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 grou p of amino acids and peptides in the solid state can easily be determi ned by the carbon chemical shielding tensors but not by the isotropic shifts. In this report the substantial variation in the sigma(22) elem ent for both protonated and deprotonated forms is shown to be a result of hydrogen bonding. We have correlated this tenser element with esta blished measures of hydrogen bonding, namely the IR stretching frequen cies of the carbonyl and the asymmetric stretching frequency for the p rotonated and deprotonated carboxy groups, respectively. We also obser ved a strong correlation between the sigma(22) values and previously r eported O...H hydrogen bonding distances from the carbonyl of protonat ed acids to the nearest proton donor. In the database, we found a fixe d geometry for the protonated acids and a variable and complicated geo metry for hydrogen bond interaction in deprotonated carboxylate. Corre spondingly, the correlation between NMR, IR, and diffraction data is m ore convincing for the protonated acids.