The CSA constants (c) of a 1H-X (X=15N or 13C) spin system increase with the field strength B0. Assuming a value of -160ppm for amide 15N CSA and 25 ppm for methine 13C CSA, the c values are:
Field strength 1H frequency (MHz) c(15N-1H) *109 (rad/sec)2 c(13C-1H) *109 (rad/sec)2 300 0.31 0.05 400 0.55 0.08 500 0.87 0.13 600 1.25 0.19 750 1.95 0.29 On the other hand, the dipole-dipole interaction (DD) is field independent. Using typical values for the NH and CH bond distances (d=1.3 and 5.37 *109 (rad/sec)2, respectively) the contribution of both mechanisms at several fields can be analyzed:
Field strength 1H frequency (MHz) c/d (15N-1H) c/d (13C-1H) 300 0.24 0.01 400 0.42 0.01 500 0.67 0.02 600 0.96 0.04 750 1.50 0.05 In summary, whereas CSA contribution plays an important role in 15N relaxation behavior, the 13C relaxation mechanism is greatly dominated by the DD mechanism. In the case of 1H, the proton CSA is -9 ppm and the contribution to relaxation mechanism can be considered as minimum.