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 image is either neutral or photo-reduced to image by illumination at 1.7 K, with samples where image is reduced by either 260 K illumination or chemical treatment. The latter preparations have the advantage of exhibiting minimal change in image 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 image, which occurs in dark-adapted samples after 1.7 K illumination. We find that spontaneous hole-filling also occurs in PSII with stable image 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 image reduction and re-oxidation. Shallow holes burnt in image samples exhibit a narrow image 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.