The Intrinsic Enzyme Activities of the Classic Polyoxometalates

The mimicking enzyme activities of eighteen classic POMs with different structures, Keggin (H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₄GeW₁₂O₄₀, K₄GeW₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiMo₁₂O₄₀ and Eu₃PMo₁₂O₄₀), Wells-Dawson (H₆P₂Mo₁₈O₆₂, α-(NH₄)₆P₂W₁₈O₆₂ and α-K₆P₂W₁₈O₆₂·14H₂O), lacunary-Keggin (Na₈H[α-PW₉O₃₄], Na₁₀[α-SiW₉O₃₄], Na₁₀[α-GeW₉O₃₄] and K₈[γ-SiW₁₀O₃₆]), the transition-metal substituted-type (α-1,2,3-K₆H[SiW₉V₃O₃₄] and H₅PMo₁₀V₂O₄₀), sandwich-type (K₁₀P₂W₁₈Fe₄(H₂O)₂O₆₈) and an isopolyoxotungstate (Na₁₀H₂W₁₂O₄₂) were screened and compared. The mechanisms and reaction conditions of POMs with mimicking enzyme-like activities were also analyzed. The results shown that the structures, the hybrid atoms, the coordination atoms, the substituted metal atoms, pH and substrate are the effect factors for the enzyme mimic activities of POM. Among the eighteen POMs, H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₄GeW₁₂O₄₀, α-(NH₄)₆P₂W₁₈O₆₂, α-K₆P₂W₁₈O₆₂·14H₂O, Na₈H[α-PW₉O₃₄], Na₁₀[α-SiW₉O₃₄], Na₁₀[α-GeW₉O₃₄], K₈[γ-SiW₁₀O₃₆], K₁₀P₂W₁₈Fe₄(H₂O)₂O₆₈ and Na₁₀H₂W₁₂O₄₂ had the peroxidase activities. Eu₃PMo₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiMo₁₂O₄₀, α-1,2,3-K₆H [SiW₉V₃O₃₄], H₆P₂Mo₁₈O₆₂ and H₅PMo₁₀V₂O₄₀ showed the oxidase-like activities. K₄GeW₁₂O₄₀ did not show the peroxidase and oxidase activities. The Na₈H[α-PW₉O₃₄], Na₁₀[α-SiW₉O₃₄] and Na₁₀[α-GeW₉O₃₄] showed intrinsic enzyme activities at alkaline conditions, which were different from other type of POMs. The sandwich-type K₁₀P₂W₁₈Fe₄(H₂O)₂O₆₈ displayed the strongest peroxidase activity, which is similar to natural horseradish peroxidase.