State selected ion-molecule reactions by a TESICO technique. VI. Vibronic-state dependence of the cross sections in the reactions O+ 2(X 2Π g , v; a 4Π u , v)+H2→ O2H++H, H+ 2+O2 J. Chem. Phys. 77, 4441 (1982) The causes of the previously reported temperature dependence of the g values of the CU(H20)~ + ions in Cu 2 + doped Zn(H 2 0)6(GeF 6 ) and the Tutton's salts M 2 Zn(H 2 0)6(S04)2' where M = K+, Rb+, NH 4 +, and Cs+, supplemented by new experimental measurements on the K + salt, have been investigated. The ground state dynamics of the complexes have been modeled on the cubic E X E J ahn-Teller Hamiltonian perturbed by an orthorhombic lattice strain. For each compound, the vibronic energy levels and associated wave functions were calculated numerically, the overall g values at any temperature being given by a thermal average of the g values of the individual vibronic energy levels, because of rapid exchange between the levels. For the Tutton's salts it was found that the low temperature g values are strongly influenced by the tetragonal component of the lattice strain, with this corresponding to an axial compression of the ligand field. The temperature dependence of the g tensors, on the other hand, was found to depend largely on the orthorhombic component of the lattice strain. For the K + and NH4+ salts, where structural data are available, the strain parameters derived using the model are in good agreement with the geometries reported for the Zn (H 2 0) ~ + host complexes. For Cu2+ doped Zn(H 2 0)6(GeF 6 ) the model implies a lattice strain of tetragonal symmetry corresponding to a slight elongation of the axial metal-ligand bonds. The results are compared with those reported previously by Silver and Getz, and other workers, who used a simple model involving temperature dependent equilibria between energy levels corresponding to different orientations of the Cu (H 2 0) ~ + ions in the host lattices to interpret the temperature dependence of the g tensors.

Purple acid phosphatases (PAPs) belong to the family of binuclear metallohydrolases and catalyze the hydrolysis of a variety of phosphoester substrates within the pH range of 4-7. 1 They are the only binuclear metallohydrolases where the necessity for a heterovalent active site (Fe III -M II , where M ) Fe, Zn, or Mn) for catalysis has been clearly established. To date, the crystal structures of PAPs from red kidney bean (rkbPAP), 2a rat, 2b,c pig, 2d human, 2e and sweet potato 2f have been reported. In the structure of rkbPAP, 2a the Fe III ion is coordinated by a tyrosine, a histidine, and an aspartate, and a Zn II ion is coordinated by two histidines and an asparagine. The Fe III Zn II ions are bridged by two oxygen atoms, one from the carboxylate group of an aspartate and the other from a modeled µ-(hydr)oxo group. Two oxygen atoms from a µ-1,3 phosphate group complete the coordination spheres of the Zn II and Fe III ions.Despite the availability of detailed structural data, the catalytic mechanism of PAPs remains a matter of controversy. For rkbPAP, a mechanism in which, in the first step of the catalytic cycle, the substrate binds in a monodentate fashion to the Zn II ion has been proposed. 2a The enzyme-substrate complex is oriented in such a way that a terminal Fe III -bound hydroxide can efficiently attack the phosphorus atom of the substrate, leading to the release of the alcohol product. 2a The monodentate binding of the substrate to Zn II is corroborated by the fact that the addition of phosphate to the Fe III Zn II derivative of bovine spleen PAP does not affect the spectroscopic properties of the Fe III ion at the pH of optimal activity (pH 6.5). 3 However, for pig 4a and sweet potato PAP, 2f,4b an alternative mechanism in which the substrate forms a µ-1,3 phosphate complex, thus placing the µ-(hydr)oxo bridge in an ideal position to act as the reaction-initiating nucleophile, has also been proposed.Homo-and heterodinuclear Fe III M II complexes which are capable of reproducing the structural, spectroscopic, and functional properties of PAPs can be very informative to evaluate the mechanism(s) of these metalloenzymes. Recently, we reported on the syntheses, characterization, and phosphatase-like activity of the heterodinuclear [LFe III (µ-OAc) 2 Zn II ] + complex (H 2 L ) 2-bis[{(2-pyridylmethyl)-aminomethyl}-6-{(2-hydroxybenzyl)-(2-pyridylmethyl)}aminomethyl]-4-methylphenol), and we have proposed that upon dissolving the complex in an aqueous solution the dissociation of the carboxylate groups leads to the formation of the catalytically active [(OH)Fe III -(µ-OH)Zn II (OH 2 )] species, 5 similar to that proposed to be present in the active site of rkbPAP. 2a Herein we report the X-ray structure, solution studies, and phosphatase activity of the first mixed-valence complex containing the Fe III (µ-OH)Zn II motif (1). The molecular structure of 1 (Figure 1) shows that in the dinuclear [L(OH 2 )Fe-(µ-OH)Zn] 2+ unit the Fe III ion is facially coordinated by the hard tridentate pendant arm of L 2-...

Purple acid phosphatase from pig uterine fluid (uteroferrin), a representative of the diverse family of binuclear metallohydrolases, requires a heterovalent Fe(III)Fe(II) center for catalytic activity. The active-site structure and reaction mechanism of this enzyme were probed with a combination of methods including metal ion replacement and biomimetic studies. Specifically, the asymmetric ligand 2-bis{[(2-pyridylmethyl)-aminomethyl]-6-[(2-hydroxybenzyl)(2-pyridylmethyl)]aminomethyl}-4-methylphenol and two symmetric analogues that contain the softer and harder sites of the asymmetric unit were employed to assess the site selectivity of the trivalent and divalent metal ions using (71)Ga NMR, mass spectrometry and X-ray crystallography. An exclusive preference of the harder site of the asymmetric ligand for the trivalent metal ion was observed. Comparison of the reactivities of the biomimetics with Ga(III)Zn(II) and Fe(III)Zn(II) centers indicates a higher turnover for the former, suggesting that the M(III)-bound hydroxide acts as the reaction-initiating nucleophile. Catalytically active Ga(III)Zn(II) and Fe(III)Zn(II) derivatives were also generated in the active site of uteroferrin. As in the case of the biomimetics, the Ga(III) derivative has increased reactivity, and a comparison of the pH dependence of the catalytic parameters of native uteroferrin and its metal ion derivatives supports a flexible mechanistic strategy whereby both the mu-(hydr)oxide and the terminal M(III)-bound hydroxide can act as nucleophiles, depending on the metal ion composition, the geometry of the second coordination sphere and the substrate.