RETINAL SCHIFF-BASES WITH AROMATIC AND ALIPHATIC AMINO-ACIDS - THE EXTREMELY DIFFERENT NATURE OF THE INTRAMOLECULAR HYDROGEN-BOND BETWEEN THE 2 TYPES OF COMPOUNDS

We studied the intramolecular hydrogen bonds in four Schiff bases of all-trans retinal with aliphatic aminocarboxylic acids, as well as four with aromatic aminocarboxylic acids. Osmometric, FT-IR, UV and H-1 NMR measurements were performed. The intramolecular hydrogen bonds formed in the aliphatic compounds always have the proton localized at the N-atom of the Schiff base, i.e. the structure O-...H+N exists; conversely, with the aromatic compounds the proton is always localized at the carboxylic acid group, i.e. the structure OH...N is the only existing one. This result is explained by the fact that the aromatic compounds have a much higher degree of electron delocalization than the aliphatic compounds, and thus the N-atoms of the Schiff base are more positively charged for the former.