Phenoxyl-carbon bond

More recently, MPO-mediated oxidation of tyrosine to dityrosine (o o -dityrosine, or 3,3 -diiyrosine) focused attention as a marker reaction of neutrophile-dependent oxidative damage of proteins and peptides (G11, H14, S3). The reaction occurs both with free tyrosine as well as with tyrosyl residues incorporated into polypeptide structures. The mechanism of dityrosine formation utilizes a relatively long-lived phenoxyl radical that cross-links to dimeric and polymeric structures by formation of carbon-carbon bonds between the aromatic moieties of phenolic tyrosine residues (H14) (Fig. 9). 

Fig. 23. Proposed mechanisms for Tyr-Cys cofactor biogenesis. (A) Initial Cu(II) binding leads to the appearance of resonance contributions from a reactive phenoxyl that electrophilically attacks the neighboring Cys-228 thiol. (B) Cu(I) binding produces an oxygen-reactive complex that drives hydrogen abstraction from Cys-228 to form a thiyl free radical which subsequently attacks the neighboring Tyr-272 ring with formation of a carbon-sulfur covalent bond.

Bond distances for phenoxyl radical, calculated by using CASSCF,(25, 34) UHF, UMP2, and various density functional methods.(2 ) Carbon atom numbering starts with Ci bonded to the oxygen and proceeds around the ring. Except for the first column of CASSCF calculations. 

Phenoxyl radicals react with each other mainly by coupling (or dimerization). Second-order decay of transient phenoxyl radicals takes place with rate constants of the order of 10 M g-i 272,296,338,352,353 leads to formation of dimeric products. Various dimers are formed by combination at the various radical sites. Since the unpaired spin is delocalized on the oxygen and on the ortho and para carbons, dimers result from combination of O with C and of C with C (equation 34). Dimers containing 0—0 bonds are less stable and generally were not detected. 

Carbon black can increase the thermal stability of many polymers because of its properties. Phenoxyl and quinoid groups on the surface of carbon black function as antioxidants. These groups also participate in the catalytic decomposition of peroxides which contributes to a reduction in degradation rate. Quinone, polynuclear structures, polyconjugated double bonds, and carbonyl groups all scavenge radicals. Many polymers and rubbers benefit from these properties of carbon black. 

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