Chlorophyll a radical ions:: A density functional study

The chlorophyll a radical cation, Chl a(+.), and the corresponding radical anion, Chl a(-.), have been investigated applying density functional methods. Furthermore, the C-10 epimer Chl a' and O-1 Chl a enol radical cations have been studied, which have been proposed to play a role in photosystem I of oxygenic photosynthesis. Isotropic hyperfine coupling constants (hfcs) and related spin density distributions calculated at the B3LYP/EPR-II//BLYP/DZVP(D) level of theory are reported. For Chl a(+.) and Chl a(-.), good agreement is observed between calculated and experimental hfes. Whereas no larger differences in the hfcs between Chl a(+.) and (Chl a')(+.) are predicted to occur, the enol forms of Chl a(+.) give rise to significantly altered hyperfine coupling constants in comparison with the respective keto form. In addition to the chlorophyll systems, the epimer and 01 enol of bacteriochlorophyll a have also been calculated. From a comparison with experimental data it is concluded that the enol forms do not occur in photosynthetic reaction centers.