Increased Efficiency of a Functional SOD Mimic Achieved with Pyridine Modification on a Pyclen-Based Copper(II) Complex

Dual-domain superoxide dismutase: In silico prediction directed combinatorial mutation for enhanced robustness and catalytic efficiency

The robustness and catalytic activity of superoxide dismutase (SOD) are still the main factors limiting their application in industrial fields. This study aims to further improve the properties of a natural thermophilic iron/manganese dual-domain SOD (Fe/Mn-SODA fused with N-terminal polypeptide) from Geobacillus thermodenitrificans NG80-2 (GtSOD) by modifying its each domain using in-depth in silico prediction analysis as well as protein engineering. First, computational analysis of the N-terminal domain and GtSODA domain was respectively performed by using homologous sequence alignment and virtual mutagenesis. Seven proposed mutation sites favoring increased robustness were screened out for single-point mutants (SPMs) construction. Enzymatic characterization of these SPMs identified the most favorable mutation sites E107 and S265 located in two different domains. Subsequently, the dual-domain site combinatorial mutant (DDSCM) E107L/S265K showed significant superposition effects and additional improvement in catalytic efficiency, with a Kcat/Km value of 145.45 %, 33.66 %, and 60.33 % higher than the wild type (WT), the SPMs E107L and S265K, respectively. Molecular dynamics simulations, structural and surface charge analysis revealed the possible mechanism by which combinatorial mutations improve the robustness and catalytic activity of GtSOD. Furthermore, DDSCM showed more significant resistance to ultraviolet B and various stress than WT, indicating its highly competitive industrial application prospects.

Use of Intramolecular Quinol Redox Couples to Facilitate the Catalytic Transformation of O2 and O2-Derived Species

ConspectusThe redox reactivity of transition metal centers can be augmented by nearby redox-active inorganic or organic moieties. In some cases, these functional groups can even allow a metal center to participate in reactions that were previously inaccessible to both the metal center and the functional group by themselves. Our research groups have been synthesizing and characterizing coordination complexes with polydentate quinol-containing ligands. Quinol is capable of being reversibly oxidized by either one or two electrons to semiquinone or para-quinone, respectively. Functionally, quinol behaves much differently than phenol, even though the pKa values of the first O-H bonds are nearly identical.The redox activity of the quinol in the polydentate ligand can augment the abilities of bound redox-active metals to catalyze the dismutation of O2-• and H2O2. These complexes can thereby act as high-performing functional mimics of superoxide dismutase (SOD) and catalase (CAT) enzymes, which exclusively use redox-active metals to transfer electrons to and from these reactive oxygen species (ROS). The quinols augment the activity of redox-active metals by stabilizing higher-valent metal species, providing alternative redox partners for the oxidation and reduction of reactive oxygen species, and protecting the catalyst from destructive side reactions. The covalently attached quinols can even enable redox-inactive Zn(II) to catalyze the degradation of ROS. With the Zn(II)-containing SOD and CAT mimics, the organic redox couple entirely substitutes for the inorganic redox couples used by the enzymes. The ligand structure modulates the antioxidant activity, and thus far, we have found that compounds that have poor or negligible SOD activity can nonetheless behave as efficient CAT mimics.Quinol-containing ligands have also been used to prepare electrocatalysts for dioxygen reduction, functionally mimicking the enzyme cytochrome c oxidase. The installation of quinols can boost electrocatalytic activity and even enable otherwise inactive ligand frameworks to support electrocatalysis. The quinols can also shift the product selectivity of O2 reduction from H2O2 to H2O without markedly increasing the effective overpotential. Distinct control of the coordination environment around the metal center allows the most successful of these catalysts to use economic and naturally abundant first-row transition metals such as iron and cobalt to selectively reduce O2 to H2O at low effective overpotentials. With iron, we have found that the electrocatalysts can enter the catalytic cycle as either an Fe(II) or Fe(III) species with no difference in turnover frequency. The entry point to the cycle, however, has a marked impact on the effective overpotential, with the Fe(III) species thus far being more efficient.

