CONFORMATIONAL EFFECTS ON THE PROTON AFFINITY OF THE SCHIFF-BASE IN BACTERIORHODOPSIN - A DENSITY-FUNCTIONAL STUDY

Density functional theory (DFT) calculations have been performed on a number of Schiff base structures related to the retinal Schiff base in bacteriorhodopsin (BR). The proton affinity (PA) of the Schiff base g roup was calculated in species with different lengths of the conjugate d double-bond system and at different cis/trans isomerization states. The results show that the length of the conjugated electronic structur e has a positive effect on the PA of the system which can be related t o the more delocalized electronic structures in the longer chains. Alt hough there is no significant difference of PA between different cis/t rans isomers at single or double bonds in the main chain of the polyen e structure, very pronounced PA changes are predicted during the rotat ion of these bonds. The calculations show that the rotation of the sin gle bond adjacent to the Schiff base group significantly decreases the PA of the Schiff base. The results of closed-shell DFT calculations i ndicate that the rotation of the second double bond in the chain has a completely contrary effect on the PA and predict an effective increas e of PA value during this rotation. The open-shell DFT calculations, o n the other hand, although indicating a different pattern of PA change s, do not show any significant decrease in the PA of the Schiff base d uring the rotation of the double bond, either. The predicted changes o f the calculated PA values are explained on the basis of bond order ch aracteristics and charge distributions along the polyene structure in each case.