Models for iron-oxo proteins. Moessbauer and EPR study of an antiferromagnetically coupled iron(III)-nickel(III) complex

Models for Unsymmetrical Active Sites in Metalloproteins: Structural, Redox, and Magnetic Properties of Bimetallic Complexes with MII-(μ-OH)-FeIII Cores

Bimetallic complexes are important sites in metalloproteins but are often difficult to prepare synthetically. We have previously introduced an approach to form discrete bimetallic complexes with M^II-(μ-OH)-Fe^III (M^II = Mn, Fe) cores using the tripodal ligand N,N',N″-[2,2',2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]^3-). This series is extended to include the rest of the late 3d transition metal ions (M^II = Co, Ni, Cu, Zn). All of the bimetallic complexes have similar spectroscopic and structural properties that reflect little change despite varying the M^II centers. Magnetic studies performed on the complexes in solution using electron paramagnetic resonance spectroscopy showed that the observed spin states varied incrementally from S = 0 through S = 5/2; these results are consistent with antiferromagnetic coupling between the high-spin M^II and Fe^III centers. However, the difference in the M^II ion occupancy yielded only slight changes in the magnetic exchange coupling strength, and all complexes had J values ranging from +26(4) to +35(3) cm^-1.

Modeling the Active Site of the Purple Acid Phosphatase Enzyme with Hetero-Dinuclear Mixed Valence M(II)-Fe(III) [M = Zn, Ni, Co, and Cu] Complexes Supported over a [N6O] Unsymmetrical Ligand

The active site of the purple acid phosphatase enzyme has been successfully modeled by a series of hetero-dinuclear M(II)-Fe(III) [M = Zn, Ni, Co, and Cu] type complexes of an unsymmetrical [N₆O] ligand that contained a bridging phenoxide moiety and one imidazoyl and three pyridyl moieties as the terminal N-binding sites. In particular, the hetero-dinuclear complexes, {L[M^II(μ-OAc)₂Fe^III]}(ClO₄)₂ [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)], were obtained directly from the phenoxy-bridged ligand (HL), namely 2-{[bis(2-methylpyridyl)amino]methyl}-6-{[((1-methylimidazol-2-yl)methyl)(2-pyridylmethyl)amino]methyl}-4-t-butylphenol (2), upon sequential addition of Fe(ClO₄)₃·XH₂O and M(ClO₄)₂·6H₂O (M = Zn and Ni) or M(OAc)₂·XH₂O (M = Co and Cu), in a low-to-moderate (ca. 32-53%) yield. The temperature-dependent magnetic susceptibility measurements indicated weak antiferromagnetic coupling interactions occurring between the two metal centers in their high-spin states. All of the 3(a-b) and 4(a-b) complexes successfully carried out the hydrolysis of the bis(2,4-dinitrophenyl)phosphate (2,4-BDNPP) substrate in a mixed CH₃CN/H₂O (v/v 1:1) medium in the pH range of 5.5-10.5 at room temperature, thereby mimicking the functional activity of the native enzyme. The spectrophotometric titration suggested a monoaquated and dihydroxo species of the type {L[(H₂O)M^II(μ-OH)Fe^III(OH)]}²⁺ to be the catalytically active species for the phosphodiester hydrolysis reaction within the pH range of ca. 5.80-7.15. Last, the kinetic studies on the hydrolysis of the model substrate, 2,4-BDNPP, divulge a Michaelis-Menten-type behavior for all complexes.

The reaction mechanism of the Ga(III)Zn(II) derivative of uteroferrin and corresponding biomimetics

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.

Site specificity of metal ions in heterodinuclear complexes derived from an "end-off" compartmental ligand

A phenol-based "end-off" compartmental ligand, 2-[N-[2-(dimethylamino)ethyl]iminomethyl]-6-[N,N-di(2-pyridylmethyl)aminomethyl]-4-methylphenol (HL), having a bidentate arm and a tridentate arm attached to the 2 and 6 positions of the phenolic ring, has afforded the following heterodinuclear M(a)(II)M(b)(II) complexes: [CuM(L)(AcO)(2)]ClO(4) (M = Mn (1), Fe (2), Co (3), Ni (4), Zn (5)), [ZnM(L)(AcO)(2)]ClO(4) (M = Co (6), Ni (7)), and [CuNi(L)(AcO)(NCS)(2)] (8). 1.MeOH (1'), 2.MeOH (2'), 3.MeOH (3'), 4.MeOH (4'), 5.MeOH (5'), and 7.MeOH (7') are isostructural and have a heterodinuclear core bridged by the phenolic oxygen atom of L(-) and two acetate groups. In 1'-5' the Cu(II) is bound to the bidentate arm and has a square-pyramidal geometry with one acetate oxygen at the apical site. The M(II) is bound to the tridentate arm and has a six-coordinate geometry together with two acetate oxygen atoms. In the case of 7' the Zn is bound to the bidentate arm and the Ni is bound to the tridentate arm. 8.2-PrOH (8') has a dinuclear core bridged by the phenolic oxygen atom of L(-) and one acetate group. The Cu bound to the bidentate arm has a square-pyramidal geometry with an isothiocyanate group at the apical site. The Ni bound to the tridentate arm has a six-coordinate geometry with further coordination of an isothiocyanate group. The site specificity of the metal ions is discussed together with the crystal structure of [Cu(4)(L)(2)(AcO)(3)](ClO(4))(3).H(2)O (9) prepared in this work.

