Our desire to understand how the individual molecules that make up cells organize, interact, and communicate to form living systems has lead to the burgeoning field of chemical biology, an interfacial area of science that combines aspects of chemistry — the study of matter and its transformations, and biology — the study of living things and their interactions with the environment. The defining feature of chemical biology is the use of chemical approaches and small molecules to interrogate or manipulate biology.1,2 These small molecules are synthetic or naturally occurring ones that, for example, bind to DNA to affect protein expression levels, bind to proteins to inhibit their function, interact with lipids to alter membrane integrity, or become fluorescent in response to a metabolic event. Because small molecules can affect biochemical function, there is a clear link between chemical biology and pharmacology and medicine.3 While small molecules are usually implied as being organic compounds,4 inorganic small molecules also have a long history in both biology and medicine. Ancient civilizations used gold and copper for healing purposes, and the modern era of drug discovery was ushered in when arsenic-containing salvarsan was discovered as an anti-syphilis agent to become the world’s first blockbuster drug.5 Inorganic compounds should therefore not be overlooked in the realm of chemical biology, since their distinctive electronic, chemical, and photophysical properties render them particularly useful for a variety of applications.6-8

What are the properties of metal ions that impart utility to biology? Because inorganic elements comprise the bulk of the periodic table, the diversity of these properties is likewise broad and has been thoroughly covered by several books in the field of bioinorganic chemistry.9-11 A brief summary of the general chemical properties of metals is given below.


Charge. Metal ions are positively charged in aqueous solution, but that charge can be manipulated depending on the coordination environment so that a metal complexed by ligands can be cationic, anionic, or neutral.


Interactions with ligands. Metal ions bind to ligands (both organic and inorganic) via interactions that are often strong and selective. The ligands impart their own functionality and can tune properties of the overall complex that are unique from those of the individual ligand or metal. The thermodynamic and kinetic properties of metal—ligand interactions influence ligand exchange reactions.


Structure and bonding. Metal—ligand complexes span a range of coordination geometries that give them unique shapes compared to organic molecules. The bond lengths, bond angles, and number of coordination sites can vary depending on the metal and its oxidation state.


Lewis acid character. Metal ions with high electron affinity can significantly polarize groups that are coordinated to them, facilitating hydrolysis reactions.


Partially filled d-shell. For the transition metals, the variable number of electrons in the d-shell orbitals (or f-shell for lanthanides) imparts interesting electronic and magnetic properties to transition metal complexes.


Redox activity. Coupled with the variability of electrons in the d-shell is the ability for many transition metals to undergo 1-electron oxidation and reduction reactions.





Biology has taken advantage of these chemical properties of metals to perform several functional roles, which are summarized in Table 1. This is by no means an exhaustive list, but rather a primer to highlight important themes. Some metal ions, particularly the alkali and alkaline earth metals, are stable in aqueous solution as cations, making Na+, K+ and Ca2+ ideal for maintaining charge balance and electrical conductivity.10 On the other hand, the distinct architectures accessible via metal—ligand bonding interactions impart important structural roles to metal ions that encompass both macroscopic structural stabilization, as in biomineralized tissues,12 as well as molecular structural stabilization, as in proteins and nucleic acids that are stabilized in a preferred fold by metal ions.13-16 Metal—ligand bonding is also significant in its reversibility. For example, Nature takes advantage of reversible binding of metal ions like Ca2+ and Zn2+ to proteins or other storage repositories in order to propagate various biochemical signals.13,17 Metal ions themselves can be their own signal to adjust DNA transcription, as in the case of metalloregulatory proteins.18,19 Reversible metal—ligand coordination is also exploited to bind and release molecules to and from a metal center, a prime example being O2 binding and release from hemoglobin.

Table 1

Functional roles of inorganic elements found in biology, with selected representative examples.

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Application of metal coordination chemistry to explore and manipulate cell biology.

