Studies of the pH dependence of /sup 13/C shifts and carbon-carbon coupling constants of (U-/sup 13/C)aspartic and -glutamic acids
/sup 13/C NMR studies of the chemical shifts and carbon--carbon spin--spin coupling constants of 90% (U-/sup 13/C)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 /sup 3/J/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 /sup 3/J/sub HH/ and /sup 3/J/sub CH/ couplings. For a given conformation of glutamic acid, it is found that /sup 3/J/sub 14/ is considerably smaller than /sup 3/J/sub 25/. This result is consistent with obsrvations on a number of other /sup 13/C-labeled amino acids. 5 figures, 4 tables.
- Research Organization:
- Los Alamos Scientific Lab., NM
- OSTI ID:
- 6977097
- Journal Information:
- J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 100:12; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400301* -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
AMINO ACIDS
ASPARTIC ACID
CARBON 13
CARBON ISOTOPES
CARBOXYLIC ACIDS
CHEMICAL BONDS
CHEMICAL SHIFT
COUPLING
ENERGY LEVELS
EVEN-ODD NUCLEI
EXCITED STATES
GLUTAMIC ACID
ISOTOPES
LABELLED COMPOUNDS
LIGHT NUCLEI
MAGNETIC RESONANCE
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
PH VALUE
RESONANCE
ROTATIONAL STATES
STABLE ISOTOPES
TITRATION