Effect of deuteration on some structural parameters of methyl groups in proteins as evaluated by residual dipolar couplings.

One bond methyl 1H-13C and 13Cmethyl-13C scalar and residual dipolar couplings have been measured at sites in an 15N, 13C, approximately 50% 2H labeled sample of the B1 immunoglobulin binding domain of peptostreptococcal protein L to investigate changes in the structure of methyl groups in response to deuterium substitution. Both one bond methyl 1H-13C and 13Cmethyl-13C scalar coupling constants have been found to decrease slightly with increasing deuterium content. Previous studies have shown that 1H-13C couplings in methyl groups are exquisitely sensitive to electronic structure, with decreases in coupling values as a function of deuteration consistent with a slight lengthening of the remaining H-C bonds. Changes in the HmethylCmethylC angle are found to be small, with average differences on the order of 0.3+/-0.1 degrees and 0.4+/-0.2 degrees between CH3, CH2D and CH3, CHD2 isotopomers, respectively. Knowledge of methyl geometry is a prerequisite for the extraction of accurate dynamics parameters from spin relaxation studies involving these groups.