Zn(II) Complex of Plant Phenolic Chlorogenic Acid: Antioxidant, Antimicrobial and Structural Studies

Abstract

The structure of the Zn(II) complex of 5-caffeoylquinic acid (chlorogenic acid, 5-CQA) and the type of interaction between the Zn(II) cation and the ligand were studied by means of various experimental and theoretical methods, i.e., electronic absorption spectroscopy UV/Vis, infrared spectroscopy FT-IR, elemental, thermogravimetric and density functional theory (DFT) calculations at B3LYP/6-31G(d) level. DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), FRAP (ferric reducing antioxidant power), CUPRAC (cupric reducing antioxidant power) and trolox oxidation assays were applied in study of the anti-/pro-oxidant properties of Zn(II) 5-CQA and 5-CQA. The antimicrobial activity of these compounds against Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, Salmonella enteritidis and Candida albicans was tested. An effect of Zn(II) chelation by chlorogenic acid on the anti-/pro-oxidant and antimicrobial activities of the ligand was discussed. Moreover, the mechanism of the antioxidant properties of Zn(II) 5-CQA and 5-CQA were studied on the basis of the theoretical energy descriptors and thermochemical parameters. Zn(II) chlorogenate showed better antioxidant activity than chlorogenic acid and commonly applied natural (L-ascorbic acid) and synthetic antioxidants (butylated hydroxyanisol (BHA) and butylated hydroxytoluene (BHT)). The pro-oxidant activity of Zn(II) 5-CQA was higher than the ligand and increased with the rise of the compound concentration The type of Zn(II) coordination by the chlorogenate ligand strongly affected the antioxidant activity of the complex.

Keywords: 5-caffeoylquinic acid; chlorogenic acid; oxidative stress; plant phenolic compounds; zinc.

Figure 1

The effect of different concentrations (0.025–0.15 μM) of (a) zinc complex of chlorogenic acid and (b) chlorogenic acid on the oxidation of trolox. Mean values from three independent experiments ± standard deviation (SD) are shown. The same letter near the means indicates no significant difference (Tukey test, p < 0.05).

Figure 2

The FT-IR spectra of: (a) Zn(II) chlorogenate and (b) chlorogenic acid registered in the range of 400–4000 cm⁻¹ for solid samples in the KBr matrix pellet.

Figure 3

TG-DTG and DSC thermal curves obtained for: (a) zinc(II) and (b) sodium chlorogenates in air atmosphere.

Figure 4

The ultraviolet (UV) spectra of the series of solutions prepared according to the Job’s method (0.1 mM 5-CQA and 0–0.09 mM ZnCl₂ in Tris-HCl, pH = 7.4). The lowest line showed the spectra of 0.1 mM ZnCl₂.

Figure 5

The UV spectra of 0.1 mM ZnCl₂, 0.1 mM chlorogenic acid (5-CQA), 0.05 mM ZnCl₂ plus 0.1 mM 5-CQA (molar ratio Zn:5-CQA 1:2) in Tris-HCl at pH = 7.4 registered in the range 200–450 nm.

Figure 6

(a) The structure of 5-CQA with atom numbering, (b) Zn 5-CQA structure I and (c) structure II; calculated at B3LYP/6-31G(d) level. The structure I of Zn 5-CQA (b) can be transformed into two different phenoxyl radicals with the participation of: –OH of the catechol group engaged in the Zn²⁺ coordination; –OH from the free catechol group.

Figure 6

(a) The structure of 5-CQA with atom numbering, (b) Zn 5-CQA structure I and (c) structure II; calculated at B3LYP/6-31G(d) level. The structure I of Zn 5-CQA (b) can be transformed into two different phenoxyl radicals with the participation of: –OH of the catechol group engaged in the Zn²⁺ coordination; –OH from the free catechol group.

Figure 7

The shapes of frontier molecular orbitals HOMO and LUMO orbitals and energy gaps (ΔE) of chlorogenic acid and Zn chlorogenates calculated at B3LYP/6-31G(d) level.