Synergic effect of two metal centers in catalytic hydrolysis of methionine-containing peptides promoted by dinuclear palladium(ii) hexaazacyclooctadecane complex?

Dalton Transactions Pub Date: 2005-03-31 DOI: 10.1039/B500210A

Abstract

The species obtained by the reaction of [Pd2([18]aneN6)Cl2](ClO4)2 (where [18]aneN6 is 1,4,7,10,13,16-hexaazacyclooctadecane) with AgBF4 have been determined by electrospray ionization mass spectrometry (ESI-MS) to be an equilibrium mixture of three major types of dinuclear Pd(II) complex cations, [Pd2(μ-O)([18]aneN6)]2+, [Pd2(μ-OH)([18]aneN6)]3+ and [Pd2(H2O)(OH)([18]aneN6)]3+, in aqueous solution. The hydroxo-group-bridged one, [Pd2(μ-OH)([18]aneN6)]3+, is a dominant species, whose crystal structure has been obtained. The crystal structure of [Pd2(μ-OH)([18]aneN6)](ClO4)3 shows that each Pd(II) ion in the dinuclear complex is tetra-coordinated by three nitrogen atoms and one hydroxo group bridge in a distorted square configuration. The two Pd(II) ions are 3.09 ? apart from each other. The dinuclear Pd(II) complex cations [Pd2(μ-OH)([18]aneN6)]3+ and [Pd2(H2O)(OH)([18]aneN6)]3+ can efficiently catalyze hydrolysis of the amide bond involving the carbonyl group of methionine in methionine-containing peptides with turnover number of larger than 20. In these hydrolytic reactions, the two Pd(II) ions are synergic; one Pd(II) ion anchors to the side chain of methionine and the other one delivers hydroxo group or aqua ligand to carbonyl carbon of methionine, or acts as a Lewis acid to activate the carbonyl group of methionine, resulting in cleavage of Met–X bond. The binding constant of dinuclear Pd(II) complex cations with AcMet–Gly and AcMet were determined by 1H NMR titration to be 282 ± 2 M?1 and 366 ± 4 M?1, respectively. The relatively low binding constants enable the catalytic cycle and the possible catalytic mechanism is proposed. This is the first artificial mimic of metallopeptidases with two metal active centers.

Graphical abstract: Synergic effect of two metal centers in catalytic hydrolysis of methionine-containing peptides promoted by dinuclear palladium(ii) hexaazacyclooctadecane complex
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