The Effect of Protonation and Electrical Interactions on the Stereochemistry of Retinal Schiff Bases

Based on quantumchemical MNDOC calculations it is shown that the ground-state properties of a retinal Schiff base depend sensitively on its protonation state and charge environment. This is exemplified for the equilibrium geometry, for the distribution of partial charges and, in particular, for the thermal isomerization barriers around the π-bonds. It is demonstrated that a protein, by protonating the retinal Schiff base and by providing one or two negative ions in its environment, can reduce double-bond isomerization barriers from 50 kcal/mol for the unprotonated compound to ∼ 5 kcal/mol and can increase single bond barriers from 5 kcal/mol to ∼ 20 kcal/mol. Thereby, the specific location of the ions relative to the polyene chain of the protonated retinal Schiff base determines the barrier heights. The results explain the ground-state isomerization reactions of retinal observed in bacteriorhodopsin and in squid retinochrome