Gaosheng Yang

New insights into the palladium-mediated selective hydrolysis of the His18–Thr19 peptide bond in cytochrome c: 1H NMR and density functional theory investigation for model compounds

The peptides, CH3CO-Met-His-GlyH, CH3CO-CysMe-His-GlyH, CH3CO-CysMe-His-Gly-OEt and its imidazole derivatives, Nτ-benzyl, Nτ-tosyl, Nτ-benzyl-Nπ-phenacyl, have been synthesized and used as model compounds for the mechanistic study of the selective cleavage of cytochrome c promoted by Pd(II) complexes. The peptide bond cleavage of these substrates by cis-[Pd(en)(solvent)2]2+ n(solvent: D2O or CD3OD) was monitored by 1H NMR spectroscopy. The results showed that the methionine-containing tripeptide differs from the S-methylcysteine-containing tripeptides in the mode of coordination to Pd(II). The former coordinates to Pd(II) through a sulfur atom, an amide nitrogen of methionine and an Nπ atom of imidazole, forming a polycyclic chelate, and is resistant to hydrolysis. The latter, as a model compound for cleavage of the His18–Thr19 bond in cytochrome c with Pd(II) complexes, coordinates to Pd(II) nvia a sulfur atom, an amide nitrogen and a carbonyl oxygen of histidine to form a polycyclic chelate in which the His–Gly peptide bond is cleaved. Kinetic studies showed that protonation of the Nπ atom of imidazole in the S-methylcysteine-containing tripeptides is one of the key factors in controlling the cleavage of the His–Gly bond. In order to obtain theoretical guidance on the cleavage reaction, the geometries of a representative Nπ protonated tripeptide cation of CH3CO-CysMe-His-GlyNMe and its Pd(II) complex with and without ancillary water molecules are optimized at the B3LYP density functional theory level using 3-21G, 6-31G(d) and LanL2DZ basis sets. Based on the experimental and theoretical results obtained from the model compounds, a mechanism is proposed for the first time to explain the nature of selective cleavage of the His18–Thr19 bond in cytochrome c promoted by Pd(II) complexes. Coordination of Pd(II) to the carbonyl oxygen of histidine and hydrogen bond formed between the CO and ancillary dimer water weaken and polarize the CO double bond of histidine, giving rise to cleavage of the peptide bond.

Dimeric water embedded within a hydrophobic cavity of tetra-(p-tert-butyl)thiacalix[4] arene

Abstract X-ray crystallographic analysis and density functional B3LYP/6-31G(d) calculation confirm that dimeric water is embedded within a hydrophobic cavity of tetra-(p-tert-butyl)thiacalix[4]arene and stabilized by hydrogen bondings of aromatic π⋯H2O(1) and methyl⋯H2O(2) in the dimeric water inclusion complex with binding energies of 1.4 and 0.9xa0kcalxa0mol−1 respectively, and by hydrogen bonding formed between H2O(2) and four phenolic OH groups from an adjacent tetra-(p-tert-butyl)thiacalix[4]arene, with binding energy of 3.8–4.2xa0kcalxa0mol−1.

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

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(mu-O)([18]aneN6)]2+, [Pd2(mu-OH)([18]aneN6)]3+ and [Pd2(H2O)(OH)([18]aneN6)](3+), in aqueous solution. The hydroxo-group-bridged one, [Pd2(mu-OH)([18]aneN6)]3+, is a dominant species, whose crystal structure has been obtained. The crystal structure of [Pd2(mu-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 A apart from each other. The dinuclear Pd(II) complex cations [Pd2(mu-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.