Fe^IIIFe^IIIand Fe^IIFe^IIIComplexes as Synthetic Analogues for the Oxidized and Reduced Forms of Purple Acid Phosphatases

Abstract

Novel Fe(III)Fe(III) and Fe(II)Fe(III) complexes [Fe(2)(BBPMP)(&mgr;-OAc)(&mgr;-X)](n)() (1, X = OAc(-), n = 1+; 2, X = OH(-), n = 1+; 3, X = OAc(-), n = 0; 4, X = OH(-), n = 0), where BBPMP(3)(-) is the anion of 2,6-bis[(2-hydroxybenzyl)(2-pyridylmethyl)aminomethyl]-4-methylphenol, and OAc(-) is acetate, were prepared in order to provide models for the active site of purple acid phosphatases (PAPs). Complex 1 was obtained by the reaction of H(3)BBPMP with Fe(ClO(4))(2).6H(2)O in methanol and sodium acetate trihydrate under ambient conditions, while complex 3 was synthesized as described for 1, under an argon atmosphere with low levels of dioxygen. 2 was isolated from 1in acetonitrile by a substitution of the bridging acetate group by hydroxide, while 4 was generated in solution during a spectropotentiostatic experiment on 2, under argon. Complex 1, [Fe(III)(2)(BBPMP)(&mgr;-OAc)(2)]ClO(4).H(2)O, has been characterized by X-ray crystallography. Crystal data: monoclinic, space group P2(1)/n, a = 14.863(5) Å, b = 12.315(3) Å, c = 20.872(8) Å, beta = 90.83(3) degrees, Z = 4. IR, Mössbauer, magnetic, electronic absorption, and electrochemical properties of 1-3 have been investigated, and some of these properties represent a contribution to the understanding of the dinuclear iron center of PAPs. Complexes 2, [Fe(III)(2)(BBPMP)(&mgr;-OAc)(&mgr;-OH)]ClO(4) (lambda(max) = 568 nm/epsilon = 4760 M(-)(1) cm(-)(1)), and 4 [Fe(II)Fe(III)(BBPMP)(&mgr;-OAc)(&mgr;-OH)] (lambda(max) = 516 nm/epsilon = 4560 M(-)(1) cm(-)(1)), constitute good synthetic analogues for the chromophoric site for the oxidized and reduced forms, respectively, of the enzyme.

Probing the role of the divalent metal ion in uteroferrin using metal ion replacement and a comparison to isostructural biomimetics

“…6 and data are given in Table 4. The broad feature between 520 and 560 nm is attributed to the terminal phenolate to Fe(III) LMCT transition, characteristic of isostructural model complexes [24][25][26][27][28][29][30][31] and PAPs [1,2,[10][11][12]. The k max (and corresponding extinction coefficients) for 1, 2 and 3 (Tables 4 and S1) are similar to those determined for both related model complexes and PAPs.…”

“…The isostructural Fe(III)M(II) biomimetics presented here and elsewhere [24][25][26][27][28][29][30][31] provide good structural, but also reasonable functional models for both types of enzymatic mechanisms. In contrast to Fe(III)Ni(II)-Uf, the three FeNi models described in this study employ a terminal Fe(III)-bound hydroxide as a nucleophile (Scheme 3), similar to the isostructural Fe(III)Zn(II) complex [28].…”

“…Biomimetics provide an additional method to probe the role of a metal ion in a catalytic cycle. A series of isostructural Fe(III)M(II) complexes (M is Fe, Mn, Cu, Zn) that mimic the coordination environment of PAPs, and that have the characteristic purple/pink color, have recently been described [24][25][26][27][28][29][30][31]. Of particular interest is the observation that at least in the Fe(III)Zn(II) complex, the best characterized of these mimics, only the terminal, Fe(III)-bound hydroxide is sufficiently strong a nucleophile to induce catalysis [28].…”

Structural and Catalytic Characterization of a Heterovalent Mn(II)Mn(III) Complex That Mimics Purple Acid Phosphatases

“…On the basis of mechanistic schemes discussed for other isostructural complexes 15,22,86,90,91,113,114 and several enzymatic systems (e.g., PAPs 18,19 ) the following reaction mechanism for the Mn(II)Mn(III) complex is proposed. At low pH (<8) the likely nucleophile involved in hydrolysis is a terminally Mn(III) bound hydroxide (Scheme 2), with the substrate bound only to the divalent metal ion.…”

Synthesis, molecular structure and spectroscopic, electrochemical and magnetic properties of a new dinuclear iron complex containing µ-sulfate-di-µ-alkoxo bridges: evaluating the influence of the sulfate bridge on the physicochemical properties of the di-µ-alkoxo-diiron unit

“…[2][3][4][5][6][7][16][17][18][19] This is due to the high ε values observed in electronic spectra of PAPs, which are attributed to a tyrosine→Fe III charge transfer transition. Thus, such dinucleating ligands hold the two metal centers in close proximity and provide phenolate coordinating groups, allowing the study of the influence of such groups on the properties exhibited by the metal centers in the complexes as well as in the enzyme.…”