Tetraazamacrocyclic derivatives and their metal complexes as antileishmanial leads

Investigation of antileishmanial, antioxidant activities, CT-DNA interaction and DFT study of novel cobalt(II) complexes derived from mesogenic aromatic amino acids based Schiff base ligands

In the present work, new Co(II) complexes were synthesized from mesogenic aromatic amino acids based Schiff base ligands, HL1 [Methyl 2-((2-hydroxy-4-(tetradecyloxy)benzylidene)amino)-3-phenylpropanoate] and HL2 [Methyl 2-((2-hydroxy-4-(tetradecyloxy)benzylidene)amino)-3-(1H-indol-2-yl)propanoate]. The compounds were thoroughly characterised using different elemental, thermogravimetric and spectroscopic studies. The in-vitro antileishmanial efficacy of the compounds against Leishmania donovani was evaluated by MTT assay and the antioxidant activity was performed by Mensor's method. The cell viability percentage and IC50 values for both the antileishmanial and antioxidant studies revealed that the cobalt(II) complexes are comparable to the standard, amphotericin B and ascorbic acid, respectively, signifying the potential applications of the biogenic compounds. The CT-DNA interaction experiments study using photophysical techniques indicated that the cobalt(II) complexes exhibited pronounced interactions as compared to the parent ligand. The parent ligands were found to possess mesogenicity as evidenced from the polarizing optical microscope (POM) and differential scanning calorimetry (DSC). The optical band gap of the compounds, as estimated from the Tauc plot of the UV-Vis spectra, lies within the domain of optoelectronic material properties, which was further supported through Density Functional Theory (DFT) study. Moreover, DFT methods have been used to explore the ground state geometry and DFT based reactivity descriptors of the two synthesised ligands, HL1 and HL2 along with their corresponding Co(II) complexes, Co(L1)2 and Co(L2)2. Reactivity descriptors obtained from Conceptual Density Functional Theory (CDFT) analysis reveal that Co(L1)2 is the most stable and Co(L2)2 is the most electrophilic.

Disclosing the Antifungal Mechanisms of the Cyclam Salt H4[H2(4-CF3PhCH2)2Cyclam]Cl4 against Candida albicans and Candida krusei

Mycoses are one of the major causes of morbidity/mortality among immunocompromised individuals. Considering the importance of these infections, the World Health Organization (WHO) defined a priority list of fungi for health in 2022 that include Candida albicans as belonging to the critical priority group and Pichia kudriavzevii (Candida krusei) to the medium priority group. The existence of few available antifungal drugs, their high toxicity, the acquired fungal resistance, and the appearance of new species with a broader spectrum of resistance, points out the need for searching for new antifungals, preferably with new and multiple mechanisms of action. The cyclam salt H4[H2(4-CF3PhCH2)2Cyclam]Cl4 was previously tested against several fungi and revealed an interesting activity, with minimal inhibitory concentration (MIC) values of 8 µg/mL for C. krusei and of 128 µg/mL for C. albicans. The main objective of the present work was to deeply understand the mechanisms involved in its antifungal activity. The effects of the cyclam salt on yeast metabolic viability (resazurin reduction assay), yeast mitochondrial function (JC-1 probe), production of reactive oxygen species (DCFH-DA probe) and on intracellular ATP levels (luciferin/luciferase assay) were evaluated. H4[H2(4-CF3PhCH2)2Cyclam]Cl4 induced a significant decrease in the metabolic activity of both C. albicans and C. krusei, an increase in Reactive Oxygen Species (ROS) production, and an impaired mitochondrial function. The latter was observed by the depolarization of the mitochondrial membrane and decrease in ATP intracellular levels, mechanisms that seems to be involved in the antifungal activity of H4[H2(4-CF3PhCH2)2Cyclam]Cl4. The interference of the cyclam salt with human cells revealed a CC50 value against HEK-293 embryonic kidney cells of 1.1 μg/mL and a HC10 value against human red blood cells of 0.8 μg/mL.

A Cyclam Salt as an Antifungal Agent: Interference with Candida spp. and Cryptococcus neoformans Mechanisms of Virulence

The importance of fungal infections, particularly those caused by yeasts, is increasing among the medical community. Candida albicans and Cryptococcus neoformans are amongst the high-priority fungal species identified by the World Health Organization (WHO) and are considered in the critical group, while Candida krusei is included in the medium-priority group. The cyclam salt H4[H2(4-CF3PhCH2)2Cyclam]Cl4 proved to be active against the growth of these three yeasts, and the aim of this work was to verify its interference with their virulence mechanisms, whether shared or unique. H4[H2(4-CF3PhCH2)2Cyclam]Cl4 significantly inhibited biofilm production and catalase activity, being able to interfere with C. albicans dimorphic transition and C. neoformans melanin production. At the minimal inhibitory concentration (MIC) values, H4[H2(4-CF3PhCH2)2Cyclam]Cl4 had no antioxidant effect, as determined by the DPPH method. When using the RAW264.7 macrophage cell line, H4[H2(4-CF3PhCH2)2Cyclam]Cl4 reduced nitric oxide (NO) detection (the Griess reaction), but this effect was associated with a significant toxic effect on the cells.