Rings of Power: Controlling SOD Mimic Activity by the Addition of Pyridine Rings within the Pyridinophane Scaffold

Superoxide dismutase enzymes are a major defense against superoxide, which is a potent reactive oxygen species. Misregulation of reactive oxygen species and subsequent neuronal damage are etiological hallmarks of neurodegenerative disease. Macrocyclic small molecules have offered inroads toward functional SOD1 mimics. Herein, we report a series of five tetra-aza macrocyclic RPy2N2 ligands, varied by 4-position substitution of the pyridine ring with both electron-donating (R = OMe) and withdrawing groups (R = Cl, I, and CF3) to offer the first comparison to well-studied RPyN3 congeners and other mimics in the literature. New ligands have been characterized by NMR, mass spectrometry, elemental analysis, and potentiometric titrations (pKa). Cyclic voltammetry and X-ray diffraction analysis of the copper(II) complexes (CuII(RPy2N2)2+) demonstrate how pyridine substitution impacts the metal center. This data, and evaluation of the log βCu(II) and log βCu(I) within the series, indicates significant improvement to the binding affinity for Cu(I) without sacrifice of Cu(II) binding. The CuII(RPy2N2)2+ series yield the highest kcat for any Cu(II)-based small molecule functional SOD1 mimic (kcat = 45.36 M-1 s-1). Furthermore, the CuII(OMePy2N2)2+ and CuII(CF3Py2N2)2+ complexes were studied in FRDA cells to determine cell toxicity as a first step toward the application of these mimics as therapeutics for neurological disease.

Chelating Properties of N6O-Donors Toward Cu(II) Ions: Speciation in Aqueous Environments and Catalytic Activity of the Dinuclear Complexes

This study focuses on the use of three isostructural N6O donor ligands, specifically known to form complexes with copper ions, to chelate Cu(II) from aqueous solutions. The corresponding Cu(II) complexes feature a dinuclear copper core mimicking the active site of natural superoxide dismutase (SOD) enzymes while also creating a coordination environment favorable for catalase (CAT) activity, being thus appealing as catalytic antioxidant systems. Given the critical role of copper dysregulation in the pathophysiology of Alzheimer's disease (AD), these complexes may help mitigate the harmful effects of free Cu(II) ions: the goal is to transform copper's reactive oxygen species (ROS)-generating properties into beneficial ROS-scavenging action. This study investigates the speciation, chelating efficiency, and metal selectivity of these ligands, as well as the antioxidant activity of the resulting complexes under aqueous and physiologically relevant conditions. Additionally, the ligands, equipped with functional groups for attaching targeting moieties, are conjugated with a small peptide that may act as an anti-aggregating agent of β-amyloid peptides, aiming to develop a multifunctional therapeutic strategy against Alzheimer's disease.

A Multipurpose Metallophore and Its Copper Complexes with Diverse Catalytic Antioxidant Properties to Deal with Metal and Oxidative Stress Disorders: A Combined Experimental, Theoretical, and In Vitro Study

We report the discovery that the molecule 1-(pyridin-2-ylmethylamino)propan-2-ol (HL) can reduce oxidative stress in neuronal C6 glioma cells exposed to reactive oxygen species (O2-•, H2O2, and •OH) and metal (Cu+) stress conditions. Furthermore, its association with Cu2+ generates [Cu(HL)Cl2] (1) and [Cu(HL)2](ClO4)2 (2) complexes that also exhibit antioxidant properties. Potentiometric titration data show that HL can coordinate to Cu2+ in 1:1 and 1:2 Cu2+:ligand ratios, which was confirmed by monocrystal X-ray studies. The subsequent ultraviolet-visible, electrospray ionization mass spectrometry, and electron paramagnetic resonance experiments show that they can decompose a variety of reactive oxygen species (ROS). Kinetic studies revealed that 1 and 2 mimic the superoxide dismutase and catalase activities. Complex 1 promotes the fastest decomposition of H2O2 (kobs = 2.32 × 107 M-1 s-1), efficiently dismutases the superoxide anion (kcat = 3.08 × 107 M-1 s-1), and scavenges the hydroxyl radical (RSA50 = 25.7 × 10-6 M). Density functional theory calculations support the formation of dinuclear Cu-peroxide and mononuclear Cu-superoxide species in the reactions of [Cu(HL)Cl2] with H2O2 and O2•-, respectively. Furthermore, both 1 and 2 also reduce the oxidative stress of neuronal glioma C6 cells exposed to different ROS, including O2•- and •OH.

Polymeric Nanozyme with SOD Activity Capable of Inhibiting Self- and Metal-Induced α-Synuclein Aggregation

Despite decades of research, Parkinson's disease is still an idiopathic pathology for which no cure has yet been found. This is partly explained by the multifactorial character of most neurodegenerative syndromes, whose generation involves multiple pathogenic factors. In Parkinson's disease, two of the most important ones are the aggregation of α-synuclein and oxidative stress. In this work, we address both issues by synthesizing a multifunctional nanozyme based on grafting a pyridinophane ligand that can strongly coordinate CuII, onto biodegradable PEGylated polyester nanoparticles. The resulting nanozyme exhibits remarkable superoxide dismutase activity together with the ability to inhibit the self-induced aggregation of α-synuclein into amyloid-type fibrils. Furthermore, the combination of the chelator and the polymer produces a cooperative effect whereby the resulting nanozyme can also halve CuII-induced α-synuclein aggregation.

