Conformational changes of the S2YZ intermediate of the S2 to S3 transition in photosystem II

The paper extends earlier studies on the S2YZ 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 S2YZ intermediate A point-dipole simulation of the spectra yields “effective distances” between the spin densities of YZ 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 S2YZ conformation, that forms during the primary proton transfer.

Research highlights
► The S2YZ radical comprises a wide component and a narrow component.
► Their respective splittings are 170 G and 120–124 G.
► Effective distances between YZ (wide/narrow) and the Mn4Ca center were computed.
► The wide signal is assigned to an electropositive conformation of S2YZ.
► The narrow signal reflects proton movement away from S2YZ.