Electronic Structure and Spectro-Structural Correlations of FeIIIZnII Biomimetics for Purple Acid Phosphatases: Relevance to DNA Cleavage and Cytotoxic Activity

Purple acid phosphatases (PAPs) are a group of metallohydrolases that contain a dinuclear (FeMII)-M-II center (M-II = Fe, Mn, Zn) in the active site and are able to catalyze the hydrolysis of a variety of phosphoric acid esters. The dinuclear complex [(H2O)Fe-III(mu-OH)Zn-II(L-H)](CIO4)(2) (2) with the ligand 2-[N-bis(2-pyridylmethyl)aminomethyl]-4-methyl-6-[N-(2-pyridylmethyl)(2-hydroxybenzyl) aminomethyl]phenol (H2L-H) has recently been prepared and is found to closely mimic the coordination environment of the (FeZnII)-Zn-III active site found in red kidney bean PAP (Neves et al. J. Am. Chem. Soc. 2007, 129, 7486). The biomimetic shows significant catalytic activity in hydrolytic reactions. By using a variety of structural, spectroscopic, and computational techniques the electronic structure of the Fe-III center of this biomimetic complex was determined. In the solid state the electronic ground state reflects the rhombically distorted (FeN2O4)-N-III octahedron with a dominant tetragonal compression align ad along the mu-OH-Fe-O-phenolate direction. To probe the role of the Fe-O-phenolate bond, the phenolate moiety was modified to contain electron-donating or -withdrawing groups (-CH3, -H, -Br, -NO2) in the 5-position. Tie effects of the substituents on the electronic properties of the biomimetic complexes were studied with a range of experimental and computational techniques. This study establishes benchmarks against accurate crystallographic struck ral information using spectroscopic techniques that are not restricted to single crystals. Kinetic studies on the hydrolysis reaction revealed that the phosphodiesterase activity increases in the order -NO2 <- Br <- H <- CH3 when 2,4-bis(dinitrophenyl)phosphate (2,4-bdnpp) was used as substrate, and a linear free energy relationship is found when log(k(cat)/k(0)) is plotted against the Hammett parameter a. However, nuclease activity measurements in the cleavage of double stranded DNA showed that the complexes containing the electron-withdrawing -NO2 and electron-donating CH3 groups are the most active while the cytotoxic activity of the biomimetics on leukemia and lung tumoral cells is highest for complexes with electron-donating groups.