Electron paramagnetic resonance radical site probes

Photosystem II (PSII) contains a remarkably stable tyrosine radical D", located at tyr-160 in the D2 polypeptide, which exhibits a well-known electron paramagnetic resonance (EPR) spectrum, EPR Signal Ilg (1). The function of D" " in the mechanism of photosynthetic H2O oxidation remains unclear despite evidence that it can oxidize the Mn complex, the H20-oxidation catalyst in PSII, from the Sq state to the normally daik-stable Si state (2). Several studies indicate that die oxidation state of the Mn complex influences the electron spin-lattice relaxation rate of D" (3-5), perhaps via a weak dipolar coupling, as suggested by Evelo et al. (5). Hence, the relaxation properties of D may provide a probe for the topology of redox-active sites in the 02-evolving center (OEC) and of the magnetic properties of the Mn complex. 

Kamnakaran, C., Zhang, H., Crow, J.P., Anthohne, W.E., Kalyanaraman, B., 2004. Direct probing of copper active site and free radical formed during bicarbonate-dependent peroxidase activity of bovine and human copper, zinc-superoxide dismutases. Low-temperature electron paramagnetic resonance and electron nuclear double resonance studies. J. Biol. Chem. 279, 32534—32540. 

The basic idea underlying the physical labelling approach is the modification of the chosen sites of the object in question by specific compounds, which are boimd covalently (labels) and/or non-covalently (probes), whose properties make it possible to trace the state of the surrounding biological matrix by appropriate physical methods. The following main types of compounds are used as labels and probes to monitor the dynamic parameters of proteins (1) centers with unpaired electrons (stable nitroxide radicals, radical pairs and paramagnetic complexes) exhibit electron spin resonance (ESR), (2) luminescent fluorescence and phosphorescence chromophores, and (3) Mossbauer atoms (e.g. Pe) which gives the nuclear y-resonance (NGR) spectra. 

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