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ELECTRON TRANSFER FROM THE EXCITED STATE OF TYROSINE TO COMPOUNDS CONTAINING DISULFIDE LINKAGES
Authors:J. FEITELSON  E. HAYON
Affiliation:Pioneering Research Laboratory, U.S. Army Natick Laboratories, Natick, Massachusetts 01760, U.S.A.
Abstract:Abstract— The flash photolysis of aqueous solutions of tyrosine has been studied in the presence of various concentrations of the cyclic disulfide sodium lipoate (thioctic acid, Na+ salt). In addition to the formation of phenoxyl radicals and hydrated electrons (and possibly H atoms) from the photoionization of tyrosine, the characteristic spectrum of the radical anion RSSR- of lipoate was also observed in neutral as well as in alkaline solutions. From the dependence of these yields upon the concentration of lipoate, it was found that a long–lived triplet excited state of tyrosine, rather than the singlet excited state, is involved in these reactions. The negative radical ions RSSR- are formed by two distinct pathways: (a) Na+–lipoate reacts with the solvated electrons which are ejected from the tyrosine triplets 3Tyr → RO.+ e -aq+ H+ followed by e -aq+ RSSR → RSSR-, and (b) by direct interaction of lipoate with triplet excited tyrosine, resulting in the transfer of a negative charge from tyrosine to the disulfide linkage. At high lipoate concentrations, the singlet excited state of lipoate is quenched, k 4= 1.6 × 1010 M -1 sec-1, but this reaction does not lead to the formation of RSSR- radical ions.
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