TAML activators

MECHANISTIC CONSIDERATIONS ON THE REACTIVITY OF GREEN Fe "-TAML ACTIVATORS OF PEROXIDES... 

Chart 1. Feni-TAML activators mentioned in this account are shown as pentacoordinated species with an axial aqua ligand as obtained in the solid state. The aqua complexes are synthesized as such or as the corresponding chloro species with Cl- instead of H20 (with two M+ = Li+, Na +, NR4 counter ions). The subscript Cl as in lei is used throughout to indicate the corresponding chloro species. 

Scheme 1. Speciation of Feni-TAML activators in aqueous solution (solid rectangle) and suggested mechanism of the H+-induced deme-talation (dashed rectangle) — — = free base ligand. From Ref. (13).

Equilibrium Constants for Fem-TAML Activators 1 Reacting with Ligands, L, in Aqueous Solution at 25°C, 0.1 M KPF0 (27)... 

ALEXANDER D. RYABOV AND TERRENCE J. COLLINS III. Kinetics and Mechanisms of Demetalation of Fem-TAML Activators A. Specific Acid Catalysis... 

V. Mechanism of Catalysis by Tetraamide Macrocyclic Fem-TAML Activators of Hydrogen Peroxide, Functional Catalase-Peroxidase... 

Scheme 6. General mechanistic picture of catalysis by Fem-TAML activators of peroxides that underscores the peroxidase-like (peroxida-tic) and catalase-like (catalatic) activities of the catalysts. Here, S(red) is an oxidizable target substrate. All rate constants are conditional (pH dependent).

The rate constants k and ku for la calculated from the data in Fig. 14 of ca. 3.5 x 103 and 1.5 x 104M 1s 1 (pH 11, 25°C), respectively, illustrate a very high activity of Fe-TAML activators particularly in terms of k. For example, Oakes and Gratton reported the value of 0.08M s1 for the oxidation of Orange II by p-sulfonated perbenzoic acid under the same conditions (57). 

Peroxidase enzymes exhibit low enthalpies of activation AHf2 for the formation of Compound I (k12 step), 12, 9.6, and 22 kJ mol 1 for horseradish (86), turnip (75), and lignin (77) peroxidases, respectively. Similarly low activation barriers have been found for Fem-TAML activators (Table III). The ks pathway for 11 is characterized by Aof 24kJmol 1, which is only... 

The catalase-like activity of Fem-TAML activators does decrease in the presence of Safranine O (Fig. 16). The inset to Fig. 16 shows that the plot of the inverse rate vs. [Safranine O] is a straight line consistent with Eq. (23). 

Fig. 16. Retardation of the catalase activity of Fe-TAML activators by the dye Safranine O as an electron donor. Conditions [H202] 2.65 x 10-3 M [lk] 1.18 x 10 6M pH 10, 25°C. Inset shows that the rate of 02 evolution is inversely proportional to [Safranine O]. From Ref. (53).

The intercept and slope equal (k + km)lkikm and ku/ i m[H202], respectively. An estimate for the ratio ku/km is derived from the slope using the known value of k (e.g., 4.4 x 103M-1s-1 for lk). The kn/km ratio equals 52 for lk and indicates that the oxidized Fe-TAML is significantly more reactive to the ED, Safranine O, than to H202. This extremely important feature of the Fe-TAML activators eliminates unproductive decomposition of H202 in the oxidation of Safranine... 

The molecular masses of heme catalases are usually significantly higher as compared with peroxidases. If expressed in Lg-1s-1, rate constants for the Fem-TAML activators when compared with catalase from beef liver, which has a molecular weight 250,000 gmol-1 (Table IV, entry 13) (89), look very impressive, viz. 17 L g 1 s-1 for 11 vs. 22 L g 1 s 1 for the enzyme. Nevertheless, the catalase-like activity of the Fem-TAML activators can be suppressed by the addition of electron donors -it is negligible in the presence of the substrates tested in this work. In Nature, catalases display only minor peroxidase-like activity (79) because electron donors bulkier than H202 cannot access the deeply buried active sites of these massive enzymes (90). The comparatively unprotected Fem-TAML active sites are directly exposed to electron donors such that the overall behavior is determined by the inherent relative reactivity of the substrates. 

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