Lisa M. Berreau

Molecules that enable the controlled delivery of carbon monoxide (CO) in biological environments are of significant current interest to probe the beneficial roles of CO for humans. Assumptions regarding the ability of molecules to reliably deliver CO continue to impact the field, including in work involving non-metal CO delivery motifs. Flavonols are drawing increasing interest as light-triggered CO release motifs due to their ease of synthesis, functionalization, and fluorescence trackability. Importantly, the light-driven CO release properties of flavonols depend on their structure and must be fully evaluated under various conditions to understand the relationship between the amount of CO delivered and the induced biological effects. Herein, we use a family of amine-functionalized π-extended flavonols to demonstrate that structural differences result in differing interactions with biomolecules, cellular uptake, and changes in subcellular localization, which can affect the amount of CO delivered intracellularly. This results in differences in the CO-induced cellular toxicity.

The π-extended flavonol Flav-1 (1) undergoes reaction with KOH or KO₂ to form 1^-, which reacts with O₂ at ambient temperature, resulting in CO release and depside formation. Mechanistic and DFT studies support a reaction pathway involving reaction of 1^- with O₂ on the triplet energy surface in the rate-determining step. Formation of a cyclic peroxide leads to CO extrusion. These studies indicate that if formed in biological environments, 1^- will release CO in the absence of light illumination.

Carbon monoxide (CO) delivery molecules are of significant current interest as potential therapeutics, including for anticancer applications. A recent approach toward generating new types of materials‐based anticancer agents involves combining the Fenton reactivity of a redox active metal ion with CO delivery. However, small molecule examples of these types of entities have not been systematically studied to evaluate the combined effect on cellular toxicity. Herein we describe a Cu(II) flavonolato complex which produces anticancer effects through a combination of copper‐mediated reactive oxygen species (ROS) generation and light‐induced flavonol CO release. Confocal microscopy studies provide evidence of enhanced flavonol uptake in the copper flavonolato system relative to the free flavonol, which leads to an increased amount of CO delivery within cells. Importantly, this work demonstrates that a metal flavonolato species can be used to produce enhanced toxicity effects resulting from both metal ion‐induced Fenton reactivity and increased cellular uptake of a flavonol CO donor.

Illumination of aerobic acetonitrile solutions of bipyridine-ligated Co(ii) chlorodiketonate complexes results in O₂-dependent aliphatic C–C bond cleavage with high ¹⁸O incorporation.

Light-triggered carbon monoxide (CO) delivery molecules are of significant current interest for evaluating the role of CO in biology and as potential therapeutics. Herein we report the first example of a metal free CO delivery molecule that can be tracked via confocal microscopy at low micromolar concentrations in cells prior to CO release. The NEt₂-appended extended flavonol (4) localizes to the endoplasmic reticulum, mitochondria, and lysosomes. Subcellular localization of 4 results in CO-induced toxicity effects that are distinct as compared to a nonlocalized analog. Anti-inflammatory effects of 4, as measured by TNF-α suppression, occur at the nanomolar level in the absence of CO release, and are enhanced with visible-light-induced CO release. Overall, the highly trackable nature of 4 enables studies of the biological effects of both a localized flavonol and CO release at low micromolar to nanomolar concentrations.

Illumination of a 3-hydroxybenzo[g]quinolone with visible light results in CO releaseviatwo different reaction pathways.

Six-coordinate Cu(ii) diketonate complexes were isolated and characterized; TPA-ligated Cu(ii) chlorodiketonate derivatives exhibit anaerobic retro-Claisen reactivity.

Carbon monoxide (CO) is a gaseous signaling molecule produced in humans via the breakdown of heme in an O₂-dependent reaction catalyzed by heme oxygenase enzymes. A long-lived species relative to other signaling molecules (e.g., NO, H₂S), CO exerts its physiological effects via binding to low-valent transition metal centers in proteins and enzymes. Studies involving the administration of low doses of CO have shown its potential as a therapeutic agent to produce vasodilation, anti-inflammatory, antiapoptotic, and anticancer effects. In pursuit of developing tools to define better the role and therapeutic potential of CO, carbon monoxide releasing molecules (CORMs) were developed. To date, the vast majority of reported CORMs have been metal carbonyl complexes, with the most well-known being Ru₂Cl₄(CO)₆ (CORM-2), Ru(CO)₃Cl(glycinate) (CORM-3), and Mn(CO)₄(S₂CNMe(CH₂CO₂H)) (CORM-401). These complexes have been used to probe the effects of CO in hundreds of cell- and animal-based experiments. However, through recent investigations, it has become evident that these reagents exhibit complicated reactivity in biological environments. The interpretation of the effects produced by some of these complexes is obscured by protein binding, such that their formulation is not clear, and by CO leakage and potential redox activity. An additional weakness with regard to CORM-2 and CORM-3 is that these compounds cannot be tracked via fluorescence. Therefore, it is unclear where or when CO release occurs, which confounds the interpretation of experiments using these molecules. To address these weaknesses, our research team has pioneered the development of metal-free CORMs based on structurally tunable extended flavonol or quinolone scaffolds. In addition to being highly controlled, with CO release only occurring upon triggering with visible light (photoCORMs), these CO donors are trackable via fluorescence prior to CO release in cellular environments and can be targeted to specific cellular locations. In the Account, we highlight the development and application of a series of structurally related flavonol photoCORMs that (1) sense characteristics of cellular environments prior to CO release; (2) enable evaluation of the influence of cytosolic versus mitochondrial-localized CO release on cellular bioenergetics; (3) probe the cytotoxicity and anti-inflammatory effects of intracellular versus extracellular CO delivery; and (4) demonstrate that albumin delivery of a photoCORM enables potent anticancer and anti-inflammatory effects. A key advantage of using triggered CO release compounds in these investigations is the ability to examine the effects of the molecular delivery vehicle in the absence and presence of localized CO release, thus providing insight into the independent contributions of CO. Overall, flavonol-based CO delivery molecules offer opportunities for triggerable, trackable, and targetable CO delivery that are unprecedented in terms of previously reported CORMs and, thus, offer significant potential for applications in biological systems.

N₃-Ligated Ni(ii) diketonate complexes are air stable and do not exhibit oxidative cleavage within the diketonate ligand under aerobic conditions.