title: Methionyl----tyrosyl radical transitions initiated by Br2-. in peptide model systems and ribonuclease A.
abstract: A series of radical transitions, Br2-.----Met(S therefore Br)----Trp(indolyl)----Tyr (phenoxyl), has been demonstrated by pulse radiolysis of N2O-saturated aqueous solutions containing Br-, Met-Gly and Trp-(Gly)2-Tyr at pH 6.7. The intramolecular Met(S therefore Br)----Trp(indolyl) transition in the dipeptide Met-Trp is shown to proceed via the Trp+. radical cation, with a rate constant of k approximately 10(7)s-1, consistent with an electron transfer. Br2-.-attack upon ribonuclease A (RNase) leads to a fast Met(S therefore Br)----Tyr(phenoxyl) process, k = (4.0 +/- 1.0) X 10(5)s-1, probably involving the solvent-exposed Met-29 and the adjacent Tyr-25. Phenoxyl dimerization in the RNase system produces the characteristic o,o'-biphenol fluorescence, but a competing interaction of the Tyr-25(phenoxyl) with the 26-84 disulphide group also appears possible.
mesh_terms: D001966:Bromine; D004579:Electron Transport; D005609:Free Radicals; D008715:Methionine; D010455:Peptides; D011675:Pulse Radiolysis; D012259:Ribonuclease, Pancreatic
chemical_list: D005609:Free Radicals; D010455:Peptides; D008715:Methionine; D012259:Ribonuclease, Pancreatic; D001966:Bromine