# Co(II) Complex of Quercetin-Spectral, Anti-/Pro-Oxidant and Cytotoxic Activity in HaCaT Cell Lines
## 1 Introduction
Both of the complexes worked actively against PC-3, HL-60, HCT-15, HeLa, Hop62, U373 MG and A2780 human cancer cell lines [27]. The results obtained by [PERSON] and others [28] confirm that Zn(II) and Cu(II) complexes were more cytotoxic than quercetin alone in PC3, Skor3, BGC-823, Bel-7402, KB and HL-60 cell lines. Moreover, recent findings propose that quercetin is able to trigger apoptosis in several tumor cells via its pro-oxidant properties, due to its generation of reactive oxygen species [29]. Therefore, both anti- and pro-oxidant activities of quercetin should be considered in relation to its tumoricidal effects. Because quercetin possesses good chelating properties many papers describe the synthesis of transition metal complexes with quercetin and their interesting biological properties, as well as their possible application in pharmacy, medicine, food technology and biotechnology [30]. Cobalt(II) stimulates the antioxidant defense and supports anti-inflammatory processes [31]. Moreover, many of the Co(II) complexes revealed interesting biological properties, including antibacterial, antiviral and antioxidant properties [32]. Although it is an essential microelement in animals, for health, e.g., as a component of Vitamin B12, an excess of cobalt ions is well-known for its toxicity [33]. For example, the exposure of rats to cobalt chloride leads to oxidative stress, observed through a significant increase of malondialdehydedehyde (MDA) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) levels, and conversely, reduced nitric oxide (NO) levels. Stress is also observed through the reduced activity of glutathione peroxidase (GPx) and reduced glutathione (GSH) content in heart and kidney tissues [34]. However, it has been noted that treatment with quercetin and/or vitamin C reverses the effects of cobalt chloride on MDA, H\({}_{2}\)O\({}_{2}\) and NO (where a synergistic effect of Q and vitamin C was observed) and strengthens the antioxidant defense system. In addition, the treatment of rats with a combination of quercetin and vitamin C resulted in a significant (\(p<0.05\)) decrease in systolic, diastolic and the mean blood pressure in rats compared to those exposed to cobalt chloride alone [34]. Therefore, the study of metal-transition-quercetin complexes are very important, in order to establish whether the physico-chemical and biological properties (e.g., solubility, bioavailability, antioxidant) of a parent ligand can be changed through its complexation with metal ions. The reported metal-quercetin complexes possessed different metal:ligand ratios of 1:1, 1:2, 2:1, and 3:2 depending on the pH of the solution and the number of reagents [27; 28; 29; 30; 31; 32; 33]. In this study the cobalt(II) complex of quercetin was synthesized in pH = 7.4. The FT-IR, elemental analysis, and the UV/Vis methods were applied to study the composition in its solid state and solution. The anti-/pro-oxidant activity of the Co(II)-quercetin complex was studied by means of DPPH, FRAP and Trolox oxidation assays and then compared with the antioxidant properties of ligand alone. The HaCaT human immortalized keratinocyte cell line has been chosen for the cellular toxicity test in mammalian cell culture.The antioxidant and cytotoxic properties of the Co(II)-quercetin complex and quercetin were compared and discussed in terms of their potential application.