Flavonoids are a family of polyphenolic compounds which are widespread in nature (vegetables) and are consumed as part of the human diet in significant amounts. There are other types of polyphenols, including, for example, tannins and resveratrol. Flavonoids and related polyphenolic compounds have significant antiinflammatory activity, among others. This short review summarizes the current knowledge on the effects of flavonoids and related polyphenolic compounds on inflammation, with a focus on structural requirements, the mechanisms involved, and pharmacokinetic considerations. Different molecular (cyclooxygenase, lipoxygenase) and cellular targets (macrophages, lymphocytes, epithelial cells, endothelium) have been identified. In addition, many flavonoids display significant antioxidant/radical scavenging properties. There is substantial structural variation in these compounds, which is bound to have an impact on their biological profile, and specifically on their effects on inflammatory conditions. However, in general terms there is substantial consistency in the effects of these compounds despite considerable structural variations. The mechanisms have been studied mainly in myeloid cells, where the predominant effect is an inhibition of NF-κB signaling and the downregulation of the expression of proinflammatory markers. At present there is a gap in knowledge of in vitro and in vivo effects, although the pharmacokinetics of flavonoids has advanced considerably in the last decade. Many flavonoids have been studied for their intestinal antiinflammatory activity which is only logical, since the gastrointestinal tract is naturally exposed to them. However, their potential therapeutic application in inflammation is not restricted to this organ and extends to other sites and conditions, including arthritis, asthma, encephalomyelitis, and atherosclerosis, among others.

Effects of Flavonoids and other Polyphenols on Inflammation

Catalytic reactions of C–C and C–X bond formation are discussed in this critical review with particular emphasis on cross coupling reactions catalyzed by palladium and wide bite angle bidentate diphosphine ligands. Especially those studies have been collected that allow comparison of the ligand bite angles for the selected ligands: dppp, BINAP, dppf, DPEphos and Xantphos. Similarities with hydrocyanation and CO/ethene/MeOH reactions have been highlighted, while rhodium hydroformylation has been mentioned as a contrasting example, in which predictability is high and steric and electronic effects follow smooth trends. In palladium catalysis wide bite angles and bulkiness of the ligands facilitate generally the reductive elimination thus giving more efficient cross coupling catalysis (174 references).

Bite angle effects of diphosphines in C-C and C-X bond forming cross coupling reactions.

Diabetes, oxidative stress and therapeutic strategies

The use of Carbon Monoxide (CO) as a therapeutic agent has already been tested in human clinical trials. Pre-clinically, CO gas administration proved beneficial in animal models of various human diseases. However, the use of gaseous CO faces serious obstacles not the least being its well-known toxicity. To fully realise the promise of CO as a therapeutic agent, it is key to find novel avenues for CO delivery to diseased tissues in need of treatment, without concomitant formation of elevated, toxic blood levels of carboxyhemoglobin (COHb). CO-releasing molecules (CO-RMs) have the potential to constitute safe treatments if CO release in vivo can be controlled in a spatial and temporal manner. It has already been demonstrated in animals that CO-RMs can release CO and mimic the therapeutic effects of gaseous CO. While demonstrating the principle of treatment with CO-RMs, these first generation compounds are not suitable for human use. This tutorial review summarises the biological and chemical behaviour of CO, the current status of CO-RM development, and derives principles for the creation of the next generation of CO-RMs for clinical applications in humans.

Developing drug molecules for therapy with carbon monoxide

The deoxy-myoglobin (deoxy-Mb)/carbonmonoxy-myoglobin (Mb-CO) UV-vis assay is the principal method used for quantifying the rates of CO release from CO-releasing molecules (CO-RMs) that might possess therapeutic benefits. Some issues emerge when the Mb-CO assay is utilized for testing CO-RMs with novel structures, which are comprehensively discussed here for the first time. Two methods for processing raw UV-vis spectroscopic data generated from the assay are presented in this paper.

Modification of the deoxy-myoglobin/carbonmonoxy-myoglobin UV-vis assay for reliable determination of CO-release rates from organometallic carbonyl complexes

