Non-conservative circular dichroism of photosystem II reaction centers: Is there an enhancement by a coupling with charge transfer states?

The non-conservative nature of the circular dichroism (CD) spectrum of the Q_y spectral region of the D1/D2/Cytb₅₅₉ reaction center complexes of photosystem II is investigated. Our theory, in addition to the usual excitonic couplings between the Q_y transitions of the chlorophyll and pheophytin pigments, takes into account the coupling of the Q_y with the higher-energy B_x, B_y and N_x+xy transitions of these pigments and with S₀→S₂ transitions of the two β-carotene pigments close to the reaction center. The higher-energy transitions are parameterized by quantum chemical calculations and an analysis of the experimental oscillator strength of the isolated pigments. The excitonic couplings between pigments are obtained with the Poisson-TrEsp method. The coupling between exciton and charge transfer states in the central P_D1-P_D2 dimer is implicitly taken into account by a scaling factor of the excitonic coupling. The observed non-conservativity of the Q_y CD spectrum can be qualitatively understood by the above described mixing of Q_y and higher-energy transitions. The analysis suggests that the non-conservativity may be enhanced by coupling to charge transfer states. We expect that the present study will stimulate explicit microscopic studies of the charge transfer states in the reaction center of photosystem II.