High Field EPR Study of the Pheophytin Anion Radical in Wild Type and D1-E130 Mutants of Photosystem II in Chlamydomonas reinhardtii

The intermediate electron acceptor in photosystem II is a pheophytin molecule. The radical anion of this molecule was studied using high field electron paramagnetic resonance in a series of Chlamydomonas reinhardtii mutants. Glutamic acid 130 of the D1 polypeptide is thought to hydrogen bond the ring V carbonyl group of this radical. Mutations at this site, designed to weaken or remove this hydrogen bond, strongly affected the g tensor of the radical. The upward shift of the g(x) component followed the decreasing hydrogen bonding capacity of the amino acid introduced. This behavior is similar to that of tyrosyl and semiquinone radicals. It is also consistent with the optical spectra of the pheophytin in similar mutants. Density functional calculations were used to calculate the g tensors and rationalize the observed trend in the variation of the g(x) value for pheophytin and bacteriopheophytin radical. The theoretical results support the experimental observations and demonstrate the sensitivity of g values to the electrostatic protein environment for these types of radicals.