Abstract
The Mg(II) and heterometallic Mn(II)/Na(I) complexes of isoferulic acid (3-hydroxy-4-methoxycinnamic acid, IFA) were synthesized and characterized by infrared spectroscopy FT-IR, FT-Raman, electronic absorption spectroscopy UV/VIS, and single-crystal X-ray diffraction. The reaction of MgCl2 with isoferulic acid in the aqueous solutions of NaOH resulted in synthesis of the complex salt of the general formula of [Mg(H2O)6]⋅(C10H9O4)2⋅6H2O. The crystal structure of this compound consists of discrete octahedral [Mg(H2O)6]2+ cations, isoferulic acid anions and solvent water molecules. The hydrated metal cations are arranged among the organic layers. The multiple hydrogen-bonding interactions established between the coordinated and lattice water molecules and the functional groups of the ligand stabilize the 3D architecture of the crystal. The use of MnCl2 instead of MgCl2 led to the formation of the Mn(II)/Na(I) complex of the general formula [Mn3Na2(C10H7O4)8(H2O)8]. The compound is a 3D coordination polymer composed of centrosymmetric pentanuclear subunits. The antioxidant activity of these compounds was evaluated by assays based on different antioxidant mechanisms of action, i.e., with •OH, DPPH• and ABTS•+ radicals as well as CUPRAC (cupric ions reducing power) and lipid peroxidation inhibition assays. The pro-oxidant property of compounds was measured as the rate of oxidation of Trolox. The Mg(II) and Mn(II)/Na(I) complexes with isoferulic acid showed higher antioxidant activity than ligand alone in DPPH (IFA, IC50 = 365.27 μM, Mg(II) IFA IC50 = 153.50 μM, Mn(II)/Na(I) IFA IC50 = 149.00 μM) and CUPRAC assays (IFA 40.92 μM of Trolox, Mg(II) IFA 87.93 μM and Mn(II)/Na(I) IFA 105.85 μM of Trolox; for compounds’ concentration 10 μM). Mg(II) IFA is a better scavenger of •OH than IFA and Mn(II)/Na(I) IFA complex. There was no distinct difference in ABTS•+ and lipid peroxidation assays between isoferulic acid and its Mg(II) complex, while Mn(II)/Na(I) complex showed lower activity than these compounds. The tested complexes displayed only slight antiproliferative activity tested in HaCaT human immortalized keratinocyte cell line within the solubility range. The Mn(II)/Na(I) IFA (16 μM in medium) caused an 87% (±5%) decrease in cell viability, the Mg salt caused a comparable, i.e., 87% (±4%) viability decrease in a concentration of 45 μM, while IFA caused this level of cell activity attenuation (87% ± 5%) at the concentration of 1582 μM (significant at α = 0.05).
Highlights
The main natural sources of antioxidants found in food are vegetables, fruits, 1
Many papers report that flavonoids and phenolic acids in combination with metal ions may exhibit higher antioxidant activity than the ligands alone because of their ability to act as free radical acceptors [39,40,41]
IC50 parameter is the concentration of antioxidant that is required to inhibit 50% of the DPPH free radicals
Summary
The main natural sources of antioxidants found in food are vegetables, fruits, 1. Many papers report that flavonoids and phenolic acids in combination with metal ions may exhibit higher antioxidant activity than the ligands alone because of their ability to act as free radical acceptors [39,40,41]. It was proven that phenolic compound complexes with transition metals (such as Fe(II) or Mn(II)) often have higher bioavailability than pure ligands, e.g., quercetin shows increasing bioavailability in combination with Cu(II) ions [45]. The extensive studies of the metal complexes with naturally occurring phenols with high antioxidant activity and well-described structure can be used in many areas, e.g., in the food, pharmaceutical, and cosmetics industries. The anti-/pro-oxidant activity of the synthesized complexes was studied and compared with the properties of ligand in the DPPH , ABTS+ , CUPRAC, hydroxyl radical scavenging, lipid peroxidation, and Trolox oxidation assays. The cellular toxicity was tested against HaCaT human immortalized keratinocyte cell line
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