Hybrid Density Functional Studies of Pheophytin Anion Radicals: Implications for Initial Electron Transfer in Photosynthetic Reaction Centers

Abstract

The electronic structure of the pheophytin a anion radical is calculated using hybrid density functional calculations at the B3LYP level. 1H, 14N, 17O, and 13C isotropic and anisotropic hyperfine couplings are calculated. Experimentally determined hyperfine couplings for 14N and 1H positions are in excellent agreement with theoretically calculated values. Comparison of the spin density distribution of the pheophytin radical with the related bacteriopheophytin form shows that the pheophytin radical exhibits a greater asymmetry in the distribution of spin around the ring system compared with its bacterial counterpart. In addition the delocalization of spin density onto the 3-acetyl group of bacteriopheophytin is greater than the delocalization calculated on to the corresponding vinyl group of pheophytin. This decreased delocalization for pheophytin is proposed to be responsible for the increase in electron-transfer time from the primary acceptor bacteriochlorophyll, BA, to the secondary electron acceptor,HA , when pheophytin replaces bacteriopheophytin in the HA site of bacterial reaction centers. Optimal orbital overlap between BA and HA facilitates rapid electron transfer from BA and may be essential in the prevention of charge recombination between BA and the primary donor bacteriochlorophyll, D.