The photosynthetic reaction center is the heart of biological solar energy conversion. Nearly all known life relies on photosynthesis to convert solar energy into chemical energy, either directly or indirectly. The reaction center (RC) is the core where this conversion occurs. The bacterial RC studied in this lab consists of two branches of chromophores, which serve as electron shuttles. When excited, the special pair of bacteriochlorophylls (P) passes an electron to the bacteriopheophytin (HL) in the active branch of chromophores in ~3 ps, with near unity quantum efficiency. The three dimensional structure of the RC exhibits a high level of symmetry with two possible, seemingly equivalent, pathways for electron transfer. Nonetheless, electron transfer only occurs along one branch of chromophores, breaking the symmetry and this is known as unidirectional electron transfer.
Our most recent work on RCs focuses on this initial electron transfer process. Why is it unidirectional? What is the role of the accessory bacteriochlorophyll (BL), which lies between P and HL?
To answer these questions we seek to understand the energetics of the resultant charge separated states. The protein environment is a key modifier of the energetics of these states via electrostatic interactions with the chromophores. A great range of perturbations to the chromophores is accessible by making mutations involving amino acids near the chromophores, and in some cases chromophores can be knocked out , . These variants are studied by a wide range of advanced spectroscopic techniques, including Stark spectroscopy, spectroelectrochemistry, ultrafast fluorescence, magnetic field effects on key intermediates, and transient absorption spectroscopy.