Evidence for two forms of the g=4.1 signal in the S-2 state of photosystem .2. Two magnetically isolated manganese dimers

Two forms of the g=4.1 signal in photosystem II (PS II) were identified from X-band and Q-band ESR signal shape and temperature dependence studies. Using ethylene glycol cryoprotected PS II illuminated at 130K, a g=4.1 signal was generated which exhibited a temperature dependence consistent with it arising from a ground state species. Using sucrose cryoprotected PS II illuminated at 200K (in the absence of monoalcohols), a g=4.1 signal was cogenerated with the multiline signal. At temperatures above similar to 20K, a signal at g similar to 6 became evident in these samples. The temperature dependencies of the multiline, g=4.1 and g similar to 6 signals were quantitatively consistent with them arising from the first 3 states (spin 1/2, 3/2, 5/2) respectively of a weakly antiferromagnetically coupled Mn III-IV dimer. The temperature dependence of the signals in these samples indicated that the g = 4.1 signal now arose from a centre displaying excited state behaviour. The two types of g=4.1 signal were very similar in shape at X-band but showed significantly different line shapes at Q-band. It is suggested that they arise from separate, near axial, S=3/2 centres in well-defined states. A model is proposed, based on the temperature dependencies, ESR line shapes and probable spin states, to suggest that the four Mn ions are arranged as two exchange coupled pairs and that each g=4.1 signal arises from a separate manganese dimer. The ground state g=4.1 signal then requires the involvement of at least one additional spin 1/2 species, coupling to each Mn of a homodimer (probably IV-IV oxidation state). The spin 1/2 centre may be an oxidised protein side chain, possibly acting as a bridging ligand between the two Mn ions. It is concluded that the Mn dimers are sufficiently spatially separated within the protein structure to exclude magnetic exchange between the dimers, but within range to allow rapid electron transfer.