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

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Abstract
The solution conformations of a series of pyridoxal-amino acid Schiff''s bases were analyzed using 13C and 1H NMR techniques. [These bases are key intermediates in many biological reactions catalyzed by pyridoxal phosphate-requiring enzymes.] The 13C.sbd.1H coupling constants were assigned based on model compounds and isotopic labeling. The predominant conformation of the C4.sbd.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 2 effects suggested an increasing contribution of the trans conformer. The conformation of the N.sbd.C.alpha. bond was found to be approximately the same for all the Schiff''s bases studied based on the long-range coupling constants 3J(C4''.sbd.H.alpha.) of these compounds, and the NOE studies indicate that there is a close spatial relationship between H4'' and H.alpha.. The conformations of the C.alpha..sbd.C.beta. 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.