Primary Processes in the Photosynthetic Reaction Center

A goal of our photosynthesis research is to achieve a molecular-level understanding of the primary charge separation process in the reaction center from purple photosynthetic bacteria. In this pigment-protein complex, light energy is converted into chemical potential energy by a series of fast electron transfers from the photoexcited bacteriochlorophyll special dimer P to BA, HA, QA, and finally QB with a quantum yield of ~1. We have been preparing and studying mutants with altered free energies of the charge separated states that modulate the yields of charge separation versus recombination and give electron transfer fully down the normally inactive B branch (see Figure 1).

To probe the electronic contribution to directionality (as opposed to the Franck Condon factor that is modulated by energetics), we are studying mutants in which the bacteriochlorophyll dimeric primary electron donor is replaced by a bacteriochlorophyll-bacteriopheophytin heterodimer (Figure 2).  This change along with other mutations in the vicinity of the primary donor creates substantial, and tunable, charge asymmetry in the excited primary donor (Figure 3), which modulates the electronic interactions with the acceptor molecules on the two sides of the RC and thus the rate constants for electron transfer.

Figure 1: Wild-Type Reaction Center and Photochemical Events

Figure 2: Wild-Type Reaction Center with amino acids altered (A)  to generate a series of reaction centers that contain a bacteriochlorphyll (green)-bacteriopheophytin (red) heterodimer primary electron donor (B).

Figure 3: Schematic of proposed localizations and relative magnitudes of charge asymmetry of the excited heterodimer primary donor  for the M-heterodimer RCs (A) and L-heterodimer RCs (B) upon addition of a hydrogen bond to the dimer (L131H), swap of PheL181 and TyrM208/Type (YF), or both.