Hydroxyl radical aromatic amino acids

The hydroxyl radical reacts at nearly a diffusion limited rate (k 1 x 10 s to 5 X 10 s ) with the aromatic amino acids, as well as with me-... 

The poly (amino acid)s with aromatic side chains behave somewhat differently. In poly(phenylalanine) the a-carbon radical is the major radical species observed, but radicals formed by hydrogen atom addition to the ring are also found. Benzyl radicals formed by side-chain cleavage are present, but only in very low yield. In poly (tyrosine) the only radical species observed is the tyrosyl phenoxyl radical formed by loss of the hydroxyl hydrogen. There is no evidence for formation of significant concentrations of a-carbon radicals. Thus, the nature of the substituents can strongly influence the radiation sensitivity of the backbone chain. 

Reactions initiated by H and OH would lead to these and other radicals. H could react at the backbone C-H and at the carbonyl group, forming the peptide radical, but is more likely to add to aromatic or heterocyclic amino acid moieties, forming side chain radicals. OH could also react by abstraction or, more likely, by addition. The formation of addition radicals facilitates crosslinking through the side chains. Studies with polyphenylalanine peptides confirm such crosslinking and show how crosslinked products with and without hydroxyl groups can be produced [15, 16]. 

Phenols.—The ring hydroxyl compounds take the class name of phenols from the simplest member, hydroxy benzene or phenol. These are true aromatic compounds and in methods of formation, reactions and properties are distinctly different from aliphatic hydroxyl compounds or alcohols. Their outstanding distinction is their marked acid character, the alcohols being neutral (p. 103). This is attributed to the influence of the phenyl radical (CeHs—). The same influence is present in the amino derivatives, for the ring amines, CeHs— NH2, /CH3... 

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