Formation and characterization of transition metal oligopeptide complexes in aqueous solution

Brigitte Elisabeth Schwederski, Purdue University

Abstract

The products of the reaction of PtCl$\sb4\sp{2-}$ or cis-Pt(NH$\sb3)\sb2$Cl$\sb2$ with $\sp{15}$N-labelled oligoglycine ligands are characterized by $\sp{195}$Pt-NMR spectroscopy. Chemical shifts as well as $\sp1$J$\sb{\rm Pt-N}$ coupling constants are indicative of the platinum coordination sphere. Among others, complexes with deprotonated peptide bonds, i.e. Pt(NH$\sb3)\sb2$(H$\sb{-1}$G$\sb2$), Pt(H$\sb{-2}$G$\sb3$)Cl$\sp{2-}$ and Pt(H$\sb{-3}$G$\sb4)\sp{2-}$, are formed. The peptide-platinum bonds form slowly, but deprotonation occurs at pH values $<$2. This is lower than observed for similar complexes of Pd(II), Cu(II), and Ni(II). A stable bis(triglycinato)palladate(II) complex, Pd(H$\sb{-1}$G$\sb3)\sp{2-}$, (K = 2 $\times$ 10$\sp6$ M$\sp{-1}$) is formed from excess triglycine and Pd(H$\sb{-2}$G$\sb3)\sp-$ via three consecutive reactions. Resolved rate constants are 600 M$\sp{-1}$ s$\sp{-1}$, 4 $\times$ 10$\sp{-3}$ s$\sp{-1}$ and 3 $\times$ 10$\sp{-4}$ s$\sp{-1}$. Only the first reaction is dependent on the concentrations of G$\sb3$ and H$\sp+$. The bis complex is proposed to have the peptide and amine nitrogens in a trans configuration in the equatorial plane and the carboxylic acid groups coordinated axially. A mechanism for its formation from the mono complex is discussed. The rate of substitution of tripeptidocuprate(II), Cu(H$\sb{-2}$L)$\sp-$, with triethylenetetramine at $-$log (H$\sp+$) = 11.5 decreases by eight orders of magnitude upon replacement of triglycine with tri-$\alpha$-aminoisobutyric acid (Aib$\sb3$). Reactions of mixed tripeptides of glycine, L-alanine, and $\alpha$-aminoisobutyric acid show that only methyl groups in the second and third amino acid residues are responsible for the steric effects and that their influence is additive. Hydroxide ion inhibits the reaction due to formation of Cu(H$\sb{-2}$L)OH$\sp{2-}$ where OH$\sp-$ replaces a carboxylate group in an equatorial coordination position. In contrast, the reaction of sterically hindered Cu(H$\sb{-2}$Aib$\sb3)\sp-$ increases in high base because OH$\sp-$ acts as a nucleophile to help displace Aib$\sb3$ from copper. A detailed mechanism for the substitution reactions and their variation with $-$log (H$\sp+$) is discussed. Nitrogen-coordinated (triglycinato)oxovanadium(IV) complexes compete with hydrolysis and oxidation reactions only in high excess of ligand and in the presence of reducing agents. Epr spectral evidence for the formation of these complexes is presented.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry

COinS