Caffeic Acid/Eu(III) Complexes: Solution Equilibrium Studies, Structure Characterization and Biological Activity

Abstract

Caffeic acid (CFA) is one of the various natural antioxidants and chemoprotective agents occurring in the human diet. In addition, its metal complexes play fundamental roles in biological systems. Nevertheless, research on the properties of CFA with lanthanide metals is very scarce, and little to no chemical or biological information is known about these particular systems. Most of their properties, including their biological activity and environmental impact, strictly depend on their structure, stability, and solution behaviour. In this work, a multi-analytical-technique approach was used to study these relationships for the Eu(III)/CFA complex. The synthesized metal complex was studied by FT-IR, FT-Raman, elemental, and thermal (TGA) analysis. In order to examine the chemical speciation of the Eu(III)/CFA system in an aqueous solution, several independent potentiometric and spectrophotometric UV-Vis titrations were performed at different M:L (metal:ligand) and pH ratios. The general molecular formula of the synthesized metal complex in the solid state was [Eu(CFA)3(H2O)3]∙2H2O (M:L ratio 1:3), while in aqueous solution the 1:1 species were observed at the optimum pH of 6 ≤ pH ≤ 10, ([Eu(CFA)] and [Eu(CFA)(OH)]−). These results were confirmed by 1H-NMR experiments and electrospray-ionization mass spectrometry (ESI-MS). To evaluate the interaction of Eu(III)/CFA and CFA alone with cell membranes, electrophoretic mobility assays were used. Various antioxidant tests have shown that Eu(III)/CFA exhibits lower antioxidant activity than the free CFA ligand. In addition, the antimicrobial properties of Eu(III)/CFA and CFA against Escherichia coli, Bacillus subtilis and Candida albicans were investigated by evaluation of the minimum inhibitory concentration (MIC). Eu(III)/CFA shows higher antibacterial activity against bacteria compared to CFA, which can be explained by the highly probable increased lipophilicity of the Eu(III) complex.

Interactions with HSA, anticancer and antiallergic activity of binuclear μ‐oxo bridged ruthenium acetate compounds

“…The free ligand only elicited significant inhibition at 200 μM (32 %) (Figure 4). These results agreed with the results of Leung et al., who reported that, compared to free ligands, their metal complexes frequently hold superior lipophilicity profiles, which allow them to cross cell membranes more easily, to exert their biological effects [70,71] . Surprisingly, 2 did not inhibit mast cell degranulation (Figure SI 11), probably because of the lower lipophilicity of the phenanthroline ligand when compared to 1 , which bears a methyl substituent in its phen ligand.…”

“…These results agreed with the results of Leung et al, who reported that, compared to free ligands, their metal complexes frequently hold superior lipophilicity profiles, which allow them to cross cell membranes more easily, to exert their biological effects. [70,71] Surprisingly, 2 did not inhibit mast cell degranulation (Figure SI 11), probably because of the lower lipophilicity of the phenanthroline ligand when compared to 1, which bears a methyl substituent in its phen ligand. To prevent false positive results concerning inhibition of mast cell degranulation, we investigated whether the action of 1 was due to inhibition of the enzymatic activity or β-hexosaminidase release.…”

Discovery of metal complexes with antibacterial properties in aqueous extracts of Radix scutellariae and a study of the antibacterial properties of the baicalin–manganese complex

“…15 However, it is common for herbal medicines to be prepared as a decoction in water, during which many organic components containing ligand groups (e.g., hydroxyl, carboxyl, carbonyl, amino, o-diphenol hydroxyl) may form complexes with metal ions, increasing their pharmacological activities or even forming new agents. For example, quercetin-Ca, 16 curcumin-Cu, 17,18 and curcumin-Fe 19 are better than their respective ligands at free radical scavenging; emodin-Cu, 20 apigenin-V, 21 and baicalin-Ce/La/Y 22 have better antitumor activity; tea polyphenols-Zn, 23 curcumin-Mn, 24 and caffeic acid-Eu 25 have significantly increased antibacterial activity against Gram-positive and Gram-negative bacteria. Therefore, metal complexes may be the active components of aqueous extracts used in traditional Chinese medicine, creating a new base for the discovery of antibacterial drug candidates.…”

Interaction of the Fungal Metabolite Harzianic Acid with Rare-Earth Cations (La3+, Nd3+, Sm3+, Gd3+)

“…Organic substances are crucial for the potential phenomena of adsorption or chelation, which may influence the impact of these metals on organisms and microorganisms sharing the same ecological niche. Thus, the last decade has seen an increasing interest among the scientific community for the coordination properties of organic ligands toward rare-earth elements [ 26 , 27 , 28 , 29 ].…”