Longgen Zhu

Hydrolytic cleavage of peptides by palladium(II) complexes is enhanced as coordination of peptide nitrogen to palladium(II) is suppressed

Abstract We report on the kinetics and mechanism of regioselective hydrolysis of amide bonds in various dipeptides and tripeptides after these substrates react with [Pd(H2O)3(OH)]+ and cis-[Pd(en)(H2O)2]2+. Peptides devoid of coordinating side chains form mononuclear palladium(II) complexes and hydrolyze slowly, over weeks. Peptides containing thio ether side chains (of methionine and S-methylcysteine) as anchors form binuclear palladium(II) complexes and hydrolyze rapidly, with half-lives that are measured in minutes. The ethylenediamine ligand stabilizes the initial complex but is released prior to hydrolysis, so that the two mononuclear complexes form similar binuclear promoter complexes with anchoring peptides. Hydrolysis requires acidic solutions, but this reaction is not catalyzed by acid; the palladium(II) promoter is required. Binding of the sulfur-anchored palladium(II) atom to the deprotonated nitrogen atom in the amide bond inhibits hydrolysis. Methylation of the amide nitrogen atom suppresses binding of palladium(II) to it and enhances the hydrolysis rate constant as much as 300-fold.

Molecular orbital study of stereoselective addition of ligands to the hydride, alkyl, and halide complexes of iridium and zirconium. The question of central versus lateral attack

Abstract Electronic structures of Ir(N(SiH3)2)(PH3)2H2, Cp2ZrMe2, Cp2ZrH2, and Cp2ZrCl2 complexes and stereoselectivity of CO addition to each of them are studied by the nonparametrized Fenske-Hall method. A new measure of the interaction between reactants, designated ΔE, is defined and used to calculate energy profiles of the carbonylation reactions. Stereoselectivity seems to be controlled by the composition and localization of the frontier orbitals. The experimental observations are explained successfully: the first two complexes undergo lateral attack, the third one undergoes central attack, and the fourth one is unreactive. The unreactivity of the chloro complex is attributed to the repulsion between the lone pairs of the Cl and CO ligands. The nature of the frontier orbitals, and consequently the preferred approach of an incoming ligand, depends markedly on the LZrL angle: the LUMO is localized laterally when the angle is acute and centrally when it is obtuse. Experiments are proposed that may test the correlation between the facility of ligand-addition reactions and the localization of the frontier orbitals.

Kinetic study of stereochemical and other factors governing hydrolytic cleavage of a peptide ligand in binuclear palladium(II) complexes

The dipeptide N-acetylmethionylglycine (MeCO-Met-Gly) reacted, via the thioether group in the methionyl side chain, with five different palladium(II) aqua complexes. The complexes cis-[Pd(en)(H2O)2]2+(en = H2NCH2CH2NH2) and cis-[Pd(pn)(H2O)2]2+(pn = H2NCH2CH2CH2NH2) yield [Pd2(µ-MeCO-Met-Gly)2(H2O)4]4+A, cis-[Pd(Met-S,N)(H2O)2]2+ yields trans-[Pd2(µ-MeCO-Met-Gly)2(H2O)2(HMet)2]6+, B and trans-[Pd2{Cys(Me)-S,N}2(H2O)2]4+[Cys(Me)=S-methylcysteine] yields trans-[Pd2{µ-HCys(Me)}2(H2O)2(MeCO-Met-Gly)2]6+C. The complex cis-[Pd(dtco)(H2O)2]2+(dtco = 1,5-dithiacyclooctane) yields [Pd2(µ-MeCO-Met-Gly)2(dtco)2]4+, D. These reactions and hydrolytic cleavage of the methionine–glycine amide bond in the co-ordinated MeCO-Met-Gly are conveniently monitored by 1H NMR spectroscopy. The rate of cleavage decreases in the order A > B≈C > D, in which the number of aqua ligands per peptide ligand decreases. Intramolecular attack by aqua ligands is more efficient than external attack by water molecules from the solvent. The peptide ligands occupying terminal and bridging positions in the binuclear palladium(II) complexes undergo hydrolysis at similar rates. This study shows the importance of polynuclear metal complexes in hydrolytic cleavage of peptide bonds.

The binary adduct of 3,6,9,16,19,22-hexaazatricyclo[22.2.2.211,14]triaconta-11,13,24,26(1),27,29-hexaene and benzene-1,2,4,5-tetracarboxylic acid.

The crystals of the title salt, 6,21-diaza-3,9,18,24-tetraazoniatricyclo[22.2.2.2(11,14)]triaconta-11,13,24,26(1),27,29-hexaene benzene-1,2,4,5-tetracarboxylate(4-) hexahydrate, C(24)H(42)N(6)(4+) x C(10)H(2)O(8)(4-) x 6H(2)O, are formed by the intermolecular interaction of a macrocyclic hexamine with a molecule of C(6)H(2)(COOH)(4) in aqueous solution. Both the cation and the anion are on inversion centres. Hydrogen bonds are formed between the four ammonium cations in the hexamine and the four carboxylate anions in the aromatic acid. Stacks exist along the crystallographic a axis in the solid state. The water molecules also take part in a hydrogen-bonding network which joins these stacks together.