Department of Chemistry, Faculty of Science, Australian National University
Previous ESR studies into the S state cycling of photosystem II (PSII) oxygen evolution have shown that, under physiological conditions, only the S2 state is paramagnetic, with two ESR signals arising from Mn containing centres observable. Temperature dependence studies on these S2 state ESR signals lead to the hypothesis that PSII contains two distinct and magnetically isolated centres containing Mn dimers (Smith 95). The two Mn dependent S2 state ESR signals are:- a signal centred near g=2 characterised by 18-22 hyperfine peaks, the multiline (ML) signal, and a broad relatively featureless resonance centred near g=4, 4.1 signal (Dismukes 81, Casey 84, Zimmermann 84). Illumination, cryoprotectant and buffer modifiers determine the relative amounts of the ML and 4.1 formed (Zimmermann 84, Pace 91). Illumination at 200K generates an S2 state featuring a ML and 4.1 signal. These signals arise as the ground S=1/2 (ML) and first excited S=3/2 (4.1) states of a centre containing a mixed valence, antiferromagnetically exchange coupled Mn dimer (ML S2 State) (Pace 91, Smith 95). Illumination of poly-alcohol cryoprotected PSII at 140K generates an S2 state with only the 4.1 signal observable (Casey 84). This signal arises as the S=3/2 ground state of a centre containing an antiferromagnetically exchange coupled (S=1) Mn homodimer, ferromagnetically coupled to a third spin centre, probably an oxidised amino acid residue (S=1/2) ligand bridging the Mn dimer (4.1 S2 State) (Smith 95). The temperature dependence of this 4.1 S2 state shows the oxidised amino acid residue behaves as the excited state species.
In PSII prepared by anion exchange of F- for Cl- (F- PSII), illumination at 200K generates only a 4.1 signal. This altered 4.1 S2* state is consistent in temperature dependence with the 4.1 S2 state from uninhibited PSII, with the exception that the exchange coupling constant between the Mn ions in the dimer is raised, reducing thermal access to the first excited state. Illumination of the F- PSII at 275K generates an ESR signal characterised by a split resonance centred near g=2 with peak to trough separation of ~ 11mT. This signal, an altered S3* state signal, follows similar temperature dependence behaviour to the 4.1 signal in the F- PSII S2* state. This S3* state signal has been proposed to arise from a spin-spin interaction between a Mn containing centre and another organic radical (Boussac 90, Hallahan 92). Illumination of F- PSII at 140K generates a broad resonance signal centred near g=2 previously not reported. This research proposes that this new signal may arise from an oxidisable amino acid residue located between P680 and YZ and may interact with the F- PSII S2* state 4.1 signal to form the split S3* state signal. Other forms of the altered S3* state developed in PSII inhibited by other procedures will also be discussed.
1. Smith, PJ and Pace, RJ Submitted to BBA (1995).
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4. Zimmermann, J-L and Rutherford, AW. (1984) BBA 767, 160-167.
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