title: Trapping of metalloradical intermediates of the S-states at liquid helium temperatures. Overview of the phenomenology and mechanistic implications.
abstract: The oxygen-evolving complex (OEC) of photosystem II (PSII) consists of a Mn cluster (believed to be tetranuclear) and a tyrosine (Tyr Z or Y(Z)). During the sequential absorption of four photons by PSII, the OEC undergoes four oxidative transitions, S(0) to S(1), ..., S(3) to (S(4))S(0). Oxygen evolves during the S(3) to S(0) transition (S(4) being a transient state). Trapping of intermediates of the S-state transitions, particularly those involving the tyrosyl radical, has been a goal of ultimate importance, as that can test critically models employing a role of Tyr Z in proton (in addition to electron) transfer, and also provide important clues about the mechanism of water oxidation. Until very recently, however, critical experimental information was lacking. We review and evaluate recent observations on the trapping of metalloradical intermediates of the S-state transitions, at liquid helium temperatures. These transients are assigned to Tyr Z(*) magnetically interacting with the Mn cluster. Besides the importance of trapping intermediates of this unique catalytic mechanism, liquid helium temperatures offer the additional advantage that proton motions (unlike electron transfer) are blocked except perhaps across strong hydrogen bonds. This paper summarizes the recent observations and discusses the constraints that the phenomenology imposes.
authors.affiliation: Institute of Materials Science, NCSR Demokritos, 15310 Aghia Paraskevi Attikis, Greece. vpetr@ ims.demokritos.gr
authors.forename: Vasili
authors.initials: V
authors.lastname: Petrouleas
authors.forename: Dionysios
authors.initials: D
authors.lastname: Koulougliotis
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Ioannidis
mesh_terms: D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D006371:Helium; D007259:Infrared Rays; D008345:Manganese / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism; D013696:Temperature; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism
chemical_list: D005609:Free Radicals; D045332:Photosystem II Protein Complex; C114979:tyrosyl radical YZ; D006371:Helium; D014443:Tyrosine; D008345:Manganese