Studies of the pH dependence of 13C shifts and carbon-carbon coupling constants of [U-13C]aspartic and -glutamic acids

13C NMR studies of the chemical shifts and carbon--carbon spin--spin coupling constants of 90% [U-13C]aspartic and -glutamic acids are reported. Effects of titration of the two carboxyl groups are separated computationally and the results compared with those for asparagine and glutamine, aspartate and glutamate containing peptides, and a series of amino-n-butyric acids. The results indicate that the carboxyl carbon shift resulting from titration of the carboxyl group is strongly dependent on its distance (number of bonds) from an amino group. Alternatively, remote methyl groups exhibit a much smaller titration induced shift than carboxyl groups in the corresponding position. Significant remote effects of pH titration on the one-bond carbon-carbon coupling are also observed, particularly for couplings involving the side-chain carboxyl carbons. These results are discussed in terms of polarization of the C--O bonds in response to titration of a remote carboxyl group. Values of 3J/sub CC/ in asparate and glutamate indicate a strong conformational dependence. Rotamer populations predicted on the basis of the observed couplings and theoretical INDO calculations are in good agreement with values based on analysis of the 3J/sub HH/ and 3J/sub CH/ couplings. For a given conformation of glutamic acid, it is found that 3J14 is considerably smaller than 3J25. This result is consistent with obsrvations on a number of other 13C-labeled amino acids. 5 figures, 4 tables