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. 2017 Aug 21;2(8):4737-4750.
doi: 10.1021/acsomega.7b00671. eCollection 2017 Aug 31.

Modeling the Active Site of the Purple Acid Phosphatase Enzyme with Hetero-Dinuclear Mixed Valence M(II)-Fe(III) [M = Zn, Ni, Co, and Cu] Complexes Supported over a [N6O] Unsymmetrical Ligand

Chandni Pathak  1 , Sandeep K Gupta  1 , Manoj Kumar Gangwar  1 , A P Prakasham  1 , Prasenjit Ghosh  1
Affiliations

Modeling the Active Site of the Purple Acid Phosphatase Enzyme with Hetero-Dinuclear Mixed Valence M(II)-Fe(III) [M = Zn, Ni, Co, and Cu] Complexes Supported over a [N6O] Unsymmetrical Ligand

Chandni Pathak et al. ACS Omega. .

Abstract

The active site of the purple acid phosphatase enzyme has been successfully modeled by a series of hetero-dinuclear M(II)-Fe(III) [M = Zn, Ni, Co, and Cu] type complexes of an unsymmetrical [N6O] ligand that contained a bridging phenoxide moiety and one imidazoyl and three pyridyl moieties as the terminal N-binding sites. In particular, the hetero-dinuclear complexes, {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)], were obtained directly from the phenoxy-bridged ligand (HL), namely 2-{[bis(2-methylpyridyl)amino]methyl}-6-{[((1-methylimidazol-2-yl)methyl)(2-pyridylmethyl)amino]methyl}-4-t-butylphenol (2), upon sequential addition of Fe(ClO4)3·XH2O and M(ClO4)2·6H2O (M = Zn and Ni) or M(OAc)2·XH2O (M = Co and Cu), in a low-to-moderate (ca. 32-53%) yield. The temperature-dependent magnetic susceptibility measurements indicated weak antiferromagnetic coupling interactions occurring between the two metal centers in their high-spin states. All of the 3(a-b) and 4(a-b) complexes successfully carried out the hydrolysis of the bis(2,4-dinitrophenyl)phosphate (2,4-BDNPP) substrate in a mixed CH3CN/H2O (v/v 1:1) medium in the pH range of 5.5-10.5 at room temperature, thereby mimicking the functional activity of the native enzyme. The spectrophotometric titration suggested a monoaquated and dihydroxo species of the type {L[(H2O)MII(μ-OH)FeIII(OH)]}2+ to be the catalytically active species for the phosphodiester hydrolysis reaction within the pH range of ca. 5.80-7.15. Last, the kinetic studies on the hydrolysis of the model substrate, 2,4-BDNPP, divulge a Michaelis-Menten-type behavior for all complexes.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Dinuclear {L[MII(μ-OAc)2FeIII]}(ClO4)2 type complexes 3(a–b) and 4(a–b) supported over unsymmetrical [N6O] ligand 2.
Scheme 1
Scheme 1. Synthesis of Unsymmetrical [N6O] Ligand 2
Scheme 2
Scheme 2. Synthesis of the Dinuclear {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)] Type Complexes Supported over Unsymmetrical [N6O] Ligand 2
Figure 2
Figure 2
Oak Ridge thermal ellipsoid plot (ORTEP) of 3a with thermal ellipsoids are shown at the 50% probability level. Two ClO4– anions have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (deg): Zn1–O14 2.010(4), Zn1–O10 2.091(3), Zn1–O11 2.111(4), Zn1–N1 2.111(5), Zn1–N2 2.149(5), Zn1–N5 2.202(5), Fe1–O13 1.957(4), Fe1–O10 1.982(4), Fe1–O12 2.023(4), Fe1–N4 2.093(5), Fe1–N6 2.205(5), Fe1–N3 2.124(5), and Fe1–O10–Zn1 113.86(16).
Figure 3
Figure 3
ORTEP of 3b with thermal ellipsoids are shown at the 50% probability level. Two ClO4– anions have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (deg): N1–Fe1 2.121(7), N3–Fe1 2.113(8), N7–Fe1 2.176(8), O1–Fe1 1.984(6), O2–Fe1 2.020(7), O3–Fe1 1.989(7), N4–Ni1 2.099(9), N5–Ni1 2.104(8), N6–Ni1 2.155(8), O1–Ni1 2.033(6), O4–Ni1 2.023(7), O5–Ni1 2.079(6), and Fe1–O1–Ni1 115.8(3).
Figure 4
Figure 4
ORTEP of 4a with thermal ellipsoids are shown at the 50% probability level. Two ClO4– anions have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (deg): Co1–O5 2.046(4), Co1–O2 2.024(4), Co1–O4 2.099(4), Co1–N1 2.178(5), Co1–N2 2.131(5), Co1–N3 2.125(5), Fe1–O5 1.971(4), Fe1–O1 2.001(4), Fe1–O3 1.954(4), Fe1–N4 2.162(4), Fe1–N5 2.106(5), Fe1–N7 2.076(5), and Fe1–O5–Co1 116.11(17).
Figure 5
Figure 5
ORTEP of 4b with thermal ellipsoids are shown at the 50% probability level. Two ClO4– anions have been omitted for the sake of clarity. Selected bond lengths (Å) and angles (°): N1−Cu1 2.220(7), N2−Cu1 2.096(7), N3−Cu1 2.060(7), N4−Fe1 2.130(6), N5−Fe1 2.201(6), N6−Fe1 2.135(7), O1−Fe1 1.973(5), O1−Cu1 2.028(5), O2−Cu1 1.969(7), O3−Fe1 2.051(6), O4−Fe1 1.933(5), O5−Cu1 2.317(6), and Fe1−O1−Cu1 118.1(2).
Figure 6
Figure 6
Overlay of the variable temperature χMT plot of the powder sample of {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)] measured at 5000 Oe (the solid line represents the best fit curve).
Scheme 3
Scheme 3. Equilibrium of Chemical Species in the CH3CN/H2O Solution Proposed for the Dinuclear {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)] Type Complexes Supported over Unsymmetrical [N6O] Ligand 2
Scheme 4
Scheme 4. Proposed Mechanism for the Hydrolysis of 2,4-BDNPP as Catalyzed by Dinuclear {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)] Type Complexes Supported over Unsymmetrical [N6O] Ligand 2
Figure 7
Figure 7
pH profile overlay of the initial rates for the hydrolysis of 2,4-BDNPP as catalyzed by {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)] type complexes in CH3CN/H2O (v/v 1:1).
Figure 8
Figure 8
Overlay of the Lineweaver–Burk plots for the hydrolysis of 2,4-BDNPP as catalyzed by {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)] type complexes in CH3CN/H2O (v/v 1:1). The solid lines represent the best fit lines.
Chart 1
Chart 1. Hydrolysis of 2,4-BDNPP as catalyzed by dinuclear {L[MII(μ-OAc)2FeIII]}(ClO4)2 [M = Zn (3a), Ni (3b), Co (4a), and Cu (4b)] type complexes supported over unsymmetrical [N6O] ligand 2
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