EPR evidence for the primary water oxidation step upon the S2 → S3 transition in the Joliot-Kok cycle of plant photosystem II

✍ Scribed by Masami Kusunoki

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 755 KB
• Volume: 239
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(95)00440-f

✦ Synopsis

A new type of 90-240 G wide EPR signal from the modified S 3 state of Ca2+-depleted photosystem II (PSII) is concluded to arise from a partially oxidized water radical with spin S = 1/2 interacting with the S = 1/2 S2-state manganese tetramer ('Mn4'). This is based exclusively on the fact that the average g value of the radical is = 2.010-2.012, a value close to that of OH" radical (2.011) and significantly larger than either one of an oxidized imidazole (2.00226) or an oxidized tyrosine (2.0046), indicating that the radical may be (HOOH)-', the most probable intermediate produced by abstracting two protons and one electron from a bound water dimer. The effective interactions between the Mn 4 and radical spins (S 1 and $2, respectively) of the form Hin t ---J12S1 • S 2 "l-S 1 • D12 • S 2 have been thoroughly investigated to find which Mnn-radical complex can reasonably make both J12 and D12 as small as = 100 G in magnitude and can, simultaneously, yield an X-ray absorption Mn K-edge energy 0.7 ___ 0.3 eV higher than that in the modified S 2 state. As the most probable model, we propose that the radical must form the third bridging ligand between di-lx-oxo or ix2-0xo-(ix3-0xo) bridged Mna(III) and Mnb(IV) ions on the opposite side of mono-lxz-oxo bridged Mn c and Mn d ions.