A THEORETICAL-EXAMINATION OF THE ELECTRONIC-STRUCTURE AND EXCITED-STATES OF THE BACTERIOCHLOROPHYLL B DIMER FROM RHODOPSEUDOMONAS-VIRIDIS

We report calculations of the electronic structure of the bacteriochlorophyll b (BChlb) dimer (P) from the reaction center of the photosynthetic bacterium, Rhodopseudomonas viridis. The calculations use the Intermediate Neglect of Differential Overlap Spectroscopic Model (INDO/S). We classify the four low-lying excited states of Q(y) origin, which determine the photophysics and photochemistry of the dimer, as mainly excitonic or charge resonance, using the excited configurations of the monomers as a basis. The calculations yield an 1850-cm-1 red shift in the lowest lying Q(y) (Q(y1) state of P relative to the average of its constituent monomers. The Q(y1) state of P contains substantial charge resonance character with only a small net transfer of charge between its BChlb components. Charge resonance contributes to the red shift of the Q(y1) state about as much as the exciton splitting, while the surrounding protein contributes only a small effect to this red shift. This work suggests that the red shift of P is mostly due to the formation of the BChlb dimer. Consideration of the protein is found to influence the charge-transfer character in Q(y1), as well as in the higher energy Q(y3) and Q(y4) states of P, and gives better agreement with the experimental Stark results. The calculated linear dichroism and the splitting between the two low-lying excitonic bands of P in Rps. viridis compare well with experimental results and support the identification of the transition at 850 nm in the experimental spectrum as the upper Q(y) exciton component. Small conformational changes in P yield calculated optical changes as large as 800 cm-1. In addition, the blue shift in the lowest Q(y) band of the BChla dimer in Rb. sphaeroides (Q(y) = 870 nm) relative to the BChlb dimer of Rps. viridis (Q(y) = 960 nm) is easily rationalized by an increase in the average spacing between the macrocyles of 0.2-0.3 angstrom.