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The 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.
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The First-Row Transition Metals in the Periodic Table of Medicine
In this manuscript, we describe medical applications of each first-row transition metal including nutritional, pharmaceutical, and diagnostic applications. The 10 first-row transition metals in particular are found to have many applications since there five essential elements among them. We summarize the aqueous chemistry of each element to illustrate that these fundamental properties are linked to medical applications and will dictate some of nature’s solutions to the needs of cells. The five essential trace elements—iron, copper, zinc, manganese, and cobalt—represent four redox active elements and one redox inactive element. Since electron transfer is a critical process that must happen for life, it is therefore not surprising that four of the essential trace elements are involved in such processes, whereas the one non-redox active element is found to have important roles as a secondary messenger.. Perhaps surprising is the fact that scandium, titanium, vanadium, chromium, and nickel have many applications, covering the entire range of benefits including controlling pathogen growth, pharmaceutical and diagnostic applications, including benefits such as nutritional additives and hardware production of key medical devices. Some patterns emerge in the summary of biological function and medical roles that can be attributed to small differences in the first-row transition metals.
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- Award ID(s):
- 1709564
- PAR ID:
- 10195427
- Date Published:
- Journal Name:
- Inorganics
- Volume:
- 7
- Issue:
- 9
- ISSN:
- 2304-6740
- Page Range / eLocation ID:
- 111-138
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3390/inorganics7090111
