Spectral hole burning at the low-energy absorption edge of photosystem II core complexes

Photosystem II (PSII) core complexes capable of efficient oxygen evolution at physiological temperatures remain strongly photo-active at low temperatures, where illumination converts the sample to very long-lived (102–5 s) charge-separated configurations via reduction of the plastoquinone, QA. Persistent spectral hole burning (PSHB) seen in the 690–704 nm range exhibits efficiencies comparable to that of QA− formation. The rate of spontaneous spectral hole filling for a hole burnt near 690 nm matches the rate for spontaneous recombination of the charge-separated configuration. This result supports the proposed hole-burning mechanism due to the charge separation that occurs following relatively slow transfer of excitation from selectively excited antenna pigments. Clipping and attenuation of the pseudo-phonon sideband hole establishes that absorption of CP47 antenna chlorophyll a (chl a) in PSII becomes negligible beyond 704 nm. Excitation in the 704–730 nm region lead to no measurable PSHB, despite this illumination inducing photo-conversion in up to 80% of all PSII centers in the sample. We also present illumination-induced changes of the absorption in the 700–730 nm region, and show that they are not associated with PSBH.