Conformational analysis of pyridoxal Schiff's bases. Nuclear magnetic resonance studies of the conformations about the C4-C4', Calpha-Cbeta, and N-Calpha bonds of the pyridoxal Schiff's bases of amino acids

The solution conformations of a series of pyridoxal-amino acid Schiff's bases were analyzed using 13C and 1H nuclear magnetic resonance techniques. The 13C--1H coupling constants were assigned based on model compounds and isotopic labeling. The predominant conformation of the the C4--C4' bond was found to be "cis" based on nuclear Overhauser effect (NOE) measurements and the "simultaneous" upfield shift of both H4' and H5' in the Schiff's bases of aromatic amino acids. Going from the monoanion (pD 8.2) to the dianion (pD 12.3), changes in these two effects suggested an increasing contribution of the "trans" conformer. The conformation of the N--Calpha bond was found to be approximately the same for all the Schiff's bases studied based on the long-range coupling constants 3J(C4'--Halpha) of these compounds, and the NOE studies indicate that there is a close spatial relationship between H4' and Halpha. The conformations of the Calpha--Cbeta bond of the Schiff's bases of aromatic amino acids were determined by stereospecific deuterium labeling at the beta position. A pi--pi interaction between the aromatic ring and the pi system of pyridoxal was observed which disappeared upon saturation of the aromatic ring.

Conformation-reactivity relationship for pyridoxal Schiff's bases. Rates of racemization and alpha-hydrogen exchange of the pyridoxal Schiff's bases of amino acids

The role of stereoelectronic effects in controlling the reaction specificity of biological reactions involving pyridoxal phosphate-amino acid Schiff's bases was tested with nonenzymatic models. The rates of racemization and Halpha exchange of a series of pyridoxal-amino acid Schiff's bases were determined. The order of these rates does not parallel the predictions based solely on electronic or steric effect, but parallels the proportions of the reactive conformers (e.g., conformers with the Calpha--Halpha bond orthogonal to the pi system) estimated by CPK models. The special reactivity of the phenylalanine Schiff's base was consistent with a special conformation in which some type of pi--pi interaction increases the proportion of exchangeable conformers, thus further substantiating the role of conformation in governing the reactivity of the Calpha--Halpha bond. Furthermore, semiempirical calculations of the conformation about the Calpha00N bond were performed using the CAMSEA conformational analysis program. The results of conformational calculations are consistent with the results of conformational analysis by nuclear magnetic resonance. The order of reactivity of the the Calpha--Halpha bond of the SB dianion, pH 12.0, predicted by calculation based on stereoelectronic effects, though not quantatively parallel to the observed rate constants, is qualitatively in agreement with the experimental results.