Steric effects, solvent effects, and turnover in hydrolytic cleavage of peptides promoted by palladium(II) aqua complexes |
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| Authors: | Xiaohua Chen L Zhu Xiaozeng You Nenad M Kosti? |
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| Institution: | (1) State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China Fax: +86-25-331-4502, CN;(2) Department of Chemistry, Iowa State University, Ames, IA 50011, USA, US |
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| Abstract: | Dipeptides and tripeptides AcMet-aaH containing N-acetyl methionine, in which the group aaH is GlyH, AlaH, ValH, or Gly-GlyH, undergo hydrolytic cleavage of the Met-aaH peptide
bond in the presence of the following complexes of palladium(II): cis-Pd(en)(H2O)2]2+, cis-Pd(tn)(H2O)2]2+, cis-Pd(en)(CH3OH)2]2+, cis-Pd(S,N-MetH)(H2O)2]2+, cis-Pd(S,N-Met-GlyH)(H2O)2]2+, and cis-Pd(S,N-Met-AlaH)(H2O)2]2+. These mononuclear complexes are precursors of binuclear palladium(II) complexes containing the substrates AcMet-aaH as bridging
thioether ligands. The rate constant for cleavage is higher when the bidentate ligand in the precursor complex is ethylenediamine
(which is completely displaced) than S,N-methionine (of which only the amino group is displaced), because the number of aqua ligands available for cleavage is greater
in the former than in the latter case. The demonstrated dependence of the rate constant on the steric bulk (volume) of the
leaving group, aaH, points the way toward achieving a degree of sequence selectivity in cleavage of peptide bonds by palladium(II)
aqua complexes. One equivalent of cis-Pd(en)(H2O)2]2+ cleaves as many as ten equivalents of AcMet-GlyH, but the rate constant decreases as the molar excess of the dipeptide over
the catalyst increases. This demonstration of catalytic turnover points the way to our ultimate goal – artificial metallopeptidases.
Received: 13 June 1997 / Accepted: 24 September 1997 |
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| Keywords: | Peptides Hydrolysis Palladium Kinetics Turnover |
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