The photochemistry in Photosystem II at 5 K is different in visible and far-red light

We have earlier shown that all electron transfer reactions in Photosystem II are operational up to 800 nm at room temperature [Thapper, A., et al. (2009) Plant Cell 21, 2391-2401]. This led us to suggest an alternative charge separation pathway for far-red excitation. Here we extend these studies to a very low temperature (5 K). Illumination of Photosystem II (PS II) with visible light at 5 K is known to result in oxidation of almost similar amounts of YZ and the Cyt b559/ChlZ/CarD2 pathway. This is reproduced here using laser flashes at 532 nm, and we find the partition ratio between the two pathways to be 1:0.8 at 5 K [the partition ratio is here defined as (yield of YZ/CaMn4 oxidation):(yield of Cyt b559/ChlZ/CarD2 oxidation)]. The result using far-red laser flashes is very different. We find partition ratios of 1.8 at 730 nm, 2.7 at 740 nm, and >2.7 at 750 nm. No photochemistry involving these pathways is observed above 750 nm at this temperature. Thus, far-red illumination preferentially oxidizes YZ, while the Cyt b559/ChlZ/CarD2 pathway is hardly touched. We propose that the difference in the partition ratio between visible and far-red light at 5 K reflects the formation of a different first stable charge pair. In visible light, the first stable charge pair is considered to be PD1+QA-. In contrast, we propose that the electron hole is residing on the ChlD1 molecule after illumination by far-red light at 5 K, resulting in the first stable charge pair being ChlD1+QA-. ChlD1 is much closer to YZ (11.3 Å) than to any component in the Cyt b559/ChlZ/CarD2 pathway (shortest ChlD1-CarD2 distance of 28.8 Å). This would then explain that far-red illumination preferentially drives efficient electron transfer from YZ. We also discuss mechanisms for accounting for the absorption of the far-red light and the existence of hitherto unobserved charge transfer states. The involvement of two or more of the porphyrin molecules in the core of the Photosystem II reaction center is proposed.