Article

Characterization of Copper(III)−Tetrapeptide Complexes with Histidine as the Third Residue

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Inorg. Chem., 1997, 36 (14), pp 3119–3124
DOI: 10.1021/ic9608713
Publication Date (Web): July 2, 1997
Copyright © 1997 American Chemical Society

Synopsis

Copper(III)−tetrapeptide complexes of Gly2HisGly and Aib2HisGly are strong oxidizing agents with reduction potentials (V vs NHE) in neutral solution of 0.978 and 0.826, respectively. The complexes do not decompose as rapidly as the corresponding tripeptide complexes that have a terminal histidine residue (where oxidative decarboxylation can occur). At pH 2−3, the half-life of Cu(H-2Gly2HisGly) is 120 s. The decomposition rate increases rapidly above pH 4 and is general-base catalyzed due to abstraction of a proton from the α-carbon of the histidyl residue. The peptide is oxidized to Gly2-α-OH-HisGly, which dehydrates to give the alkene Gly2-α,β-dehydro-HisGly.

Abstract

Abstract Image

Copper(III) complexes of Gly2HisGly and Aib2HisGly are characterized, where Gly is glycine, His is l-histidine, and Aib is α-aminoisobutyric acid. Their respective reduction potentials (V vs NHE) are 0.978 and 0.826. Both Cu(III) complexes undergo amine deprotonation with pKa values of 8.79 and 8.81, respectively. The influence of the 5−5−6 membered chelate rings on the E°‘ and pKa values is examined relative to the Cu(III)−tripeptide complexes without histidine that have 5−5−6, 5−6−5, 6−5−5, and 5−5−5 membered linked-consecutive rings. The presence of a six-membered ring in the third peptide residue causes a decrease of ∼1.0 pKa unit relative to a 5−5−5 membered ring system. Imidazole coordination from histidine compared to carboxylate coordination causes an additional decrease of 1.3 pKa units. Decompositions of CuIII(H-2Gly2HisGly) and CuIII(H-2Aib2HisGly) complexes are measured over the pH range 0.3−4.7. The kinetics and the reaction products show that abstraction of a proton at the α carbon of the histidyl residue is the rate-determining step. The initial decomposition product of Gly2HisGly is a tetrapeptide with an α-hydroxyhistidyl residue that dehydrates to give an alkene with an α,β-dehydrohistidyl residue. UV−vis spectral properties are reported for the alkene as well as the Cu(III) complexes of Gly2HisGly and Aib2HisGly.

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Article Views: 569 Times
Received 24 July 1996
Published online 2 July 1997
Published in print 1 July 1997
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