The Electronic Structure of the Bacteriopheophytin a Anion Radical, in Vivo

Abstract

Density functional calculations are used to calculate the electronic structure of the bacteriopheophytin a radical anion, φA, formed in the initial electron-transfer reactions of bacterial photosynthesis. Using the hybrid B3LYP functional together with the double ζ basis set EPR−II, 13C, 1H, 17O, and 14N isotropic and anisotropic hyperfine couplings are calculated and explained by reference to the electron density of the highest occupied molecular orbital and of the unpaired spin distribution around the radical. Good agreement is observed between calculated and experimental hyperfine couplings. Hydrogen bonding to the carbonyl group of ring E leads to minor changes in the unpaired spin density distribution and the resultant hyperfine couplings. The electronic structure of the anion radical form of the other bacteriopheophytin a molecule, φB, found along the inactive-electron-transfer (B) branch, is also studied, and the calculated electronic properties are compared with φA. It is shown that, whereas the e...