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-...

The new heterodinuclear mixed valence complex [Fe(III)Mn(II)(BPBPMP)(OAc)(2)]ClO(4) (1) with the unsymmetrical N(5)O(2) donor ligand 2-bis[((2-pyridylmethyl)-aminomethyl)-6-((2-hydroxybenzyl)(2-pyridylmethyl))-aminomethyl]-4-methylphenol (H(2)BPBPMP) has been synthesized and characterized. Compound 1 crystallizes in the monoclinic system, space group P2(1)/c, and has an Fe(III)Mn(II)(mu-phenoxo)-bis(mu-carboxylato) core. Two quasireversible electron transfers at -870 and +440 mV versus Fc/Fc(+) corresponding to the Fe(II)Mn(II)/Fe(III)Mn(II) and Fe(III)Mn(II)/Fe(III)Mn(III) couples, respectively, appear in the cyclic voltammogram. The dinuclear Fe(III)Mn(II) center has weakly antiferromagnetic coupling with J = -6.8 cm(-1) and g = 1.93. The (57)Fe Mössbauer spectrum exhibits a single doublet, delta = 0.48 mm s(-1) and DeltaE(Q) = 1.04 mm s(-1) for the high spin Fe(III) ion. Phosphatase-like activity at pH 6.7 with the substrate 2,4-bis(dinitrophenyl)phosphate reveals saturation kinetics with the following Michaelis-Menten constants: K(m) = 2.103 mM, V(max) = 1.803 x 10(-5) mM s(-1), and k(cat) = 4.51 x 10(-4) s(-1).

The new heterodinuclear complex [Fe(III)Zn(II)(BPBPMP)(OAc)(2)]ClO(4) (1) with the unsymmetrical N(5)O(2) donor ligand 2-bis[((2-pyridylmethyl)-aminomethyl)-6-[(2-hydroxybenzyl)(2-pyridylmethyl)]-aminomethyl]-4-methylphenol (H(2)BPBPMP) has been synthesized and characterized by X-ray crystallography, which reveals that the complex cation has an Fe(III)Zn(II)(mu-phenoxo)-bis(mu-carboxylato) core. Solution studies of 1 indicate that a pH-induced change of the bridging acetate occurs, and the formation of an active [(OH)Fe(III)Zn(II)(OH(2))] species as a highly efficient catalyst under weakly acidic conditions for phosphate diesters hydrolysis is proposed.

Species 1-6 are [M(III)(L)2]ClO4 complexes formed with the PhO--CH=N-CH2-Py imines, (L(I))- and (L(tBuI))-, and PhO--CH2-NH-CH2-Py amines, (L(A))- and (L(tBuA))-, in which PhO- is a phenolate ring and Py is a pyridine ring and the prefix tBu indicates the presence of tertiary butyl groups occupying the positions 4 and 6 of the phenol ring. Monometallic species with d5 high-spin iron (1, 2, 3, 4) and d10 gallium (5, 6) were synthesized and characterized to assess the influence of the ligand rigidity and the presence of tertiary butyl-substituted phenol rings on their steric, electronic, and redox behavior. Characterization by elemental analysis, mass spectrometry, IR, UV-visible, and EPR spectroscopies, and electrochemistry has been performed, and complexes [FeIII(L(tBuI))2]ClO4 (2), [FeIII(L(tBuA))2]ClO4 (4), and [Ga(III)(L(tBuI))2]ClO4 (5) have been characterized by X-ray crystallography. The crystal structures show the imine ligands meridionally coordinated to the metal centers, whereas the amine ligands are coordinate in a facial mode. Cyclic voltammetry shows that the complexes with the ligands (L(tBuI))- and (L(tBuA))- were able to generate ligand-based phenoxyl radicals, whereas unsubstituted ligands displayed ill-defined redox processes. EPR spectroscopy supports high-spin configurations for the iron complexes. UV-visible spectra are dominated by charge-transfer phenomena, and imine compounds exhibit dramatic hyperchromism when compared to equivalent amines. The tertiary butyl groups on the phenolate ring enhance this trend. Detailed B3LYP/6-31G(d)-level calculations have been used to account for the results observed.

Abstract. Of particular interest is the observation that cleavage of double-stranded plasmid DNA occurs even at very low concentrations of 1 (2.5 µM), under physiological conditions (optimum pH of 7.0), with a rate enhancement of 2.7×10 7 over the uncatalyzed reaction. Thus, 1 is one of the most effective model complexes to date, mimicking the function of nucleases.

In order to understand and predict structural, redox, magnetic, and optical properties of more complex and potentially mesogenic electroactive compounds such as [Co(III)(L(t-BuLC))2]ClO4 (1), five archetypical complexes of general formula [Co(III)(L(RA))2]ClO4, where R = H (2), tert-butyl (3), methoxy (4), nitro (5), and chloro (6), were obtained and studied by means of several spectrometric, spectroscopic, and electrochemical methods. The complexes 2, 4, and 6 were characterized by single-crystal X-ray diffraction, and show the metal center in an approximate D2h symmetry. Experimental results support the fact that the electron donating or withdrawing nature of the phenolate-appended substituents changes dramatically the redox and spectroscopic properties of these compounds. The 3d6 electronic configuration of the metal ion dominates the overall geometry adopted by these compounds with the phenolate rings occupying trans positions to one another. Formation of phenoxyl radicals has been observed for 1, 3, and 6, but irreversible ligand oxidation takes place upon bulk electrolysis. These data were compared to detailed B3LYP/6-31G (d)-level computational calculations and have been used to account for the results observed. A comparison between compound 1 and archetype 3, validates the approach of using archetypical models to study metal-containing soft materials.

Three new iron(III) complexes with the ligand N,N'-bis(2-hydroxybenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine, H2bbpen, containing electron-donating and -withdrawing groups (Me, Br, NO2) in the 5-position of the phenol rings were synthesized and fully characterized by IR spectroscopy, ESI mass spectrometry, and CHN elemental analyses. X-ray structures of the iron(III) complexes containing NO2 and Me groups were determined. The effects of the substituents on the electronic properties of the complexes were detected by UV-visible spectroscopy, cyclic voltammetry, and X-ray crystallography. Linear correlations between the Hammett parameter for the substituents (sigma(p)) and the Fe(III)/Fe(II) redox potentials or ligand-metal charge-transfer (LMCT) processes of the complexes were obtained. A theoretical study using DFT is presented and shows good agreement between the experimental and calculated data.

A new asymmetric pentadentate ligand was designed to impose low symmetry to trivalent ions. Five-coordinate Fe3+ and Ga3+ complexes were investigated by crystallographic, electrochemical, and electron paramagnetic resonance methods showing enhanced redox reversibility. Calculations were performed to account for the observed trends.