Synthesis, analytical characterization and some biological applications of metal complexes of Flavonoids

Abstract

This study deals with the exploration of new sources of natural antioxidants as well as developing effective antioxidants. It is a collaborative project between two universities, i.e. University of Sindh, Jamshoro, Pakistan and St. Cloud State University, U.S.A.; concerning with (i) investigation of antioxidant potential of some herbs and spices in Pakistan (ii) exploring antioxidant activity of flavonoids and their metal complexes after the complexation. Among the food scientists, natural origin is focus of powerful research for exploitation of newer sources of antioxidants. In extension of the investigations, in the first phase of the study, a number of botanical materials have been selected for evaluation of their antioxidant probability. Antioxidant potential of different spices and herbs such as Ajwain, Cinnamon, Coriander, Cumin, Fennel, Fenugreek and Kalonji was investigated. The extracts of herbs and spices were prepared by electrical shaking and soxhlet extraction methods. A number of solvents ranging from polar to non- polar were used as extractant. Total phenolic content and antioxidant potentional by 2, 2-diphenyl-1- picryhydrazyl radical (DPPH·) were used as the parameters to study the effectiveness of extraction medium, i.e. solvent. Among the extracts of herbs and spices, ethanolic extract was found to be richer in phenolics and antioxidant activity. Moreover, total phenolic content, total flavonoid content, reducing power, chelating activity and DPPH· scavenging activity parameters were used for investigation of their antioxidant potential. From the results it has been deduced that most of the herbal spice materials exhibited appreciable antioxidant power as; compared to previously exploited sources of natural antioxidants. The Kalonji was found to be the richest source of antioxidants among the herbs and spices analyzed. The order of spices in antioxidant potential is Kalonji> Fenugreek> Cinnamon> Ajwain> Coriander> Cumin> Fennel. The flavonoids compose one of the most ubiquitous groups of all plant phenolics and they are an important class of defense antioxidants. The organoleptic properties and human health scientific studies have revealed their importance in food for maintaining human health. Flavonoids reduce the risk of stroke and heart disease, protect against relieve hay fever, age- related vision disorders, asthma symptoms, sinusitis, alleviate inflammatory skin conditions, reduce inflammation in joints and muscles, common to rheumatoid arthritis, minimize menopausal hot flushes, shrink hemorrhoids, reduce varicose veins and battle viral infections. Transition metal ions participate in the initiation of free radical processes. The antioxidant action of flavonoids has been considered to be via two possible modes of action, metal chelation and radical scavenging. Radical scavenging arises by donation of hydrogen atom from the free hydroxyls on the flavonoids nucleus and other possible antioxidant mechanism for flavonoids is metal chelation through phenolic OH groups, preventing metal- mediated generation of free radicals and, accordingly, may shelter the potential biological target from oxidative stress. However, in the second phase of study, among the most common dietary and biologically active flavonoids, Quercetin was selected. In order to study the chelation mechanisms of flavonoids with metal ions that permit a better understanding of their complexation and antioxidant properties; it has been decided to synthesize their metal (Mn Co 2+ , Ni 2+ , Cu 2+ 2+ , and Zn2+ ) complexes. This phase of study also consist on the characterization and investigation of antioxidant activity and the microbial activity of flavonoid-metal complexes. The thermal analyses as TGA and DSC. UV–Vis, IR, 1H-NMR spectroscopic technique were also used to asses relevant interactions of flavonoids and metal ions, the chelation sites are dependence upon the complex structure on the metal/ligand ratio, the capacity of flavonoids in binding metal ions, and etc. The outcome largely depends on the experimental condition and the type of assay employed. The Job's method (continual variation method) was used to validate the stoichiometric composition of the chelate for the synthesis of metal flavonoid complexes. The complex formation reactions of M(II) Resulted in the formation of 2:1 metal:ligand complexes quercetin respectively. 70+ % yield have been found in the method employed for their synthesis. UV–vis spectroscopy remained the main tool for structural analysis of flavonoids. Both of the complexes formation and subsequent electron transfer reactions of M(II) with these ligands were examined by using UV–vis spectrophotometry. The 1H-NMR results indicate that the quercetin coordinate with two M(II) ion by replacing two of its protons. The coordination sites and the binding property of quercetin have also been evaluated by IR spectroscopy, which shows the shift of bands due to the formation of complex. The antioxidant and antimicrobial potentional of the complexes were studied and compared with the activity of free quercetin. The quercetin-M(II) complexes are found more potent free radical scavenger than the parent quercetin ligands and play a prominent role in protecting from oxidative stress and microbial organisms. This study demonstrates antioxidant potential of herbs and spices, and shows that the antioxidant activity of a flavonoid may be enhanced through chelation with metals.