Carbon monoxide-releasing molecules (CO-RMs) are compounds capable of delivering controlled amounts of CO within a cellular environment. Ruthenium-based carbonyls [tricarbonyldichloro ruthenium(II) dimer and tricarbonylchloro-(glycinato)ruthenium(II)] and boronacorbonates (sodium boranocarbonate) have been shown to promote vasodilatory, cardioprotective, and anti-inflammatory activities in a variety of experimental models. Here, we extend our previous studies by showing that eta-4-(4-bromo-6-methyl-2-pyrone)tricarbonyl iron (0) (CORM-F3), an irontricarbonyl complex that contains a 2-pyrone motif, liberates CO in vitro and exerts pharmacological actions that are typical of CO gas. Specifically, CORM-F3 caused vasorelaxation in isolated aortic rings and inhibited the inflammatory response (e.g., nitrite production) of RAW264.7 macrophages stimulated with endotoxin in a dose-dependent fashion. By analyzing the rate of CO release, we found that when the bromide at the 4-position of the 2-pyrone CORM-F3 is substituted with a chloride group [eta-4-(4-chloro-6-methyl-2-pyrone)tricarbonyl iron (0) (CORM-F8)], the rate of CO release is significantly decreased (4.5-fold), and a further decrease is observed when the 4- and 6-positions are substituted with a methyl group [eta-4-(4-methyl-6-methyl-2-pyrone)tricarbonyl iron (0) (CORM-F11)] or a hydrogen [eta-4-(4-chloro-2-pyrone)tricarbonyl iron (0) (CORM-F7)], respectively. Interestingly, the compounds containing halogens at the 4-position and the methyl at the 6-position of the 2-pyrone ring (CORM-F3 and CORM-F8) were found to be less cytotoxic compared with other CO-RMs when tested in RAW246.7 macrophages. Thus, iron-based carbonyls mediate pharmacological responses that are achieved through liberation of CO and the nature of the substituents in the organic ligand have a profound effect on both the rate of CO release and cytotoxicity.

https://jpet.aspetjournals.org/content/jpet/early/2006/04/07/jpet.106.101758.full.pdf

Bioactive Properties of Iron-Containing Carbon Monoxide-Releasing Molecules

η4-Pyrone iron(0)carbonyl complexes as effective CO-releasing molecules (CO-RMs)

An evaluation of the CO releasing ability of iron(II) and molybdenum(II) complexes has facilitated the discovery of the most rapid CO releaser, namely [Mo(CO)3(η5-C5H5)(η1-{O}–C{O}–O–CMeCH-COMeCBr)]BF4 (CORM-F10), reported to date. The rate of CO release is related to the overall solution phase stability of the transition metal carbonyl complex. The cytotoxity and vasodilatory properties of CORM-F10 have been determined.

η1-2-Pyrone metal carbonyl complexes as CO-releasing molecules (CO-RMs): A delicate balance between stability and CO liberation

Chalcones are naturally occurring flavonoids composed of two aromatic rings connected by a three-carbon unit forming an alpha-beta unsaturated carbonyl group. They are pharmacologically relevant because of their ability to exert anticarcinogenic, antimicrobial, and anti-inflammatory activities. Recent evidence indicates that the bioactivity of hydroxy-chalcones is correlated with their intrinsic property to induce the antioxidant and cytoprotective enzyme heme oxygenase-1 (HO-1). In the present study, we assessed how the methoxy substituents positioned on the two aromatic rings affect the anti-inflammatory action of different chalcones in relation to their ability to increase heme oxygenase in RAW246.7 macrophages. Structure-activity relationships of methoxychlacones were qualitatively and quantitatively examined and correlated with inhibition of endotoxin-mediated nitrite production and cytotoxic effects. Our data indicate that (i) a progressive increase in heme oxygenase activity is obtained by sequentially increasing the number of methoxy substituents in the 3,4,5- and 3',4',5'-positions of the aromatic rings; (ii) methoxy substituents placed either in the 2,4,6-positions or alone in the 4- or 4'-position are ineffective; (iii) increased heme oxygenase activity by chalcones is lost when the alpha-beta double bond and the carbonyl group are reduced or protected; (iv) the anti-inflammatory activity and cytotoxicity profiles of the chalcones examined correlate with their potency as HO-1 inducers; and (v) chalcone-mediated HO-1 induction is reduced by thiols or inhibitors of phosphatidylinositol-3 kinase (PI3K) pathway. This study provides additional information on the structural features that methoxychalcones and natural antioxidants need to possess to be considered as therapeutic agents for maximizing HO-1 expression and activity.

Benjamin E. Moulton

Papers

Modification of the deoxy-myoglobin/carbonmonoxy-myoglobin UV-vis assay for reliable determination of CO-release rates from organometallic carbonyl complexes

Bioactive Properties of Iron-Containing Carbon Monoxide-Releasing Molecules

η4-Pyrone iron(0)carbonyl complexes as effective CO-releasing molecules (CO-RMs)

η1-2-Pyrone metal carbonyl complexes as CO-releasing molecules (CO-RMs): A delicate balance between stability and CO liberation

Structure-activity relationships of methoxychalcones as inducers of heme oxygenase-1.