title: Thermodynamics of proton transfer in carboxylic acid-retinal Schiff base hydrogen bonds with large proton polarizability.
abstract: During the photocycle of bacteriorhodopsin (BR) the chromophore, a retinal Schiff base, is deprotonated. Simultaneously an asp residue is protonated. These results suggest that this deprotonation occurs via a Schiff base - asp hydrogen bond. Therefore, we studied carboxylic acid - retinal Schiff base model systems in CCl4 using IR spectroscopy. The IR spectra show that double minimum proton potentials are present in the OH ... N in equilibrium with O- ... HN+ H-bonds formed and that the proton can easily be shifted in these bonds by local electrical fields. The thermodynamic data of H-bond formation and proton transfer within these H-bonds are determined. On the basis of these data a hypothesis is developed with regard to the molecular mechanism of the deprotonation of the Schiff base of BR.
mesh_terms: D000085:Acetates; D001436:Bacteriorhodopsins / Q000378:metabolism*; D002338:Carotenoids / Q000378:metabolism*; D006860:Hydrogen Bonding; D011522:Protons; D012172:Retinaldehyde; D012545:Schiff Bases; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics
chemical_list: D000085:Acetates; D011522:Protons; D012545:Schiff Bases; D002338:Carotenoids; D001436:Bacteriorhodopsins; D012172:Retinaldehyde