Retinal Schiff bases with aromatic and aliphatic amino acids — the extremely different nature of the intramolecular hydrogen bond between the two types of compounds

Abstract

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 1H 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.