A 13C and 15N nuclear magnetic resonance study of the protonation of a retinal Schiff base by acids of different pKas and in solvents of different polarities

Abstract

13C and 15N nuclear magnetic resonance spectra of mixtures of all-trans-retinylidene tert-butylamine (RtBA) and substituted acetic acids in equimolar concentrations in both CDCl3 and CD3OD were studied in relation to two questions: the nature of the state of protonation of a retinal Schiff base in the presence of carboxylic and mineral acids in solvents of different polarities, and secondly, to determine the nature of the interactions (if any) between the polar groups located on the acids and the polyenic chain of the retinal derivative. Using carboxylic acids of different pKas and three mineral acids, we present results that indicate that, in solvents of low polarity, weak acids can only partially protonate RtBA while, in a more polar milieu, the protonation percentages are higher. With polar groups such asNO2, Cl, Br, I, and CN, no specific interaction could be found between the polyenic chain of the imine and these groups. Variations in the intensity of certain peaks of RtBA were noted and these could possibly be related to the presence of the polar groups on the acids. Comparisons of our results with those obtained on rhodopsin and bacteriorhodopsin indicate that the milieu surrounding the chromophore must be polar if protonation is complete, while in a less polar environment partial protonation can be expected.