Comparative Study of the Reactivity of Zirconium(IV)-Substituted Polyoxometalates towards the Hydrolysis of Oligopeptides

Abstract

The hydrolytic activity of the ZrIV-substituted Keggin-type (Et₂NH₂)₈[{α-PW₁₁O₃₉Zr-(μ-OH)(H₂O)}₂]·7H₂O (1), Lindqvist-type (Me₄ N)₂[W₅O₁₈Zr(H₂O)₃] (2), and Wells–Dawson-type Na₁₄[Zr₄(P₂W₁₆O₅₉)₂(μ₃-O)₂(OH)₂(H₂O)₄]·57H₂O (3) polyoxometalates (POMs) towards the peptide bonds in the oligopeptides triglycine, tetraglycine, glycylglycylhistidine, and glycylserylphenylalanine was investigated by kinetic methods and multinuclear NMR spectroscopy. ³¹P NMR and UV/Vis spectroscopy showed that 1–3 were stable under the conditions used to study peptide bond hydrolysis. The reactivity of 1–3 towards oligopeptides was compared on the basis of the amount of free glycine produced at a certain time increment. In the presence of 1–3, rate constants in the range 6.25 × 10⁻⁷ to 10.14 × 10⁻⁷ s⁻¹ were obtained, whereas no hydrolysis was observed after one month in the absence of these POMs. The results showed that the Keggin-type complex 1 was the most active towards peptide bond hydrolysis in tri- and tetrapeptides. ¹H and ¹³C NMR spectroscopy showed that triglycine, tetraglycine, and glycylserylphenylalanine interact with 1 and 2 preferentially through the amine nitrogen atom and the N-terminal amide oxygen atom to activate the peptide bond towards hydrolysis. The coordination of glycylglycylhistidine resulted in multiple complexes with 1–3 as a result of additional imidazole coordination to the Zrᶦᵛ centers.