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 PheoD1. These shifts arise from the formation of metastable plastoquinone anion (QA−) 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’ PD1–PD2. The opportunities offered by these observations towards the understanding of the nature of P680 and PSII fluorescence are discussed.