Earth Abundant Oxidation Catalysts for Removal of Contaminants of Emerging Concern from Wastewater: Homogeneous Catalytic Screening of Monomeric Complexes

Twenty novel Mn, Fe, and Cu complexes of ethylene cross-bridged tetraazamacrocycles with potentially copolymerizable allyl and benzyl pendant arms were synthesized and characterized. Multiple X-ray crystal structures demonstrate the cis-folded pseudo-octahedral geometry forced by the rigidifying ethylene cross-bridge and show that two cis coordination cites are available for interaction with substrate and oxidant. The Cu complexes were used to determine kinetic stability under harsh acidic and high-temperature conditions, which revealed that the cyclam-based ligands provide superior stabilization with half-lives of many minutes or even hours in 5 M HCl at 50-90 °C. Cyclic voltammetry studies of the Fe and Mn complexes reveal reversible redox processes indicating stabilization of Fe2+/Fe3+ and Mn2+/Mn3+/Mn4+ oxidation states, indicating the likelihood of catalytic oxidation for these complexes. Finally, dye-bleaching experiments with methylene blue, methyl orange, and rhodamine B demonstrate efficient catalytic decolorization and allow selection of the most successful monomeric catalysts for copolymerization to produce future heterogeneous water purification materials.

Catalytic oxidation properties of an acid-resistant cross-bridged cyclen Fe(II) complex. Influence of the rigid donor backbone and protonation on the reactivity

The catalytic properties of an iron complex bearing a pentadentate cross-bridged ligand backbone are reported. With H₂O₂ as an oxidant, it displays moderate conversions in epoxidation and alkane hydroxylation and satisfactory ones in aromatic hydroxylation. Upon addition of an acid to the reaction medium, a significant enhancement in aromatic and alkene oxidation is observed. Spectroscopic analyses showed that accumulation of the expected Fe^III(OOH) intermediate is limited under these conditions, unless an acid is added to the mixture. This is ascribed to the inertness induced by the cross-bridged ligand backbone, which is partly reduced under acidic conditions.

A Bridge too Far? Comparison of Transition Metal Complexes of Dibenzyltetraazamacrocycles with and without Ethylene Cross-Bridges: X-ray Crystal Structures, Kinetic Stability, and Electronic Properties

Tetraazamacrocycles, cyclic molecules with four nitrogen atoms, have long been known to produce highly stable transition metal complexes. Cross-bridging such molecules with two-carbon chains has been shown to enhance the stability of these complexes even further. This provides enough stability to use the resulting compounds in applications as diverse and demanding as aqueous, green oxidation catalysis all the way to drug molecules injected into humans. Although the stability of these compounds is believed to result from the increased rigidity and topological complexity imparted by the cross-bridge, there is insufficient experimental data to exclude other causes. In this study, standard organic and inorganic synthetic methods were used to produce unbridged dibenzyl tetraazamacrocycle complexes of Co, Ni, Cu, and Zn that are analogues of known cross-bridged tetraazamacrocycles and their transition metal complexes to allow direct comparison of molecules that are identical except for the cross-bridge. The syntheses of the known tetraazamacrocycles and the new transition metal complexes were successful with high yields and purity. Initial chemical characterization of the complexes was conducted by UV-Visible spectroscopy, while cyclic voltammetry showed more marked differences in electronic properties from bridged versions. Direct comparison studies of the unbridged and bridged compounds' kinetic stabilities, as demonstrated by decomposition using high acid concentration and elevated temperature, showed that the cyclen-based complex stability did not benefit from cross-bridging. This is likely due to poor complementarity with the Cu2+ ion while cyclam-based complexes benefited greatly. We conclude that ligand-metal complementarity must be maintained in order for the topological and rigidity constraints imparted by the cross-bridge to contribute significantly to complex robustness.

