Cu(II)Gly2HisGly Oxidation by H2O2/Ascorbic Acid to the Cu(III) Complex and Its Subsequent Decay to Alkene Peptides

Protonation and stability constants for Gly2HisGly and its Cu(II) complexes ((beta)mhl), determined at 25.0 C and (mu)= 0.10 M (NaClO4), have values of log (beta)011 = 7.90, log (beta)021 = 14.51, log (beta)031 = 17.55, log (beta)101 = 7.82, log (beta)1-21 = -0.80, and log (beta)1-31 = -12.7 (where the subscripts refer to the number of metal ions, protons, and ligands, respectively). The reaction of CuII(H-2Gly2HisGly)- with L-ascorbic acid and H2O2 at p[H+] 6.6 rapidly generates CuIII(H-2Gly2HisGly) with (lambda)max at 260 and 396 nm, which is separated chromatographically. The Cu(III) complex decomposes to give alkene peptide isomers of glycylglycyl-(alpha),(beta)-dehydrohistidylglycine that are separated chromatographically and characterized. These alkene peptide species are present as geometric isomers with imidazole tautomers that have distinct spectral and chemical characteristics. The proposed major isomeric form (94%), Z-N(tau)-H, has a hydrogen on the nitrogen of the imidazole ring and three conjugated double bonds. It has absorption bands at 295 and 360 nm at p[H+] 6.6 in the absence of copper. The proposed minor isomeric form (6%) is E-N(pi)-H with a hydrogen on the (pi) nitrogen of the imidazole ring. This isomer has four conjugated double bonds and strongly binds Cu(II) to give a complex with intense absorption bands at 434 and 460 nm.