Morin (2’,3,4’,5,7-pentahydroxyflavone) is a natural plant dye belonging to flavonol class of the flavonoids a group of low cytotoxicity polyphenols. The natural compounds have a broad pharmacological activity: antibacterial, anticancer, antioxidant, anti-inflammatory, and antiallergic [1, 2]. The ability to chelate metal ions and scavenge free radicals renders the flavonoids very good antioxidants. Most of the studies of metal complexation were done in non-aqueous solvents or mixed solvent systems due to the limited solubility of flavonoids in purely aqueous solution. Much better solubility in water is shown by some sulfonic derivatives of flavonoids (quercetin and morin water-soluble derivative has been synthesized [3]), and, at the same time, these retain the properties of the parent compound [4]. The last studies indicated that substitution of the sulfo group at position 5’ on the lateral phenyl ring enhances anti-staphylococcal activity of flavonoids [5]. Furthermore, to date, a crystal structure of flavonoids, and their metal complexes has been limited reported.

The present study we described a modified synthesis of the water soluble sodium salt of morin-5’-sulfonic acid (NaMSA hydrate), and investigate the behavior of morin-5’-sulfonic ligand (MSA) at water environment in the presence Zn(II) ions. To date, crystalization experiments carried out in other conditions have not led to obtained single crystal suitable for laboratory diffraction measurements. Herein, single crystal of zinc(II) ions with sulfonated morin from aqueous solution first time were isolated and described. New structures were confirmed with IR, UV-Vis and NMR spectroscopy, elemental and X-ray diffraction analyses. Furthermore, the aim was examination and comparison of antioxidant properties of the synthesized compounds by DPPH radical scavenging activity method.

Our results indicated that composition of the compounds Zn(II)-MSA depends on the excess of either metal cations or ligand during precipitation. If there is an excess of metal cations in the solution Zn(C₁₅H₈O₁₀S)(H₂O)₂·1,5 H₂O is formed. However, in the case of MSA ligand excess, sodium salt of bischelate Na₂Zn(C₁₅H₈O₁₀S)₂(H₂O)₂·4H₂O is obtained. Moreover, the free radical scavenging activity of NaMSA and ZnMSA is higer than for morin investigated in the same conditions. In addition, the radical DPPH scavenging activity of Na₂Zn(MSA)₂ is the largest, about 90% (Fig. 1). This suggest that the introduction of the sulfo group and metal ions significantly change the chemical properties of the morin. In DPPH radical reaction for investigated compounds a hydrogen atom is abstracted from Zn(II)-MSA complexes to give a semiquinone complexes stabilized by the metallic center and by conjugation with the 3-OH group [6].

Fig. 1. Relative antioxidative potential of morin, NaMSA and Zn(II)-MSA complexes 

In addition, the Na₂Zn(MSA)₂ bischelate complex is more effective free radical scavenging than corresponding compounds due to the acquisition of additional superoxide dismutating centres. The complexation with metal ions decrease the oxidation potential of the flavonoids; so the complexed morin-5’-sulfonic acid ligand (MSA) is relatively more effective antioxidant than the uncomplexed one.

These data have important role to a better understanding of the chemistry of sulfonated flavonoids ligands in the presence of metal ions involved with biological system.

References:

1. Cheng F., Kevin B. (2000). On the ability of four flavonoids, baicilein, luteolin, naringenin, and quercetin, to suppress the fenton reaction of the iron-ATP complex. Biometals, Vol.13, pp. 77-83.
2. Torreggiani A., Tamba M., Trinchero A., Bonora S. (2005). Copper(II)–quercetin complexes in aqueous solutions: spectroscopic and kinetic properties. Journal of Molecular Structure, Vol.744, pp. 759-766.
3. Kopacz M. (2003). Quercetin and morinsulfonates as analytical reagents. Zhurnal Analliticheskoi Khimii, Vol.58, pp. 258-262.
4. Heneczkowski M., Kopacz M., Nowak D., Kuźniar A. (2001). Infrared spectrum analysis of some flavonoids. Acta Poloniae Pharmaceutica - Drug Research, Vol.58, pp. 415-420.
5. Woźnicka E., Kuźniar A., Nowak D., Nykiel E., Kopacz M., Gruszecka J., Golec K. (2013). Comparative study on the antibacterial activity of some flavonoids and their sulfonic derivatives. Acta Pol. Acta Poloniae Pharmaceutica - Drug Research, Vol.70, pp. 567-571.
6. Bukhari S.B., Memon Sh., Tahir M.M., Bhanger M.I. (2008). Synthesis, characterization and investigation of antioxidant activity of cobalt-quercetin complex. Journal of Molecular Structure. Vol.892, pp. 39-46.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/31043 must be provided.

1. Potentiometric study of Pd(II) complexes of some flavonoids in water-methanol-1,4-dioxane-acetonitrile (MDM) mixture