AB-INITIO QUANTUM-CHEMICAL ANALYSIS OF SCHIFF-BASE WATER INTERACTIONSIN BACTERIORHODOPSIN

The light-driven proton pump, bacteriorhodopsin, contains a photoactiv atable retinal molecule. The Schiff base moiety of the retinal lies in a transmembrane channel through which the protons are pumped. Water m olecules are strongly suspected to be closely associated with the Schi ff base moiety and to play important roles in the proton transfer. We examine protonated Schiff base-water interactions using ab initio quan tum mechanical calculations. Calculations on isolated protonated and d eprotonated Schiff bases confirm that in the protonated species the C= C and C-C bonds close to the nitrogen atom are more equal in length th an in the neutral species. The positive charge of the protonated Schif f base is delocalized over the molecule. The geometries and binding en ergies of two protonated Schiff base-water complexes are examined; in one complex the water molecule hydrogen bonds to the NH group of the S chiff base, and in the other it hydrogen bonds to a CH group on the op posite side of the molecule. The binding energies for both complexes a re strong. Further calculations on a protonated Schiff base-: 2H2O com plex indicate that the binding of the two water molecules is non-coope rative. The optimized complex is transferred into the three-dimensiona l structure of bacteriorhodopsin, where a number of additional water-p rotein hydrogen bonds are found to be possible.