Oxidation of copper(II) complexes of histidine-containing tri- and tetrapeptides by L-ascorbic acid and hydrogen peroxide to copper(III) and its subsequent decay to geometric isomers of α,β-dehydrohistidyl tautomers

Samantha Kaye Burke, Purdue University

Abstract

The reaction of CuII(H−2Gly 2HisGly)− with L-ascorbic acid (H2A) and H2O2 at p[H+] 6.6 rapidly generates CuIII(H−2Gly2HisGly), which is separated chromatographically and has λmax at 260 and 396 nm. The combination of both H2O2 and H2A are essential to form the Cu(III)-intermediate, which is believed to assist in site-specific DNA cleavage. The Cu(III) complex decomposes to give two forms of glycylglycyl-α,β-dehydrohistidylglycine, which are characterized. The alkene species are present as geometric isomers and imidazole tautomers of one another and were identified by mass spectrometry, 1H-NMR, and FT-IR. The major isomeric form (Z-Nτ-H) has three conjugated double bonds and λmax at 295 and 360 nm at p[H+] 6.6. The minor alkene (E-Nπ-H) has a system of four conjugated double bonds with its λmax at 295 and 370 nm at p[H+] 6.6. Interconversion between geometric isomers is not observed, but imidazole tautomer interconversion for each geometric isomer results in sixteen isomeric forms. An increase (pH > 9) or decrease (pH < 2) in pH, then adjustment to neutral pH, gives ratios of 7:3 ( Z isomer) and 9:1 (E isomer) for chromatographically-separated imidazole tautomers. Low pH alteration favors Nπ-H, while high pH favors Nτ-H. Imidazole protonation constants for Z-Nτ-H, E-Nτ-H, E-Nτ-H, and Z-Nπ-H isomers are 3.7(1)/8.4(1), 2.2(8)/7.1(6), 2.4(6)/7.5(4), and 3.3(9)/7.8(8) (μ = 1.0 M (NaClO4)), respectively. Additionally, reduction potentials for Cu(III/II)-Z-Nτ-H and Cu(III/II)- E-Nπ-H are 1.24 and 0.946 V (vs. NHE) at p[H +] 6.73, respectively. The conjugation in the structure creates increased electron delocalization in the peptide; thus, resulting in lower imidazolium nitrogen pKa values, higher reduction potentials, and enhanced Cu(II) binding. It also leads to a rapid hydrolysis reaction that gives Gly2NH2 and imidazolylpyruvylglycine as products. The oxidation reaction of Cu(II)Gly2HisNH2 by H 2O2/H2A also produces Cu(III) and alkene isomers with the same properties as Gly2-α,β-dehydro-HisGly.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry|Biochemistry|Chemistry

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