A Heterodinuclear Fe^IIIZn^II Complex as a Mimic for Purple Acid Phosphatase with Site-Specific Zn^II Binding

Purple acid phosphatases (PAPs) are the only dinuclear metallohydrolases for which the necessity for a heterovalent active site (Fe-III-M-II; M = Fe, Zn or Mn) for catalysis has been established. A major goal for the synthesis of PAP biomimetics is to design a ligand in which the two coordination sites exhibit discrimination between the trivalent and divalent metal ions. With this goal in mind the ligand 2-{[bis(2-methoxyethyl)amino]methyl}-6-{[(2-hydroxybenzyl)(2-pyridylmethyl)amino]methyl}-4-methylphenol (BMMHPH2), with two distinct coordination sites, N2O2 () and NO3 (), has been prepared. Although not exactly mimicking the active site of PAP, the ligand facilitated the formation of the complex [(FeZnII)-Zn-III(BMMHP)(CH3COO)(2)](BPh4), which exhibited regioselectivity in the two metal binding sites. The phosphoesterase-like activity of the complex in 50:50 acetonitrile/water was investigated by using the substrate bis(2,4-dinitrophenyl) phosphate (BDNPP) yielding kinetically relevant pK(a) values of 6.89, 7.37 and 9.00, a K-M of 10.8 +/- 2.1 mM and a k(cat) of 3.20 +/- 0.38x10(-3) s(-1) (at pH = 7.5). Attempts to prepare a diiron analogue resulted in a centrosymmetric dimer, [Fe-2(III)(BMMHPH)(2)(-OH)(2)](BPh4)(2), with one six-coordinate Fe-III atom in each of the -sites, connected by two -hydroxido groups. In this Fe(-OH)(2)Fe diamond core the Fe-III ions are weakly antiferromagnetically coupled, with J = -1.76 +/- 0.03 cm(-1). The -sites were vacant. Attempts to replace the Zn-II ion with Mg-II resulted in the formation of a centrosymmetric trimer, i.e. [(Fe2MgII)-Mg-III(BMMHPH)(2)(CH3COO)(2)(CH3O)(2)](BPh4)(2)