Deuterium Spin Probes of Backbone Order in Proteins: 2H NMR Relaxation Study of Deuterated Carbon α Sites

H-2 spin relaxation NMR experiments to study the dynamics of deuterated backbone alpha-positions, D-alpha, are developed. To date, solution-state H-2 relaxation measurements in proteins have been confined to side-chain deuterons-primarily (CH2D)-C-13 or (CHD2)-C-13 methyl groups. It is shown that quantification of H-2 relaxation rates at D-alpha backbone positions and the derivation of associated order parameters of C-alpha-D-alpha bond vector motions in small [U-N-15, C-13, H-2]-labeled proteins is feasible with reasonable accuracy. The utility of the developed methodology is demonstrated on a pair of proteins-ubiquitin (8.5 kDa) at 10, 27, and 40 degrees C, and a variant of GB1 (6.5 kDa) at 22 degrees C. In both proteins, the D-alpha-derived parameters of the global rotational diffusion tensor are in good agreement with those obtained from N-15 relaxation rates. Semiquantitative solution-state NMR measurements yield an average value of the quadrupolar coupling constant, QCC, for D-alpha sites in proteins equal to 174 kHz. Using a uniform value of QCC for all D-alpha sites, we show that C-alpha-D-alpha bond vectors are motionally distinct from the backbone amide N-H bond vectors, with H-2-derived squared order parameters of C-alpha-D-alpha bond vector motions, S-C alpha D alpha(2), on average slightly higher than their N-H amides counterparts, S-NH(2). For ubiquitin, the H-2-derived backbone mobility compares well with that found in a 1-mu s molecular dynamics simulation.