Effect of Ligand Substituents on Spectroscopic and Catalytic Properties of Water-Compatible Cp*Ir-(pyridinylmethyl)sulfonamide-Based Transfer Hydrogenation Catalysts

Transition-metal-based hydrogenation catalysts have applications ranging from high-value chemical synthesis to medicinal chemistry. A series of (pyridinylmethyl)sulfonamide ligands substituted with electron-withdrawing and -donating groups were synthesized to study the influence of the electronic contribution of the bidentate ligand in Cp*Ir piano-stool complexes. A variable-temperature NMR investigation revealed a strong correlation between the electron-donating ability of the substituent and the rate of stereoinversion of the complexes. This correlation was partially reflected in the catalytic activity of the corresponding catalysts. Complexes with electron-withdrawing substituents followed the trend observed in the variable-temperature NMR study, thereby confirming the rate-determining step to be donation of the hydride ligand. Strongly electron-donating groups, on the other hand, caused a change in the rate-determining step in the formation of the iridium-hydride species. These results demonstrate that the activity of these catalysts can be tuned systematically via changes in the electronic contribution of the bidentate (pyridinylmethyl)sulfonamide ligands.

Coordination Chemistry of Sulfur-Containing Bifunctional Chelators: Toward in Vivo Stabilization of 64Cu PET Imaging Agents for Alzheimer's Disease

The broader utilization of 64Cu positron emission tomography (PET) imaging agents has been hindered by the unproductive demetalation induced by bioreductants. To advance the development of 64Cu-based PET imaging tracers for Alzheimer's Disease (AD), there is a need for novel ligand design strategies. In this study, we developed sulfur-containing dithiapyridinophane (N2S2) bifunctional chelators (BFCs) as well as all nitrogen-based diazapyridinophane (N4) BFCs to compare their abilities to chelate Cu and target Aβ aggregates. Through spectrophotometric titrations and electrochemical measurements, we have demonstrated that the N2S2-based BFCs exhibit >10 orders of magnitude higher binding affinity toward Cu(I) compared to their N4-based counterparts, while both types of BFCs exhibit high stability constants toward Cu(II). Notably, solid state structures for both Cu(II) and Cu(I) complexes supported by the two ligand frameworks were obtained, providing molecular insights into their copper chelating abilities. Aβ binding experiments were conducted to study the structure-affinity relationship, and fluorescence microscopy imaging studies confirmed the selective labeling of the BFCs and their copper complexes. Furthermore, we investigated the potential of these ligands for the 64Cu-based PET imaging of AD through radiolabeling and autoradiography studies. We believe our findings provide molecular insights into the design of bifunctional Cu chelators that can effectively stabilize both Cu(II) and Cu(I) and, thus, can have significant implications for the development of 64Cu PET imaging as a diagnostic tool for AD.

Hydrogen Peroxide Disproportionation Activity Is Sensitive to Pyridine Substitutions on Manganese Catalysts Derived from 12-Membered Tetra-Aza Macrocyclic Ligands

The abundance of manganese in nature and versatility to access different oxidation states have made manganese complexes attractive as catalysts for oxidation reactions in both biology and industry. Macrocyclic ligands offer the advantage of substantially controlling the reactivity of the manganese center through electronic tuning and steric constraint. Inspired by the manganese catalase enzyme, a biological catalyst for the disproportionation of H2O2 into water and O2, the work herein employs 12-membered tetra-aza macrocyclic ligands to study how the inclusion of and substitution to the pyridine ring on the macrocyclic ligand scaffold impacts the reactivity of the manganese complex as a H2O2 disproportionation catalyst. Synthesis and isolation of the manganese complexes was validated by characterization using UV-vis spectroscopy, SC-XRD, and cyclic voltammetry. Potentiometric titrations were used to study the ligand basicity as well as the thermodynamic equilibrium with Mn(II). Manganese complexes were also produced in situ and characterized using electrochemistry for comparison to the isolated species. Results from these studies and H2O2 reactivity showed a remarkable difference among the ligands studied, revealing instead a distinction in the reactivity regarding the number of pyridine rings within the scaffold. Moreover, electron-donating groups on the 4-position of the pyridine ring enhanced the reactivity of the manganese center for H2O2 disproportionation, demonstrating a handle for control of oxidation reactions using the pyridinophane macrocycle.