitivity of these measurements to the actual CSA values. Here we present nitrogen-detected NMR experiments for measuring 15N relaxation in deuterated amide groups in proteins, where the dipolar contribution to 15N relaxation is significantly reduced by the deuteration. Under these conditions nitrogen spin relaxation becomes a sensitive probe for variations in 15N chemical shielding tensors. Using the nitrogen direct-detection experiments we measured the rates of longitudinal and transverse 15N relaxation for backbone amides in protein G in D(2)O at 11.7 T. The measured relaxation rates are validated by comparing the overall rotational diffusion tensor obtained from these data with that from the conventional 15N relaxation measurements in H(2)O. This analysis revealed a 17-24 degree angle between the NH-bond and the unique axis of the 15N chemical shielding tensor.