Investigation of the identity of the nucleophile initiating the hydrolysis of phosphate esters catalyzed by dinuclear mimics of metallohydrolases

Abstract

di-Zinc(II) complexes of the ligands 2,6-bis((bis(2-methoxyethyl)amino)methyl)-4-methylphenol (HL1), 2,6-bis(bis(hydroxyethyl)aminomethyl)-4-methylphenol (HL2) and 2,6-bis((hydroxyethyl)(methoxyethyl)-aminomethyl)-4-methylphenol (HL3) have been prepared and characterized. The three ligands differ in their donor types, having ether donors (HL1), alkoxido donors (HL2) and both ether and alkoxido donors (HL3). These differences allowed an investigation into the role of the potential nucleophiles in the hydrolysis reaction with the phosphodiester substrate bis(2,4-dinitrophenyl)phosphate (BDNPP). In addition, the di-Mg(II) complex of ligand HL2 was prepared in order to examine the potential for Mg(II) to replace Zn(II) in these biomimetic systems. Kinetically relevant pK values for the three di-Zn(II) complexes were determined to be 7.14 and 9.21 for [Zn(L1)(CHCOO)](PF), 7.90 and 10.21 for [Zn(L2)(CHCOO)](BPh) and 8.43 and 10.69 for [Zn(L3)(CHCOO)](BPh). At the respective pH optima the relevant catalytic parameters are k=5.44(0.11)×10s (K=5.13(0.92) mM), 2.60(0.87)×10s (K=5.49(1.51) mM) and 1.53(0.27)×10s (K=2.14(0.50) mM) for [Zn(L1)(CHCOO)](PF), [Zn(L2)(CHCOO)](BPh) or [Zn(L3)(CHCOO)](BPh), respectively. The di-Mg(II) complex was found to be unreactive in the hydrolysis reaction with BDNPP under the conditions employed. Computational methods using the [Zn(L2)(CHCOO)](BPh) complex were used to discriminate between different possible mechanistic pathways. The DFT calculations indicate that an alkoxido-mediated pathway in the complexes formed with ligands L2 or L3 is unlikely, because it induces significant distortion of the Zn(L) unit; a direct attack by a coordinated hydroxide is preferred in each of the three systems studied here. The calculations also revealed the important role of ligand structural rigidity.

Metal‐coordinated Hydroxide as a Nucleophile: a Brief History

“…For these types of complexes the core of the active form was proposed to be the dimeric hydroxido/aqua species [M(OH)( 7 )M(H 2 O)] + , in line with many previous studies of complexes of this type . The efficacy in the order of conversion of HPNP to the product, p ‐nitrophenolate ion, followed the sequence Zn II > Co II > Mn II , a trend that followed the decrease of the p K a values of the coordinated water (Mn II 7.67 ± 0.08, Co II 7.09 ± 0.05, and Zn II 6.20 ± 0.04).…”

“…However, the exact nature of the active entity in these reactions is often questioned , , . The hydrolysis at the metal site (in the example below represented as a bimetallic site) is assumed to involve coordination of the substrate (in this example a phosphoester) to one or both of the central metal atoms, deprotonation of an aqua ligand, and nucleophilic attack at the coordinated substrate.…”

“…Three mechanistic scenarios have been considered, i.e. hydrolysis by (a) a bridging, (b) a terminal or (c) a non‐coordinated hydroxide:, , …”

Mechanistic Studies of Homo- and Heterodinuclear Zinc Phosphoesterase Mimics: What Has Been Learned?

“…Based on kinetic data, X-ray analysis of the complex cocrystallized with a phosphodiester and binding studies, the following mechanisms have been assigned to dinuclear zinc(II) catalysts; (i) monodentate coordination of the phosphodiester to one Zn(II) and nucleophilic attack by OH bound to the other Zn(II) (Bazzicalupi et al, 2004; Jarenmark et al, 2010; Pathak et al, 2018) or to the same Zn(II) (Massoud et al, 2016); (ii) nucleophilic attack by Zn-OH on the bridging substrate (Bazzicalupi et al, 1997, 2004; Daumann et al, 2012, 2013; Brown et al, 2016) and (iii) nucleophilic attack of the bridging substrate by a bridging hydroxide (Das et al, 2014, 2018; Montagner et al, 2014; Daver et al, 2016). As discussed above, a shift of μ-OH to a terminal position in mechanism (iii) would render the attacking hydroxide a better nucleophile.…”

Heterodinuclear Zn(II)−Fe(III) and Homodinuclear M(II)−M(II) [M = Zn and Ni] complexes of a Bicompartmental [N 6 O] ligand as synthetic mimics of the hydrolase family of enzymes

“…Various ligands both of the symmetrical [39][40][41][42][43][44][45][46][47][48][49] and unsymmetrical [31,42,[50][51][52][53][54][55][56][57] [90] types have been reported in literature. In this regard, we intend to design synthetic mimics of the dinucleating enzymes namely, PAP and zinc phosphoesterase using a symmetrical ligand containing a phenoxy bridge and tetra pyridine side arms.…”