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

Abstract

The non-conservative nature of the circular dichroism (CD) spectrum of the Qy 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 Qy transitions of the chlorophyll and pheophytin pigments, takes into account the coupling of the Qy with the higher-energy Bx, By and Nx+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 PD1-PD2 dimer is implicitly taken into account by a scaling factor of the excitonic coupling. The observed non-conservativity of the Qy CD spectrum can be qualitatively understood by the above described mixing of Qy 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.