Conversion of the g=4.1 EPR signal to the multiline conformation during the S2 to S3 transition of the oxygen evolving complex of Photosystem II

The oxygen evolving complex of Photosystem II undergoes four light-induced oxidation transitions, S-0-S-1,..., S-3-(S-4)S-0 during its catalytic cycle. The oxidizing equivalents are stored at a (Mn)(4)Ca cluster, the site of water oxidation. EPR spectroscopy has yielded valuable information on the S states. S-2 shows a notable heterogeneity with two spectral forms: a g = 2 (S = 1/2) multiline, and a g = 4.1 (S = 5/2) signal. These oscillate in parallel during the period-four cycle. Cyanobacteria show only the multiline signal, but upon advancement to S-3 they exhibit the same characteristic g = 10 (S = 3) absorption with plant preparations, implying that this latter signal results from the multiline configuration. The fate of the g = 4.1 conformation during advancement to S-3 is accordingly unknown. We searched for light-induced transient changes in the EPR spectra at temperatures below and above the half-inhibition temperature for the S-2 to S-3 transition (ca 230 K). We observed that, above about 220 K the g = 4.1 signal converts to a multiline form prior to advancement to S-3. We cannot exclude that the conversion results from visible-light excitation of the Mn cluster itself. The fact however, that the conversion coincides with the onset of the S-2 to S-3 transition, suggests that it is triggered by the charge-separation process, possibly the oxidation of tyr Z and the accompanying proton relocations. It therefore appears that a configuration of (Mn)(4)Ca with a low-spin ground state advances to S-3. (C) 2010 Elsevier HIM. All rights reserved.