Charge separation in photosystem II core complexes induced by 690-730 nm excitation at 1.7 K

The illumination of oxygen-evolving PSII core complexes at very low temperatures in spectral regions not expected to excite P680 leads to charge separation in a majority of centers. The fraction of centers photoconverted as a function of the number of absorbed photons per PSII core is determined by quantification of electrochromic shifts on Pheo_D1. These shifts arise from the formation of metastable plastoquinone anion (Q_A⁻) configurations. Spectra of concentrated samples identify absorption in the 700-730 nm range. This is well beyond absorption attributable to CP47. Spectra in the 690-730 nm region can be described by the ‘trap’ CP47 absorption at 689 nm, with dipole strength of ∼1 chlorophyll a (chl a), partially overlapping a broader feature near 705 nm with a dipole strength of ∼0.15 chl a. This absorption strength in the 700-730 nm region falls by 40% in the photoconverted configuration. Quantum efficiencies of photoconversion following illumination in the 690-700 nm region are similar to those obtained with green illumination but fall significantly in the 700-730 nm range. Two possible assignments of the long-wavelength absorption are considered. Firstly, as a low intensity component of strongly exciton-coupled reaction center chlorin excitations and secondly as a nominally ‘dark’ charge-transfer excitation of the ‘special pair’ P_D1-P_D2. The opportunities offered by these observations towards the understanding of the nature of P680 and PSII fluorescence are discussed.