Synthesis, structure and catalase-like activity of dimanganese(III) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity

Several lines of antioxidant defense against oxidative stress: antioxidant enzymes, nanomaterials with multiple enzyme-mimicking activities, and low-molecular-weight antioxidants

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are well recognized for playing a dual role, since they can be either deleterious or beneficial to biological systems. An imbalance between ROS production and elimination is termed oxidative stress, a critical factor and common denominator of many chronic diseases such as cancer, cardiovascular diseases, metabolic diseases, neurological disorders (Alzheimer's and Parkinson's diseases), and other disorders. To counteract the harmful effects of ROS, organisms have evolved a complex, three-line antioxidant defense system. The first-line defense mechanism is the most efficient and involves antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). This line of defense plays an irreplaceable role in the dismutation of superoxide radicals (O2•-) and hydrogen peroxide (H2O2). The removal of superoxide radicals by SOD prevents the formation of the much more damaging peroxynitrite ONOO- (O2•-  + NO• → ONOO-) and maintains the physiologically relevant level of nitric oxide (NO•), an important molecule in neurotransmission, inflammation, and vasodilation. The second-line antioxidant defense pathway involves exogenous diet-derived small-molecule antioxidants. The third-line antioxidant defense is ensured by the repair or removal of oxidized proteins and other biomolecules by a variety of enzyme systems. This review briefly discusses the endogenous (mitochondria, NADPH, xanthine oxidase (XO), Fenton reaction) and exogenous (e.g., smoking, radiation, drugs, pollution) sources of ROS (superoxide radical, hydrogen peroxide, hydroxyl radical, peroxyl radical, hypochlorous acid, peroxynitrite). Attention has been given to the first-line antioxidant defense system provided by SOD, CAT, and GPx. The chemical and molecular mechanisms of antioxidant enzymes, enzyme-related diseases (cancer, cardiovascular, lung, metabolic, and neurological diseases), and the role of enzymes (e.g., GPx4) in cellular processes such as ferroptosis are discussed. Potential therapeutic applications of enzyme mimics and recent progress in metal-based (copper, iron, cobalt, molybdenum, cerium) and nonmetal (carbon)-based nanomaterials with enzyme-like activities (nanozymes) are also discussed. Moreover, attention has been given to the mechanisms of action of low-molecular-weight antioxidants (vitamin C (ascorbate), vitamin E (alpha-tocopherol), carotenoids (e.g., β-carotene, lycopene, lutein), flavonoids (e.g., quercetin, anthocyanins, epicatechin), and glutathione (GSH)), the activation of transcription factors such as Nrf2, and the protection against chronic diseases. Given that there is a discrepancy between preclinical and clinical studies, approaches that may result in greater pharmacological and clinical success of low-molecular-weight antioxidant therapies are also subject to discussion.

Dinuclear manganese(iii) complexes with bioinspired coordination and variable linkers showing weak exchange effects: a synthetic, structural, spectroscopic and computation study

High-resolution HFEPR indicates weak exchange interactions between Mn^III ions in agreement with DFT calculations.

Synthesis, characterization, and reactivity studies of a water-soluble bis(alkoxo)(carboxylato)-bridged diMn(III) complex modeling the active site in catalase

A new diMn(III) complex, Na[Mn2(5-SO3-salpentO)(μ-OAc)(μ-OMe)(H2O)]·4H2O, where 5-SO3-salpentOH = 1,5-bis(5-sulphonatosalicylidenamino)pentan-3-ol, has been prepared and characterized. ESI-mass spectrometry, paramagnetic (1)H NMR, EPR and UV-visible spectroscopic studies on freshly prepared solutions of the complex in methanol and 9 : 1 methanol-water mixtures showed that the compound retains the triply bridged bis(μ-alkoxo)(μ-acetato)Mn2(3+) core in solution. In the 9 : 1 methanol-water mixture, slow substitution of acetate by water molecules took place, and after one month, the doubly bridged diMn(III) complex, [Mn2(5-SO3-salpentO)(μ-OMe)(H2O)3]·5H2O, formed and could be characterized by X-ray diffraction analysis. In methanolic or aqueous basic media, acetate shifts from a bridging to a terminal coordination mode, affording the highly stable [Mn2(5-SO3-salpentO)(μ-OMe)(OAc)](-) anion. The efficiency of the complex in disproportionating H2O2 depends on the solvent and correlates with the stability of the complex (towards metal dissociation) in each medium: basic buffer > aqueous base > water. The buffer preserves the integrity of the catalyst and the rate of O2 evolution remains essentially constant after successive additions of excess of H2O2. Turnovers as high as 3000 mol H2O2 per mol of catalyst, without significant decomposition and with an efficiency of k(cat)/K(M) = 1028 M(-1) s(-1), were measured for the complex in aqueous buffers of pH 11. Kinetic and spectroscopic results suggest a catalytic cycle that runs between Mn(III)2 and Mn(IV)2 oxidation states, which is consistent with the low redox potential observed for the Mn(III)2/Mn(III)Mn(IV) couple of the catalyst in basic medium.

