A Comparison of Primary Donors of Type I and Type II Photosynthetic Reaction Centers*

Photosynthesis is one of the most important processes in nature as it powers the planet with sunlight. Light-driven electron transfer in photosynthesis originates in the reaction center (RC). Reaction centers are large, multi-subunit pigment-protein complexes that harvest light energy through a network of internal or external chlorophyll (Chl) or bacteriochlorophyll (BChl) molecules and store the energy through charge separation and multi-step electron transfer. Here, highly coupled chlorophyll molecules serve as both primary electron donors and acceptors in the charge separation reactions. There are two types of RCs in nature, Type I and Type II, that differ in the identity of the terminal electron acceptors. Recent advances in experimental methodology and computational modeling provide an opportunity to conduct a detailed comparison of the charge separation and electron transfer reactions in Type I and Type II RCs. In the present study, we examine the structures of primary electron donors in Type I and Type II RCs in relation to the vast body of spectroscopic research that has been performed on them to date. Further, we perform density functional theory calculations on each oxidized primary donor to study both their electronic properties and our ability to model experimental spectroscopic data.