Conformational changes of the intermediate of the S2 to S3 transition in photosystem II

The paper extends earlier studies on the S 2 Y Z intermediate that is trapped by illumination in the temperature range 77 K to 190 K of untreated samples poised in the S2…QA state. X-band EPR experiments on untreated and glycerol (50% v/v) treated samples at 10 K indicate that the intermediate consists of two components. A wide one with a splitting of ca 170 G, and a narrow one characterized by a splitting of ca 120 G (untreated), or 124 G (glycerol-treated samples). Lower temperatures of illumination in the above temperature range favor the wide component, which at 10 K decays faster than the narrow one. Re-illumination at 10 K after decay of the signal trapped at 77–190 K induces only the narrow component. Rapid scan experiments in the temperature range 77–190 K reveal high resolution spectra of the isolated tyz Z radical and no evidence of alternative radicals. The two split signals are accordingly assigned to different conformations of the S 2 Y Z intermediate A point-dipole simulation of the spectra yields “effective distances” between the spin densities of Y Z and the Mn4Ca center of 5.7 Å for the wide and 6.4 Å for the narrow component. The results are discussed on the basis of a molecular model assuming two sequential proton transfers during oxidation of tyr Z. The wide component is assigned to a transient S 2 Y Z conformation, that forms during the primary proton transfer.