THE IMPORTANCE OF CHARGE-TRANSFER BETWEEN THE RETINAL CHROMOPHORE ANDTHE PROTEIN ENVIRONMENT IN BACTERIORHODOPSIN - A THEORETICAL-ANALYSISON REDUCED AND OXIDIZED CHROMOPHORES

The importance of charge transfer (CT) between the retinal chromophore and the protein environment in the ground state of bacteriorhodopsin (BR) has been verified by using ab initio and semiempirical molecular orbital methods, We hypothesize that the chromophore is stabilized in BR by highest occupied molecular orbital-lowest unoccupied molecular o rbital (HOMO-LUMO) interaction with the protein environment, If suffic ient charge is transferred between two sites due to the strong HOMO-LU MO interaction, the chromophore might be treated as a one-electron red uced species (when it behaves as an electron acceptor), or as a one-el ectron oxidized one (when it acts as an electron donor), In both optim ized geometries, the Ir-conjugated systems exhibit a drastic decrease in bond alternation, To estimate the rotational barrier for thermal is omerization between the all-trans and the 13,15-dicis form, the potent ial energy curve around these two bonds was computed. The first pi-pi transition energy was also calculated for an inspection of the opsin shift, The barrier height and the transition energy became much lower as a result of the chromophore reduction, The site selectivity in phot o-and thermal isomerization and the opsin shift in BR can be well expl ained by considering CT from the protein environment to the chromophor e.