A purple acid phosphatase from sweet potato contains an antiferromagnetically coupled binuclear Fe-Mn center

A purple acid phosphatase from sweet potato is the first reported example of a protein containing an enzymatically active binuclear Fe-Mn center. Multifield saturation magnetization data over a temperature range of 2 to 200 K indicates that this center is strongly antiferromagnetically coupled. Metal ion analysis shows an excess of iron over manganese. Low temperature EPR spectra reveal only resonances characteristic of high spin Fe(III) centers (Fe(III)-apo and Fe(III)-Zn(II)) and adventitious Cu(II) centers. There were no resonances from either Mn(II) or binuclear Fe-Mn centers. Together with a comparison of spectral properties and sequence homologies between known purple acid phosphatases, the enzymatic and spectroscopic data strongly indicate the presence of catalytic Fe(III)-Mn(II) centers in the active site of the sweet potato enzyme. Because of the strong antiferromagnetism it is likely that the metal ions in the sweet potato enzyme are linked via a mu-oxo bridge, in contrast to other known purple acid phosphatases in which a mu-hydroxo bridge is present. Differences in metal ion composition and bridging may affect substrate specificities leading to the biological function of different purple acid phosphatases.

Mono- and dinuclear zinc complexes derived from unsymmetric binucleating ligands: synthesis, characterization, and formation of tetranuclear arrays

Zinc complexes of the unsymmetric, binucleating Schiff base ligands 3-(N-[2-(dimethylamino)ethyl]iminomethyl)-salicylic acid (H2L1) and 3-[N-(2-pyridylmethyl)iminomethyl]-salicylic acid (H2L2) have been studied in the solid state as well as in solution. Reaction of ZnX2 (X = NO3-, CH3CO2-) with 3-formylsalicylic acid and N,N-dimethylethylenediamine at neutral or slightly acidic pH afforded the dinuclear complexes [Zn2(HL1)2(H2O)2](NO3)2.2H2O (1a) and [Zn2(HL1)2(CH3CO2)2].6H2O (1b). The Zn ions, which are 3.126(1) A (1a) and 3.2665(7) A (1b) apart, are bridged by two phenolate oxygens. Further coordination sites of the ligand are the imine nitrogen and carboxylate oxygen, while the amino nitrogen is protonated. On dissolution in DMSO or DMF, 1a and 1b are converted into the mononuclear species [Zn(HL1)]+. Cleavage of the dinuclear complexes is accompanied by migration of the ammonium proton to the carboxylate group and coordination of the amino nitrogen to Zn. Reaction of 1b with base yielded the novel tetranuclear Zn complex [Zn4(L1)4].6.5H2O (2) that exhibits coordination number asymmetry. The four Zn ions having N2O3 and N2O4 coordination environments are located at the corners of a nearly square-planar rectangle. H2L2 binds Zn via the phenolate oxygen and, imine and pyridine nitrogens in acidic solution. Deprotonation of the carboxyl group in alkaline solution gave the tetranuclear compound [Zn4(L2)4].4.5H2O (4) with a cubane-like Zn4O4 core.

Model Compounds for Iron Proteins. Structures and Magnetic, Spectroscopic, and Redox Properties of Fe(III)M(II) and [Co(III)Fe(III)](2)O Complexes with (&mgr;-Carboxylato)bis(&mgr;-phenoxo)dimetalate and (&mgr;-Oxo)diiron(III) Cores

A series of heterobimetallic complexes of the type [Fe(III)M(II)L(&mgr;-OAc)(OAc)(H(2)O)](ClO(4)).nH(2)O (2-5) and [{Fe(III)Co(III)L(&mgr;-OAc)(OAc)}(2)(&mgr;-O)](ClO(4))(2).3H(2)O (6) where H(2)L is a tetraaminodiphenol macrocyclic ligand and M(II) = Zn(2), Ni(3), Co(4), and Mn(5) have been synthesized and characterized. The (1)H NMR spectrum of 6 exhibits all the resonances between 1 and 12 ppm. The IR and UV-vis spectra of 2-5 indicate that in all the cases the metal ions have similar coordination environments. A disordered crystal structure determined for 3 reveals the presence of a (&mgr;-acetate)bis(&mgr;-phenoxide)-Ni(II)Fe(III) core, in which the two metal ions have 6-fold coordination geometry and each have two amino nitrogens and two phenolate oxygens as the in-plane donors; aside from the axial bridging acetate, the sixth coordination site of nickel(II) is occupied by the unidentate acetate and that of iron(III) by a water molecule. The crystal structure determination of 6 shows that the two heterobinuclear Co(III)Fe(III) units are bound by an Fe-O-Fe linkage. 6 crystallizes in the orthorhombic space group Ibca with a = 17.577(4) Å, b = 27.282(7) Å, c = 28.647(6) Å, and Z = 8. The two iron(III) centers in 6 are strongly antiferromagnetically coupled, J = -100 cm(-1) (H = -2JS(1).S(2)), whereas the other two S(1) = S(2) = (5)/(2) systems, viz. [Fe(2)(III)(HL)(2)(&mgr;-OH)(2)](ClO(4))(2) (1) and the Fe(III)Mn(II) complex (5), exhibit weak antiferromagnetic exchange coupling with J = -4.5 cm(-1) (1) and -1.8 cm(-1) (5). The Fe(III)Ni(II) (3) and Fe(III)Co(II) (4) systems, however, exhibit weak ferromagnetic behavior with J = 1.7 cm(-1) (3) and 4.2 cm(-1) (4). The iron(III) center in 2-5 exhibits quasi-reversible redox behavior between -0.44 and -0.48 V vs Ag/AgCl associated with reduction to iron(II). The oxidation of cobalt(II) in 4 occurs quasi-reversibly at 0.74 V, while both nickel(II) and manganese(II) in 3 and 5 undergo irreversible oxidation at 0.85 V. The electrochemical reduction of 6 leads to the generation of 4.