Insight into Recent Drug Discoveries against Trypanosomatids and Plasmodium spp Parasites: New Metal-based Compounds

Scaffolds of metal-based compounds can act as pharmacophore groups in several ligands to treat various diseases, including tropical infectious diseases (TID). In this review article, we investigate the contribution of these moieties to medicinal inorganic chemistry in the last seven years against TID, including American trypanosomiasis (Chagas disease), human African trypanosomiasis (HAT, sleeping sickness), leishmania, and malaria. The most potent metal-based complexes are displayed and highlighted in figures, tables and graphics; according to their pharmacological activities (IC50 > 10µM) against Trypanosomatids and Plasmodium spp parasites. We highlight the current progresses and viewpoints of these metal-based complexes, with a specific focus on drug discovery.

Dual use of colorimetric sensor and selective copper removal from aqueous media with novel p(HEMA-co-TACYC) hydrogels: Cyclen derivative as both monomer and crosslinker

Within the scope of this study, p(2-hydroxyethyl methacrylate-co-tetraacrylic cyclen) (p(HEMA-co-TACYC)) hydrogels were synthesized for the first time in the literature using a tetraacrylic cyclen (TACYC) as both functional monomer and crosslinker. The hydrogels designed especially for Cu²⁺ ions showed colorimetric sensor behavior selective for Cu²⁺ ions in all aqueous media (deionized, tap, river and sea water) and in metal ion mixtures. The p(HEMA-co-TACYC) hydrogels forming a stable complex with Cu²⁺ ions simultaneously showed properties of being a good adsorbent material. The hydrogels have reuse capacity as both sensor and adsorbent material. Changing the amount of TACYC in the hydrogel structure changes the maximum adsorption capacity for Cu²⁺ ions. The Langmuir and Freundlich adsorption constants for Cu²⁺ ion adsorption of the hydrogels, acting as selective adsorbent in all aqueous media and metal ion mixtures, were determined.

Investigations into the Structure/Antibacterial Activity Relationships of Cyclam and Cyclen Derivatives

A series of cyclam- and cyclen-derived salts are described in the present work; they were designed specifically to gain insights into their structure and antibacterial activity towards Staphylococcus aureus and Escherichia coli, used respectively, as Gram-positive and Gram-negative model organisms. The newly synthesized compounds are monosubstituted and trans-disubstituted tetraazamacrocycles that display benzyl, methylbenzyl, trifluoromethylbenzyl, or trifluoroethylbenzyl substituents appended on the nitrogen atoms of the macrocyclic ring. The results obtained show that the chemical nature, polarity, and substitution patterns of the benzyl groups, as well as the number of pendant arms, are critical parameters for the antibacterial activity of the cyclam-based salts. The most active compounds against both bacterial strains were the trans-disubstituted cyclam salts displaying CF₃ groups in the para-position of the aromatic rings of the macrocyclic pendant arms. The analogous cyclen species presents a lower activity, revealing that the size of the macrocyclic backbone is an important requirement for the antibacterial activity of the tetraazamacrocycles. The nature of the anionic counterparts present on the salts was found to play a minor role in the antibacterial activity.

Acetate as a model for aspartate-based CXCR4 chemokine receptor binding of cobalt and nickel complexes of cross-bridged tetraazamacrocycles

A number of disease states including WHIM syndrome, HIV infection and cancer have been linked to the chemokine receptor CXCR4. High-affinity CXCR4 antagonist transition metal complexes of configurationally restricted bis-tetraazamacrocyclic ligands have been identified in previous studies. Recently synthesised and structurally characterised Co2+/Co3+ and Ni2+ acetate complexes of mono-macrocycle cross-bridged ligands have been used to mimic their known coordination interaction with the aspartate side chains on binding to CXCR4. Here, X-ray crystal structures for three Co2+/Co3+ acetate complexes and five Ni2+ acetate complexes are presented and demonstrate flexibility in the mode of binding to the acetate ligand concomitantly with the requisite cis-V-configured cross-bridged tetraazamacrocyle. Complexes of the smaller Co3+ metal ion exclusively bind acetate by chelating both oxygens of acetate. Larger Co2+ and Ni2+ metal ions in cross-bridged tetraazamacrocycles show a clear tendency to coordinate acetate in a monodentate fashion with a coordinated water molecule completing the octahedral coordination sphere. However, in unbridged tetraazamacrocycle acetate structures reported in the literature, the coordination preference is to chelate both acetate oxygens. We conclude that the short ethylene cross-bridge restricts the equatorial bulk of the macrocycle, prompting the metal ion to fill the equator with the larger monodentate acetate plus water ligand set. In unbridged ligand examples, the flexible macrocycle expands equatorially and generally only allows chelation of the sterically smaller acetate alone. These results provide insight for generation of optimised bis-macrocyclic CXCR4 antagonists utilising cobalt and nickel ions.