New bis[MoO2] and [MoO(O2)] compounds: An artificial enzyme with peroxidase activity against o-phenylenediamine and dopamine

Peroxidase, as a biocatalyst, exhibits high efficacy, specificity, and operate under mild reaction circumstances [1,2]. These enzymes are extensively distributed in nature and play a crucial role in organism metabolism [3]. They have been utilized in various diagnostic and detection assays, including the screening of hydrogen peroxide, glucose, and ascorbic acid [4,5]. However, the usage of natural enzymes is limited due to the high cost of synthesis and purification, tight storage requirements, and lower stability [4,[6], [7], [8], [9]]. Moreover, peroxidases exhibit optimal activity under specific temperature and pH conditions [2,10,11]. In view of these constraints, researchers have shifted their attention to the development of artificial peroxidase mimics as a means of overcoming these limitations and expanding the value of peroxidase-based assays [[12], [13], [14], [15], [16]]. Gao and colleagues were the first to report the use of Fe3O4 nanoparticles (NPs) for the screening of hydrogen peroxide (H2O2) and their demonstration of peroxide-like activity similar to that of the natural enzyme i.e. horseradish peroxidase (HRP) [17]. These findings have led to the progress of a diversity of nanomaterials as enzyme mimics, known as nanozymes, for H2O2 detection. These include magnetic NPs [[18], [19], [20]], graphene oxide [[21], [22], [23]], silver alloy nanostructures [24], [email protected] nanostructures [25], and ruthenium nanomaterials [26]. Qu and his colleagues have investigated the use of graphene quantum dots as peroxidase mimics [21,22], characterized their substrate binding sites [22], and thoroughly studied and categorised nanozymes in the literature [23]. The peroxidase-like activity of dioxido molybdenum(VI) [27] and oxovanadium(IV) [28,29] complexes has been documented in recent literature. However, there is currently no reported instance of the utilization of bis-dioxido- and oxidoperoxidomolybdenum(VI) complexes with H2O2 for mimicking the peroxidase activity of o-peheneylenediamine and dopamine.

There has been a rise of interest in the discovery and usage of dioxidomolybdenum(VI) complexes as catalysts in the synthesis of valuable compounds in recent years [30,31]. These complexes consist of a [MoVIO2]2+ core, which remains stable during catalytic reactions. However, the presence of H2O2 triggers the formation of non-separable oxidoperoxido species, serving as active catalytic entities and enabling reaction efficiently. The activity of numerous dioxidomolybdenum(VI) complexes has been examined in a variety of reactions, including the epoxidation of alkenes [[32], [33], [34], [35], [36], [37], [38]], oxidation of alcohols [39,40], oxidative bromination of organic substrates [[41], [42], [43]], and multicomponent reactions [44]. These complexes were found to exhibit desirable enzyme-mimicking and oxygen atom transfer activities after extensive evaluation [27].

The current research seeks to comprehend the chemistry involved in the formation of bis-dioxidomolybdenum and bis-oxidoperoxidomolybdenum complexes, as well as their application as catalysts in catalytic reactions. Earlier, dihydrazone [45] and semicarbazide [46] ligands were used in the production of bis-[MoO2]2+ complexes. The Schiff base ligand was used in this investigation to create the desired binuclear complexes. The peroxide activity of the resulting dihydrazone-based binuclear molybdenum complexes is evaluated using o-phenylenediamine and dopamine as substrates. The catalytic oxidation of o-phenylenediamine (OPD) leads to the creation of numerous coloured products, among which 2,3-diaminophenazine(DAP) is the major product, which is coloured and fluorescent in the existence of H2O2 and the specific outcome of the reaction may depend on the experimental condition [47,48]. The activity of the complexes is determined by spectrometrically monitoring the rate of product formation. Moreover, the oxidation of dopamine to aminochrome, a precursor of neuromelanin, is also being studied. The oxidation of dopamine has the potential to cause various adverse effects, including neurotoxicity, endoplasmic reticulum stress, proteasome autophagy, lysosomal malfunction, and oxidative stress [49]. Therefore, the detection of dopamine is crucial for biological systems. The current approach provides a suitable way for converting dopamine to aminochrome, utilizing H2O2 and a catalyst. Further studies are necessary to clarify the mechanism of this reaction.

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