Molecular species of Schiff bases derived from o-hydroxyaromatic aldehydes. III. Schiff bases of pyridoxal and its analogs with unsaturated amino acids.

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The analyses of the electronic absorption spectra and the band assignments to the molecular species were carried out on the methanol solutions of the Schiff bases of pyridoxal (PL), pyridoxal N-methochloride (PLM), and related aromatic aldehydes with α, β-, β, γ-, and γ, δ-unsaturated amino acids and their methyl esters. The Schiff bases of PL with propargylglycine, allylglycine, and vinylglycine had spectra similar to those with a saturated amino acid, showing the unsaturated bond is not conjugated with the azomethine bond. In neutral and alkaline solutions of the Schiff bases of PLM with esters of alanine, propargylglycine, and allylglycine, 2-4% of the Schiff bases was deprotonated at the α-carbon atom, forming a quinoid species absorbing at 500 nm. With ester of vinylglycine, a species absorbing at 534 nm was formed, indicating the quinoid structure with an extended conjugation. It is suggested from the spectral analysis of the Schiff bases of methyl dehydrovalinate that in species protonated at the phenolate or the azomethine the molecule is not planar and the planarity is achieved through the metal chelation.

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