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

Abstract

Purple acid phosphatases (PAPs) are a group of metallohydrolases that contain a dinuclear Fe III 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 dinuclearhas recently been prepared and is found to closely mimic the coordination environment of the Fe III Zn II 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 Fe III N 2 O 4 octahedron with a dominant tetragonal compression aligned along the μ-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 (-CH 3 , -H, -Br, -NO 2 ) in the 5-position. The 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 structural 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 -NO 2 rBr rH rCH 3 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 σ. However, nuclease activity measurements in the cleavage of double stranded DNA showed that the complexes containing the electron-withdrawing -NO 2 and electron-donating -CH 3 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.

Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes

“…139 A similar effect has been reported in a PAP model with a Fe(III)Zn(II) centre. 141 In neither case, however, were similar correlations found with K m suggesting that the substituent in para-position is primarily affecting nucleophilic activation. The nucleophile in zinc complex-catalysed hydrolysis reactions is in most cases proposed to be a terminal water molecule ( pK a = 6.60-9.5; (A) and (B) in Fig.…”

“…Peralta et al reported a linear correlation between the pK a values attributed to deprotonation of the Fe(III)-bound water for complexes with different substituents, methyl, H, Br and NO 2 respectively. 141 These studies suggest that subtle structural changes in the ligand can influence catalytic activity and the acidity of the Cd(II) ions in nucleophilic activation.…”

Synthesis, structure, magnetism, and hydrolase and catecholase activity of a new trinuclear copper(II) complex

“…The reactions were accompanied by monitoring the increase in the 2,4-dinitrophenolate characteristic absorption band at 400 nm (pH/e L mol À1 cm À1 = 3.5/2125; 4.0/3408; 4.5/7182; 5.0/10 078; 5.5/11 405; 6.0/ 12 004; 6.5-10.0/12 100) at 50°C for the hydrolase-like activity [18] and the formation of 3,5-di-tert-butylquinone (3,5-DTBQ) at 400 nm (e = 1900 M À1 cm À1 ) for the catecholase-like activity at 25°C. Less than 5% of the conversion of the substrate to the product was monitored and the data were treated using the initial rate method as previously described [9].…”

Asymmetric zinc(ii) complexes as functional and structural models for phosphoesterases

“…Throughout the pH range studied (4.75-8.5), the extinction coefficient of this product at 400 nm varies from 7180 at pH 4.5, 10 080 at 5.0; 11 400 at pH 5.5 to 12 000 at 6.0 and 12 100 at pH 6.5-8.5. 30 All assays were measured in 50 : 50 acetonitrile-buffer with the substrate and complex initially dissolved in acetonitrile. The aqueous multicomponent buffer was comprised of 50 mM 2-(N-morpholino)-ethanesulfonic acid (MES), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 2-(N-cyclohexylamino)ethane sulfonic acid (CHES) and N-cyclohexyl-3-aminopropanesulfonic acid (CAPS) with controlled ionic strength (LiClO 4 ) 250 mM.…”