Rapid hydrolysis of phosphate ester promoted by Ce(iv) conjugating with a β-cyclodextrin monomer and dimer

Abstract

Two N-donor ligands (L¹ and L²) derived from a β-cyclodextrin (βCD) monomer and dimer were employed to mediate the hydrolytic activity and stability of the Ce(IV) ion in aqueous solution. Complexes Ce(IV)–L¹ and Ce(IV)–L² were prepared in situ and characterized by means of UV-vis and NMR measurements. Ce(IV)–L¹ catalyzed the hydrolysis of a DNA model, bis(4-nitrophenyl)phosphate (BNPP) with k_cat = 5.2 × 10⁻³ s⁻¹ (half-life t_1/2 ≈ 2 minutes) under mild conditions, which represented an approximate 130 million-fold acceleration with respect to the spontaneous hydrolysis of BNPP. The dinuclear species, [Ce₂L¹₂(OH)₅]³⁺, contributed splendidly to the catalytic efficiency which echoed the active species postulation of [Ce₂(OH)₇]⁺ in the literature. Ce(IV)–L² exhibited efficient binding with BNPP giving 1/K_M = 2.1 × 10⁵ M⁻¹ which exceeded other Ce(IV) species, e.g. [Ce₄(OH)₁₅]⁺, by 2 orders of magnitude, which highlighted the hydrophobicity effect of βCDs. Such a highly binding affinity leads to the second-order rate constant, k_cat/K_M = 2.3 × 10² M⁻¹ s⁻¹, which probably ranks as the highest in the non-enzymatic cleavage of BNPP under similar conditions. Additionally, Ce(IV)–L² showed favorable tolerance to basic aqua owing to the bulky protection of double βCD pendants.