Optical and electro-optical properties of bacterial photosynthetic reaction centers

The bacterial photosynthetic reaction center (RC) is responsible for the initial photo-induced electron transfer in photosynthesis. The special pair P in the RC is a pair of closely interacting bacteriochlorophylls (BChl). Its electronic absorption spectrum is very broad and qualitatively different from that of monomeric BChl. The P band becomes even broader when one of the BChls is replaced by a bacteriopheophytin (BPhe) to form a heterodimer. When hydrogen bonds from the protein to conjugated carbonyl groups of the heterodimer are added/removed, the absorption of heterodimer exhibits further systematic variations. We postulate an intradimer charge resonance interaction for the excited state of the special pair and this leads to predictions for the absorption line shape. We conclude that an intermediate coupling limit applies to the heterodimer, the coupling and energetics of an intradimer charge transfer (CT) state are obtained directly from the absorption spectrum. This model satisfactorily explains the heterodimer absorption line shapes in various heterodimer/hydrogen bond mutants. A novel resonance Stark effect is discovered for the monomeric BChl absorption in the RC. Its molecular origin is explored by comparing results from RCs in which neighboring chromophores or the immediate protein environment is modified. These results collectively demonstrate that this resonance Stark effect is related to both the monomeric BChl and BPhe on the functional pathway of the RC. A theory of the resonance Stark effect is developed based on a weak coupling limit for a donor-acceptor complex. It predicts a series of Stark line shapes depending primarily on the driving force for electron transfer. They closely resemble the series of experimental resonance Stark spectra for the B band of several RC variants. Analysis of the trends leads to the conclusion that these resonance Stark effects are due to coupling between the {dollar}sp1{lcub}rm B{rcub}sb{lcub}rm L{rcub}{dollar} and B{dollar}rmsb{lcub}L{rcub}sp+Hsb{lcub}L{rcub}sp-{dollar} states. The lifetime of this reaction in WT Rb. sphaeroides RC is estimated to be 3.3 ps. These results have implications for the mechanism of the initial electron transfer in the RC. The resonance Stark spectroscopy should be generally applicable to study photo-induced electron transfer.