Photosystem tyrosyl radical

Un, S., M. Atta et al. (1995). g-values as a probe of the local protein environment High-field EPR of tyrosyl radicals in ribonucleotide reductase and photosystem II. J. Am. Chem. Soc. 117 10713-10719. 

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

It is interesting to note that a manganese-organic radical system critical for catalysis is not unique. A related Mn-tyrosyl radical(s) ensemble is known to operate in Photosystem II (discussed later). 

In an important step to mimic the natural photosystem, tyrosine residues tethered to a Rubpy sensitizer as in 15 have been shown to reduce the Rum center obtained after oxidative quenching with methylviologen or [Co(NH3)5C1]2+.187 Formation of the resulting tyrosyl radical is a proton coupled process and it has been shown to be a concerted process in which the reorganization energy associated with deprotonation can be tuned by H-bonding and pH.188191 Similar results are observed for tyrosyl residues tethered to Re(I)diimine based chromophores.192... 

The next illustration of how DFT can be appHed to bioinorganic systems is the oxidation of water in photosystem II. Two different tyrosyl radicals have been detected with one of them, Tyrz, near the water oxidising complex (woe). The woe involves a Mn4 cluster which mediates the evolution of one molecule of O2 from a molecule of water for every four photons absorbed. 

Figure 9 ESR spectra of free thyrosyl radical TyrD photogenerated in Mn-depleted photosystem II at 4K (A) and warming to 200 K (B). The spectral change near 0.3362T is attributed to a thermally activated deprotonation of the Yd radical. (Reproduced with permission from Faller P, Goussias C, Rutherford AW, and Un S (2003) Resolving intermediates in biological proton-coupled electron transfer A tyrosyl radical prior to proton movement. Proceedings of the National Academy of Sciences of the USA 100 8732-8735.)...

Examples of the application of pulsed techniques include the distance between spin-labels attached to p-93 cysteine in hemoglobin (70 the distance between the Mn4 cluster and the dark stable tyrosyl radical in photosystem II (83), and distances between spin labels in model compounds and polypeptides (72, 73, 75, 76, 78, 79). Several papers have examined the effect of distributions of distances and orientations on the 2+1 and DEER ESE results (48, 67, 76). 

The realization of the widespread occurrence of amino acid radicals in enzyme catalysis is recent and has been documented in several reviews (52-61). Among the catalytically essential redox-active amino acids glycyl [e.g., anaerobic class III ribonucleotide reductase (62) and pyruvate formate lyase (63-65)], tryptophanyl [e.g., cytochrome peroxidase (66-68)], cysteinyl [class I and II ribonucleotide reductase (60)], tyrosyl [e.g., class I ribonucleotide reductase (69-71), photosystem II (72, 73), prostaglandin H synthase (74-78)], and modified tyrosyl [e.g., cytochrome c oxidase (79, 80), galactose oxidase (81), glyoxal oxidase (82)] are the most prevalent. The redox potentials of these protein residues are well within the realm of those achievable by biological oxidants. These redox enzymes have emerged as a distinct class of proteins of considerable interest and research activity. 

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