Reevaluation of the Kinetics of Polynuclear Mimics for Manganese Catalases

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• 作者：Sandra Signorella ; Annette Rompel ; Klaudia Bü ; ldt-Karentzopoulos ; Bernt Krebs ; Vincent L. Pecoraro ; Jean-Pierre Tuchagues
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：December 10, 2007
• 年：2007
• 卷：46
• 期：25
• 页码：10864 - 10868
• 全文大小：71K
• 年卷期：v.46,no.25(December 10, 2007)
• ISSN：1520-510X

文摘

Considerable effort has been expended in order to understand the mechanism of manganese catalases and todevelop functional mimics for these enzymes. For many years, the most efficient reactivity mimic was [Mn^IVsalpn(-O)]₂ [H₂salpn = 1,3-bis(salicylideneiminato)propane], a compound that cycles between the Mn^IV₂ and Mn^III₂oxidation levels instead of the Mn^II₂ and Mn^III₂ oxidation states used by the enzyme, with k_cat = 250 s^-1 and k_cat/K_M= 1000 M^-1 s^-1. Recently, a truly exceptional high value of k_cat was reported for the complex [Mn(bpia)(-OAc)]₂²⁺[bpia = bis(picolyl)(N-methylimidazol-2-yl)amine]. On the basis of a calculated k_cat value of 1100 s^-1 and an efficiencyk_cat/K_M of 34 000 M^-1 s^-1, this complex has been suggested to represent a significant breakthrough in catalyticefficiencies of manganese catalase mimics. However, a plot of r_i/[cat]_T vs [H₂O₂]₀, where the saturation valueapproaches 1.5 s^-1, is inconsistent with the 1100 s^-1 value tabulated for k_cat. Similar discrepancies are observedfor two other families of manganese complexes containing either a Mn₂(-OPh)₂²⁺ core and different substitutedtripodal ligands or complexes of methyl and ethyl salicylimidate, with an Mn₂(-OPh)₂⁴⁺ core. Reevaluation of thekinetic parameters for these three systems reveals that the originally reported values were overestimated by afactor of ~1000 for both k_cat and k_cat/K_M. We discuss the origin of the discrepancy between the previously publishedkinetic parameters and the newly derived values. Furthermore, we provide a short analysis of the existing manganesecatalase mimics in an effort to provide sound directions for future investigations in this field.