Novel characteristics of persistent spectral hole-burning and hole-filling in Photosystem II core complexes

We report spectral hole-burning as well as spontaneous and photo-induced hole-filling in Photosystem II (PSII) core complexes. A comparison is made between measurements on initially dark-adapted PSII cores in which the plastoquinone Q_A is either neutral or photo-reduced to Q_A^- by illumination at 1.7 K, with samples where Q_A is reduced by either 260 K illumination or chemical treatment. The latter preparations have the advantage of exhibiting minimal change in Q_A^- with either time or illumination at 1.7 K. This stability allowed us to investigate the association of rapid hole-filling reported for PSII core complexes with the spontaneous re-oxidation of Q_A^-, which occurs in dark-adapted samples after 1.7 K illumination. We find that spontaneous hole-filling also occurs in PSII with stable Q_A^- configurations. In either class of sample, spontaneous hole-filling occurs at significantly greater rates than for 'normal' two level system hole-burning processes in chlorophyll-protein antenna complexes. It is suggested that the rapid spontaneous hole-filling is associated with protein relaxation within the energy landscape of the PSII core proteins. This landscape may also be involved in influencing other processes such as low-temperature Q_A reduction and re-oxidation. Shallow holes burnt in Q_A samples exhibit a narrow (∼ 2 cm^{- 1}) photoproduct on the low energy side of the hole. Subsequent illumination of holes burnt at 690 nm with 705 nm light induces strong hole-filling. These results are discussed with respect to charge and excitation energy transfer processes in PSII cores.