HYDROGEN-BONDING OF CARBOXYL GROUPS IN SOLID-STATE AMINO-ACIDS AND PEPTIDES - COMPARISON OF CARBON CHEMICAL SHIELDING, INFRARED FREQUENCIES, AND STRUCTURES

We previously reported that the protonation state of the carboxyl group of amino acids and peptides in the solid state can easily be determined by the carbon chemical shielding tensors but not by the isotropic shifts. In this report the substantial variation in the sigma(22) element for both protonated and deprotonated forms is shown to be a result of hydrogen bonding. We have correlated this tenser element with established measures of hydrogen bonding, namely the IR stretching frequencies of the carbonyl and the asymmetric stretching frequency for the protonated and deprotonated carboxy groups, respectively. We also observed a strong correlation between the sigma(22) values and previously reported O...H hydrogen bonding distances from the carbonyl of protonated acids to the nearest proton donor. In the database, we found a fixed geometry for the protonated acids and a variable and complicated geometry for hydrogen bond interaction in deprotonated carboxylate. Correspondingly, the correlation between NMR, IR, and diffraction data is more convincing for the protonated acids.