What controls the magnetic interaction in bis-μ-alkoxo Mn(III) dimers? A combined experimental and theoretical exploration

The synthesis and magnetic characterisation of a series of bis-μ-alkoxide bridged Mn(III) dinuclear complexes of general formula [Mn(III)(2)(μ-OR)(2)(biphen)(2)(ROH)(x)(L)(y)] (where R = Me, Et; H(2) biphen = 2,2'-biphenol and L = terminally bonded N-donor ligand) is described, doubling the literature basis set for this type of complex. Building on these findings we have categorised all known μ-OR bridged Mn(III) dinuclear complexes into one of three classifications with respect to their molecular structures. We have then employed DFT and MO calculations to assess all potential magneto-structural correlations for this class of compound in order to identify the structural requirements for constructing ferromagnetic family members. Our analysis indicates that the most influential parameter which governs the exchange interaction in this class of compounds is the relative orientation of the JT axes of the Mn(III)  atoms. A perpendicular orientation of the JT axes leads to a large ferromagnetic contribution to the exchange. These results also suggest that a large ferromagnetic interaction and a large anisotropy are unlikely to co-exist in such structural types.

Catalase and glutathione peroxidase mimics

Overproduction of the reactive oxygen species (ROS) superoxide (O(2)(-)) and hydrogen peroxide (H(2)O(2)) are increasingly implicated in human disease and aging. ROS are also being explored as important modulating agents in a number of cell signaling pathways. Earlier work has focused on development of small catalytic scavengers of O(2)(-), commonly referred to as superoxide dismutase (SOD) mimetics. Many of these compounds also have substantial abilities to catalytically scavenge H(2)O(2) and peroxynitrite (ONOO(-)). Peroxides have been increasingly shown to disrupt cell signaling cascades associated with excessive inflammation associated with a wide variety of human diseases. Early studies with enzymatic scavengers like SOD frequently reported little or no beneficial effect in biologic models unless SOD was combined with catalase or a peroxidase. Increasing attention has been devoted to developing catalase or peroxidase mimetics as a way to treat overt inflammation associated with the pathophysiology of many human disorders. This review will focus on recent development of catalytic scavengers of peroxides and their potential use as therapeutic agents for pulmonary, cardiovascular, neurodegenerative and inflammatory disorders.

Structure and Properties of Dinuclear Manganese(III) Complexes with Pentaanionic Pentadentate Ligands Including Alkoxo, Amido, and Phenoxo Donors

Doubly bridged mu-alkoxo-mu-X (X = pyrazolato or acetato) dinuclear MnIII complexes of 2-hydroxy-N-{2-hydroxy-3-[(2-hydroxybenzoyl)amino]propyl}benzamide) (H5L1) and 2-hydroxy-N-{2-hydroxy-4-[(2-hydroxybenzoyl)amino]butyl}benzamide (H5L2), [Mn2(L)(pz)(MeOH)4].xMeOH (1, L = L1, x = 0.5; 2, L = L2, x = 0; Hpz = pyrazole) and [Mn2(L1)(OAc)(MeOH)4] (3), have been prepared, and their structure and magnetic properties have been studied. The X-ray diffraction analysis of 1 (C24.5H34Mn2N4O9.5, triclinic, P, a = 12.2050(7) A, b = 12.7360(8) A, c = 19.2780(10) A, alpha = 99.735(5) degrees , beta = 96.003(4) degrees , gamma = 101.221(5) degrees , V = 2867.6(3) A3, Z = 4), 2 (C25H34Mn2N4O9, triclinic, P, a = 9.4560(5) A, b = 11.0112(5) A, c = 13.8831(6) A, alpha = 90.821(4) degrees , beta = 92.597(4) degrees , gamma = 93.403(4) degrees , V = 1441.29(12) A3, Z = 2), and 3 (C23H32Mn2N2O11, triclinic, P, a = 10.511(5) A, b = 11.713(5) A, c = 13.135(5) A, alpha = 64.401(5) degrees , beta = 74.000(5) degrees , gamma = 66.774(5) degrees , V = 1329.3(10) A3, Z = 2) revealed that all complexes consist of dinuclear units which are further extended into 1D (1 and 3) and 2D (2) supramolecular networks via hydrogen-bonding interactions. Magnetic susceptibility data evidence antiferromagnetic interactions for all three complexes: J = -3.6 cm-1, D approximately 0 cm-1, g = 1.93 (1); J = -2.7 cm-1, D = 0.8 cm-1, g = 1.93 (2); J = -4.9 cm-1, D = 3.8 cm-1, g = 1.95 (3).