MODELING ELECTRON-TRANSFER IN BIOCHEMISTRY - A QUANTUM-CHEMICAL STUDYOF CHARGE SEPARATION IN RHODOBACTER-SPHAEROIDES AND PHOTOSYSTEM-II

High-level quantum chemical methods (hybrid density-functional type) a re applied to the light-driven charge-separation process in the photos ynthetic reaction centers of both bacteria and photosystem II in green plants. Structural information on the bacterial system provides the b asis for choosing the models used in the calculations. The energetics of the electron transfer from the (bacterio)chlorophyll to the quinone are calculated as well as those of the intermediate step involving th e (bacterio)pheophytin. The surrounding protein is treated as a dielec tric medium, and the cavities around the solute molecules are determin ed by isodensity surfaces. The dielectric effects on the charge-separa tion processes are calculated to be as large as 50 kcal/mol. It is sho wn that hydrogen bonding between the chromophores and certain peptide residues as well as the axial histidine ligand on the (bacterio)chloro phylls contributes substantially to the energetics. Good agreement wit h experimentally estimated driving forces of the different steps is ob tained within 2 kcal/mol for the bacteriosystem and within 8 kcal/mol for photosystem II. The results for photosystem II have a lower accura cy, as expected, due to the lack of detailed structural information on this system. Recent low-resolution data indicating a weaker coupling between the chlorophylls in P680 as compared to that of the special pa ir in the bacterial systems are taken into account in the calculations . In the bacterial system, charge separation to the accessory bacterio chlorophyll is essentially thermoneutral and the P865(+)BPheo(-) state is stable by 7.5 kcal/mol. In photosystem II, charge separation to th e P680(+)Pheo(-) state is much less strongly driven, and the absence o f an axial histidine ligand to the accessory chlorophyll appears neces sary to allow its formation. The creation of the tyrosine radical (Y-z ) by proton-coupled electron transfer to the photoionized reaction cen ter chlorophyll in photosystem II is also studied. In this case as wel l, hydrogen bonding to other peptide residues plays an important role in the overall energetic balance of the reaction.