Simulation of the Absorption and Circular Dichroism Spectra for the Primary Electron Donor in Reaction Centers of Purple Bacteria and Photosystem II

Abstract

When different derivatives of bacteriochlorophylls (Bchls) are introduced to a mixture of formamide and water that contains micelles of Triton X-100 (TX-100) they form dimers with spectral properties strongly resembling those of P-860, the primary electron donor of Rhodobacter sphaeroides. In all these dimers, the monomeric lowest-energy (Qy) transition is replaced by two bathochromically shifted and optically active transitions. The energy difference between the two new transitions, their oscillator strengths and their rotational strengths may be calculated for Bchl dimers with the special pair’s geometry using the point monopole approximation. The bathochromic shift of the excitonic transition’s center of gravity cannot be explained in terms of charge resonance (CR) states admixing into the Qy transitions but must involve electrostatic and dispersive interactions among the paired molecules. Weak dimerization which is accompanied by the replacement of the monomeric Qy transition with two bathochromically shifted and optically active transitions is also observed for two chlorophyll a (Chl a) in the D1-D2-cyt b559 complex. This complex is believed to be the reaction center of photosystem II (PSII). It is likely that these Chls make up P-680, the PSII primary electron donor.KeywordsB559 ComplexBathochromic ShiftPurple BacteriumExcitonic TransitionDipolar StrengthThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.