Abstract
Two redox-active tyrosine residues located on the D1 and D2 polypeptides of the photosystem II reaction center (PS II) have been extensively characterized by EPR spectroscopy. TyrZ (D1 161), which is involved in the electron transfer pathway between P680 and the oxygen-evolving complex, can form a transient radical. The symmetry-related TyrD (D2 160), which has a still mostly unknown function, easily forms the stable radical TyrD° (see [1] for a review). This different behaviour of the two tyrosines may be due to local structural asymmetry. FTIR difference spectroscopy can probe the structure and bonding interactions of a cofactor which changes ionization state upon illumination even in large membrane protein complexes such as PS II. Due to the specificity of the FTIR sample conditions, this approach requires stringent controls of the state generated. Furthermore, the interpretation of the IR difference spectra in vivo is based on comparison with model compound spectra and isotope or mutagenesis effects. In this paper, the TyrD°/TyrD FTIR difference spectrum obtained in PS II membranes is compared to FTIR difference spectra of tyrosine and phenol radicals generated by UV irradiation at low temperature.
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