Corrie, Andrew Robert; (1991) Studies of the acceptor complex of Photosystem II from the thermophilic cyanobacterium Phormidium laminosum and of the manganese cluster of the water-oxidising complex of Photosystem II. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis reports studies of the components of the electron acceptor complex of photosystem II isolated from the thermophilic cyanobacterium Phormidium laminosum. The principal technique used has been electron paramagnetic resonance spectrometry (epr): epr signals are characterised by lineshape and g-value. This thesis reports the first detection of the g = 1.9 signal (assigned to the interaction Qa Fe2+) by photoreduction at 77K of Qa in PS II from P. laminosum. This signal could be replaced by the g = 1.8 signal by treatment with formate, an inhibitor of electron transfer between the two quinones, which displaces bicarbonate from its ligation site at the non-heme iron. A third signal, with g ≈ 1.6 was detected and assigned as described below. Using epr signals, the midpoint potential of Qa/Qa was measured with either bicarbonate or formate bound to the iron. At pH 7.8, both were found to have midpoint potentials of ≈ +25mV. This represented the first direct determination of the redox potential of Qa in the presence of bicarbonate, and suggested that formate does not inhibit simply by affecting the redox potential of Qa. Unlike titrations of in higher plant PS II, there was no indication of further low-potential quinone acceptors. However, the behaviour of a signal due to the interaction of the iron-semiquinone and photoreduced pheophytin indicated some involvement of another electron acceptor, with midpoint potential of -250mV. These results were supported by kinetic optical spectrophotometry. The assignment of the epr signal with g ≈ 1.6 to an interaction of Qa -Fe2+ with Qb- was made. Using this signal, a direct estimate of the midpoint potential for the couple in PS II (≈ +60mV) was made; evidence was found also for pH- dependence of the signal which indicated the possible second reduction of at around this potential. It was found that the g ≈ 1.6 can be either 77K or 200K photoinduced following saturating illumination at room temperature, indicating the dark stability of the Qb- species, . Applying this protocol, an epr signal due to -Fe2+ was proposed; and interactions of a (phenyl- para-benzoquinone) with the non-heme iron could be monitored. Similar signals are seen following 77K illumination of both cyanobacterial and spinach PS II treated with the analogue, tribromotoluquinone. The technique extended X-ray absorption fine-structure (EXAFS) was used to probe of the structure of the manganese complex of PS II from spinach. The results were consistent with an arrangement of the four manganese of the oxygen-evolving complex either as two μ-oxo bridged dimers, or a μ-oxo-bridged trimer with a single monomeric manganese.

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