Synthesis, characterization, spectroscopic and antioxidation studies of Cu(II)–morin complex
Introduction
Flavonoids (flavus-yellow), or bioflavonoids, are a group of polyphenolic substances which are present in the majority of plants, found in the seeds, fruit coat, bark and flowers [1]. The name flavonoid is functional to all those molecules whose composition is based on that of flavones (2-phenylchromone) [2]. They have a fundamental structure, Fig. 1, of 2-phenylbenzo-γ-pyrones [3]. Flavones comprising of two benzene rings (A and B) linked together by a three-carbon linkage which is constructed into a γ-pyrone ring (C). The different classes of flavonoids vary one from another only by the state of oxidation of the 3-C link (C6-C3-C6). And, the individual compounds within each class are differentiated essentially by the number and position of hydroxy, methoxy and other groups substituted in two phenyl rings [4], [5], [6]. Therefore, based on the oxidation state of the heterocyclic ring, flavonoids are classified as flavones, flavanols, flavonols, flavanones or isoflavones [7].
In recent years, these molecules have attracted the concentration of many researchers because flavonoids exhibit a significant range of biological and pharmacological activities as antioxidant, anti-inflammatory, antimicrobial, anticancer and cardiovascular protection etc. [8]. To explain the antioxidant activity, two possibilities arise: (i) scavenge of free radicals or (ii) chelation of metal ions. Due to their specific chemical structure, flavonoids easily chelate metal ions and create complex compounds. Chelation is accomplished by carbonyl and hydroxyl groups present in flavonoids molecules. Besides metal chelation capabilities, flavonoids exhibit reducing properties (via electron or H-atom donation). In particular, it is widely known that flavonoids can react with harmful free radicals such as hydroxyl or peroxyl, thus preventing the oxidation of lipids and other important molecular agents. Furthermore, transition metal ions (especially copper) in their reduced oxidation state can promote the formation of free radicals [9], [10]. Redox reactions are also observed through the change of the oxidation state of the metal, jointly with the oxidation of the flavonoids by loss of hydrogen.
Copper have a major role in the production of the very reactive hydroxyl radical (HO) through the Fenton and Haber–Weiss reactions [11]. Copper (II) can bind and manifests large hyperchromic and bathochromic shifts in the molecular absorption spectra [12]. The aim of these studies was to investigate the interactions between Cu(II) ions and the flavonoids [13].
Morin (Fig. 2) is a light yellowish natural plant dye belonging to flavonol (Fig. 3) class of the flavonoids that shows many of the biological activities such as antioxidant, anti-inflammatory, anticancer, cardiovascular protection etc. The bioactivity of morin is associated with its structural features which can be involved in complex formation with metals. Due to well-situated location of the 5OH and 4CO as well as 3OH and 4CO groups in a molecule, morin forms chelate complexes with ions of p-, d-, and f-electron metals. Morin has been used as a colorimetric reagent for spectrophotometric determination of metal ions [14], [15], [16]. Morin hydrate can frequently be used as a food preservative and has been shown to prevent lipid per oxidation. It is also known to protect against oxyl-radicals. The chelation of morin with metal cations is an important factor both in its bioactivity and its role as a carrier and monitoring metal concentration [17]. As there are some of previously reported methods found in the literature about morin and its complexes [18], [19], [20], [21].
Herein, a modified method for the complexation of copper with morin has been adopted as well as the characterization and comparative study of ligand and its metal complex has been carried out by spectroscopic techniques along with investigation of their antioxidant activities.
Section snippets
Materials
All reagents and solvents were of analytical or chemically pure grade. Morin hydrate (2-(2,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one) by Sigma, CuSO4·5H2O by Fluka, HPLC grade methanol by Fisher scientific UK Ltd., KBr and DPPH were purchased from Aldrich Chemical Co. All reagents were weighed with an accuracy of ±0.0001 g.
Instrumentation
UV–visible spectra were obtained by Perkins Elmer Lambda 35 UV–visible double beam spectrophotometer using standard 1.00 cm quartz cells in methanol solvent. 1H
UV–vis studies
Job’s method of continuous variation was used to find out the stoichiometric composition of chelate complex. After adding the CuSO4·5H2O, the absorbance band of the morin was shifted from 358 nm to a new characteristic band of the complex at 424 nm. The absorbance plots at 424 nm, Fig. 4, against the mole fraction of morin (X) have a maximum absorbance at XL = 0.5, confirming that the stoichiometric ratio for the complexation of Cu(II) and morin is 1:1.
The flavonoids display two major absorption
Conclusion
The study describes a convenient synthesis as well as the structure elucidation and the antioxidant activity of morin copper complex. From UV–vis spectroscopic studies and elemental analysis for metal content of complex by titration with EDTA, it has been revealed that the complex has got 1:1 ligand/metal stoichiometric composition. It has also been noticed that in the complexation, 3-OH group shows more activity for the co-ordination. Besides this, it has been found that the negative values
Acknowledgement
Thanks to the National Center of Excellence in Analytical Chemistry, University of Sindh, Jamshoro/Pakistan for supporting through financial assistance and providing the necessary facilities.
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