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Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes.

ABSTRACT: Four cobalt(iii) complexes of the general formula [Co(Schiff base)(L)2]+, where L is ammonia (NH3) or 3-fluorobenzylamine (3F-BnNH2), were synthesized. The complexes were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Their electrochemical properties, ligand substitution mechanisms, and ligand exchange rates in aqueous buffer were investigated. These physical properties were correlated to the cellular uptake and anticancer activities of the complexes. The complexes undergo sequential, dissociative ligand substitution, with the exchange rates depending heavily on the axial ligands. Eyring analyses revealed that the relative ligand exchange rates were largely impacted by differences in the entropy, rather than enthalpy, of activation for the complexes. Performing the substitution reactions in the presence of ascorbate led to a change in the reaction profile and kinetics, but no change in the final product. The cytotoxic activity of the complexes correlates with both the ligand exchange rate and reduction potential, with the more easily reduced and rapidly substituted complexes showing higher toxicity. These relationships may be valuable for the rational design of Co(iii) complexes as anticancer or antiviral prodrugs.

SUBMITTER: King AP

PROVIDER: S-EPMC6504617 | biostudies-literature | 2019 May

REPOSITORIES: biostudies-literature

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Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes.

King A Paden AP Gellineau Hendryck A HA MacMillan Samantha N SN Wilson Justin J JJ

Dalton transactions (Cambridge, England : 2003) 20190501 18

Four cobalt(iii) complexes of the general formula [Co(Schiff base)(L)2]+, where L is ammonia (NH3) or 3-fluorobenzylamine (3F-BnNH2), were synthesized. The complexes were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Their electrochemical properties, ligand substitution mechanisms, and ligand exchange rates in aqueous buffer were investigated. These physical properties were correlated to the cellular uptake and anticancer activities of the complexes. The comple ...[more]

PMID: 30672949

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Project description:New Schiff base ligand, derived from antiviral valacyclovir, and its novel Cr(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II) complexes have been synthesized. By using a variety of analytical and spectroscopic techniques, the type of bonding between the ligand and the metal ions in the recently formed complexes was clarified. The Schiff base ligand act as a bidentate and coordinated with the metal ions through the azomethine-N and the phenolic-O centers, in a mono-deprotonated form. Except for the Zn(II) complex, which displayed a tetrahedral geometry, all complexes displayed octahedral geometry. The TGA findings supported that the stability and decomposition properties of the metal complexes were entirely distinct from one another. The thermogram showed decomposition of all investigated metal complexes above 200 °C in three, four or five steps, and indicated the high thermal stability of these complexes. According to XRD patterns, the particles of these complexes were located at the nanoscale. Moreover, for all the samples analyzed, the TEM images showed uniform and homogeneous surface morphology. The biological activity revealing the high efficiencies of the screened complexes as antibacterial and antitumor agents. The antimicrobial activity of the ligand and its complexes was examined against a variety of pathogenic bacteria and fungi including Escherichia coli, Staphylococcus aureus and Candida albicans. The data obtained revealed that the metal ion in the complexes enhanced the antimicrobial activity compared to the free ligand. The high efficiencies toward S. aureus, E. coli, and C. albicans appeared by Cu(II) complex 23, Ni(II) complex 20, and Ni(II) complex 19, respectively. The antitumor activity of the ligand and its complexes was tested against Hepatocellular carcinoma cell line (HepG-2 cells), the residue 28 which produced after heating the Cu(II) complex 25 at 200 °C for 1 h, exhibited strong inhibition of HepG-2 cell growth. The results of the DNA cleavage investigation demonstrated the ability of investigated Cu(II) complex to degrade DNA. The docking findings showed strong interactions of both the ligand and its examined Cu(II) complex, revealing their ability to cleavage DNA and their potent inhibitory effects on tumor cells. The electrical conductivity study confirmed that the ligand and its investigated complexes had semiconducting properties.

Project description:The kinetic and thermodynamic ligand exchange dynamics are important considerations in the rational design of metal-based therapeutics and therefore, require detailed investigation. Co(III) Schiff base complex derivatives of bis(acetylacetone)ethylenediimine [acacen] have been found to be potent enzyme and transcription factor inhibitors. These complexes undergo solution exchange of labile axial ligands. Upon dissociation, Co(III) irreversibly interacts with specific histidine residues of a protein, and consequently alters structure and causes inhibition. To guide the rational design of next generation agents, understanding the mechanism and dynamics of the ligand exchange process is essential. To investigate the lability, pH stability, and axial ligand exchange of these complexes in the absence of proteins, the pD- and temperature-dependent axial ligand substitution dynamics of a series of N-heterocyclic [Co(acacen)(X)(2)](+) complexes [where X = 2-methylimidazole (2MeIm), 4-methylimidazole (4MeIm), ammine (NH(3)), N-methylimidazole (NMeIm), and pyridine (Py)] were characterized by NMR spectroscopy. The pD stability was shown to be closely related to the nature of the axial ligand with the following trend toward aquation: 2MeIm > NH(3) ≫ 4MeIm > Py > Im > NMeIm. Reaction of each [Co(III)(acacen)(X)(2)](+) derivative with 4MeIm showed formation of a mixed ligand Co(III) intermediate via a dissociative ligand exchange mechanism. The stability of the mixed ligand adduct was directly correlated to the pD-dependent stability of the starting Co(III) Schiff base with respect to [Co(acacen)(4MeIm)(2)](+). Crystal structure analysis of the [Co(acacen)(X)(2)](+) derivatives confirmed the trends in stability observed by NMR spectroscopy. Bond distances between the Co(III) and the axial nitrogen atoms were longest in the 2MeIm derivative as a result of distortion in the planar tetradentate ligand, and this was directly correlated to axial ligand lability and propensity toward exchange.

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Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes.

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Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes.

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