pH-tuned metal coordination and peroxidase activity of a peptide dendrimer enzyme model with a Fe(ii)bipyridine at its core

Abstract

Peptide dendrimer BP1 was obtained by double thioether bond formation between 5,5′-bis(bromomethyl)-2,2′-bipyridine and two equivalents of peptide dendrimer N1 (Ac-Glu-Ser)₈(Dap-Glu-Ala)₄(Dap-Amb-Tyr)₂Dap-Cys-Asp-NH₂ (Dap = branching 2,3-diaminopropanoic acid, Amb = 4-aminomethyl-benzoic acid). At pH 4.0 BP1 bound Fe(II) to form the expected tris-coordinated complex [Fe^II(BP1)₃] (K_f = 2.1 × 10¹⁵ M⁻³). At pH 6.5 a monocoordinated complex [Fe^II(BP1)] was formed instead (K_f = 2.1 × 10⁵ M⁻¹) due to electrostatic repulsion between the polyanionic dendrimer branches, as confirmed by the behavior of three analogues where glutamates were partially or completely replaced by neutral glutamines or positive lysines. [Fe^II(BP1)] catalyzed the oxidation of o-phenylenediamine with H₂O₂ with enzyme-like kinetics (k_cat = 1.0 min⁻¹, K_M = 1.5 mM, k_cat/k_uncat = 90 000) and multiple turnover, while Fe²⁺ or [Fe(bipy)₃]²⁺ were inactive. The labile coordination positions allowing coordination to H₂O₂ and to the substrate are likely responsible for the enhanced peroxidase activity of the metallopeptide dendrimer.