the Institute of Experimental Physics, University of Bialystok
and the Institute of Atomic Energy, Swierk
under the auspices
of the European Crystallographic Association;
Committee of Crystallography, the Polish Academy of Sciences;
and the Polish Neutron Scattering Society
The School is financially supported in part by
the International Union of Crystallography;
the European Crystallographic Association;
the European Office of Aerospace Research & Development;
the Ministry of Science and Information Society Technologies;
the Warsaw University of Technology;
and the University of Bialystok.
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Kvìtoslava Burda1,2 and Jan Stanek2
1 Institute of Nuclear Physics PAS, ul. Radzikowskeigo 152, 31-342 Kraków, Poland;
2 Institute of Physics, Jagiellonian University, ul. Reymonta 4, 30-059 Kraków, Poland
Topics
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Photosynthetic organisms convert light energy into chemical energy in their reaction centers. The appearance of oxygen-producing organisms that used water as a reductant for photosynthesis was a major breakthrough in the evolution of life on Earth. The invention of the water splitting mechanism is responsible for conversion of our atmosphere from anaerobic to its present molecular oxygen (O2) rich composition. Without oxygen, life could not have evolved to its contemporary level of complexity.
Knowledge of the underlying principles of the natural systems, which are able to exploit solar energy in highly efficient way can inspire construction of biomimetic devices and development of new technologies of energy production and its storage.
Photosystem II (PSII), being a place of water cleavage, is a multimeric protein complex, which is incorporated into the thylakoid membranes. There are many redox cofactors participating in the photosynthetic electron chain within PSII. Although, the tertiary structure of PSII is presently known, the action of some of its redox components is not recognized. For example, the action of cytochrome b559 has not yet been explained. Participation of cytochrome b559 in the cyclic electron transport or in a side path of electron flow through PSII has been suggested as a protection against photoinhibition. Its role in stabilization of oxygen evolution and/or proton acceptor during photoactivation has been discussed, as well as its action as a scavenger of photogenerated free radicals. Cytochrome b559 has an unusually high and variable midpoint reduction potential, which can be influenced by the spin and valence state of the heme-iron. The function of cytochrome b559 is probably associated with the conversion between the high and low potential forms, but the mechanism of the switch between these two potential forms is not known. Another intriguing component of photosystem II is the non-heme iron located between two quinone sites. The non-heme iron appears in a reduced high spin state in the reaction centers of photosynthetic organisms (type II). It has been shown that in higher plants it can undergo redox changes Fe+2/ Fe+3 due to the reduction /oxidation process induced by some quinones. It is difficult to remove the non-heme iron. There are some discrepancies as regards to the influence of the iron depletion on the kinetics of linear electron transport within PSII.
To gain more insight into the role of non-heme iron and cytochrome b559 in the electron and proton transport process within photosystem II, it is important to know the electronic and structural properties of these two iron binding sites. EPR and Mössbauer spectroscopy have already proven to be powerful tools in such studies. The Mössbauer method permits to study of diamagnetic states of Fe+2, which are EPR silent. Temperature measurements of the Mössbauer recoil-free fraction yield information on lattice dynamics. Investigations of the vibrational and collective motions of the Mössbauer probe give information on local modes, which are unobtainable by other techniques.
We present a thorough study of new valence and spin states of the non-heme and heme iron as well as the characterization of their collective modes in intact thylakoid membranes. We indicate the role of the non-heme iron and cytochrome b559 in the cyclic electron flow around photosystem II and in the process of energy dissipation.