NMR EXPERIMENTS FOR THE MEASUREMENT OF CARBON RELAXATION PROPERTIES IN HIGHLY ENRICHED, UNIFORMLY C-13,N-15-LABELED PROTEINS - APPLICATION TO C-13(ALPHA) CARBONS

New two-dimensional NMR experiments with high sensitivity and resolution are presented for the measurement of T-1, T-1 rho, and steady-state H-1-C-13 NOE values for CH C-13(alpha) spin systems in highly enriched, uniformly C-13-labeled proteins. Using a sample consisting of approximately equimolar amounts of 99% C-13(alpha)-alanine and 99% uniformly C-13-labeled alanine (C-13(3)-alanine) dissolved in perdeuterated glycerol, high signal-to-noise C-13(alpha) relaxation measurements of both singly and uniformly C-13-labeled alanine have been made. This allows an investigation of the influence of both carbon-carbon scalar coupling effects and dipolar relaxation effects on the measurement of relaxation properties of carbon spins. T-1, T-1 rho, and steady-state H-1(alpha)-C-13(alpha) NOE values have been measured over a range of temperatures from 10 degrees C to 40 degrees C, with the correlation time for molecular tumbling varying from similar to 17 to similar to 1 ns. The results indicate that, for macromolecules, the contributions to the longitudinal carbon relaxation from neighboring carbons must be included in the interpretation of T-1 data in terms of motional models. The H-1(alpha)-C-13(alpha) steady-state NOE can be influenced significantly by C-13(alpha)-C-13 beta cross relaxation, and because of the small H-1(alpha)-C-13(alpha) NOE in proteins, it may not be possible to measure H-1(alpha)-C-13(alpha) NOE values with high accuracy. Theoretical results are presented which indicate that it is possible to measure accurate (CT1 rho)-C-13-T-alpha values in all residues, with the exception of serine and threonine when the C-13(alpha) and C-13(beta) chemical shifts are nearly equivalent, and experimental verification is provided for the case of alanine. A strategy is proposed for obtaining accurate dynamics of C-13(alpha) carbons based on the measurement of C-13(alpha) T-1 values using at least two field strengths and T-1 rho values measured at a single field.