Effect of nonexcitonic interactions among the paired molecules on the Qy transition of bacteriochlorophyll dimers. Applications to the primary electron donors P-860 and P-960 in bacterial reaction centers

Abstract

X-ray diffraction of reaction centers (RCs) from two strains of purple bacteria, Rhodobacter sphaeroides and Rhodopseudomonas viridis, revealed that their primary electron donors, P-860 and P-960, are tight dimers of bacteriochlorophyll (Bchl) a and b, respectively. The geometry of the paired molecules is such that their transition monopoles strongly interact, resulting in a bathochromic shift of the Q{sub y} bands. Recent calculations based upon interactions among point-charge monopoles (PCM) showed that exciton coupling among the localized Q{sub y} transitions of the paired molecules can account for part of the shift. The origin of the remaining nonexcitonic shift is controversial. In this communication the authors show that there is a ratio of 1.2-1.5 between the nonexcitonic shift and the exciton splitting of the Q{sub y} transition in several Bchl dimers in vitro.