Primary events in biology probed by ultrafast laser spectroscopy. Chapter 3: Electron transfer reactions in photosynthetic bacteria

Both bacteria and plants use arrays of pigments as antennae to harvest the Sun's energy. This harvested energy is transferred to sites called reaction centers (RCs). The elementary chemical reactions in RCs are electron transfer (ET). The present understanding of RC electron transfer steps is described. Modeling work was performed to correlate electrochemical, spectroscopic, and geometric parameters to help predict the likelihood of electron transfer reactions. To summarize the modeling work, the cofacial diporphyrin models compare favorably with natural photosystems. The distances and rates of ET are similar, and both store greater than 90% of the incident photons energy in ET products. These conclusions lend strong support to RC electron transfer schemes that employ an electron transfer intermediate between BChl)2 and BPheo. The modeling studies suggest that a close to cofacial arrangement may be necessary for rapid electron transfer even at donor to acceptor separations of greater than 7A.