# Co(II) Complex of Quercetin-Spectral, Anti-/Pro-Oxidant and Cytotoxic Activity in HaCaT Cell Lines
## 3 Results  
### Spectral Analysis  
The assignment of selected bands, from the FT-IR spectra of the quercetin and the Co(II) complex of quercetin, are gathered in Table 1 and shown in Figure 2. The strong broad-band derived from the stretching vibrations of the -OH groups were located in the spectral range of 3426-3250 cm\({}^{-1}\). The strong band assigned to the stretches of the C=O in the quercetin molecule (1672 cm\({}^{-1}\)) was slightly shifted towards a lower wavenumber in the spectra of the Co(II) complex. The stretching \(\
u\)(OH) and \(\
u\)(C=O) vibrations moved towards lower wavenumbers due to the breakdown of the inter- and intramolecular hydrogen bonds, and a metal complex formation with the participation of carbonyl and hydroxyl groups. Moreover, such an occurrence in the FT-IR spectra of the complex, in the bands assigned to the stretching vibrations of the C-O catechol group at 1460 and 1422 cm\({}^{-1}\) and Co(II)-O, was caused by the participation of catechol moiety in the metal ion coordination. The other bands, which were derived from the aromatic ring of the ligand, changed their position as well when comparing the spectra of quercetin to its metal _complex._  
In the UV/Vis spectrum of quercetin, recorded in a Tris-HCl buffer solution with pH = 7.4 and concentration C = 5 \(\times\) 10\({}^{-5}\) M (Figure 3), two main absorption bands were observed corresponding to the \(\pi\)-\(\pi\) * transitions. Band I, present at the wavelength \(\lambda_{\max}\) = 375 nm, corresponds to the \(\pi\)-\(\pi\) * transitions in the ring marked as B, and band II present at the wavelength \(\lambda_{\max}\) = 256 nm, corresponds to the \(\pi\)-\(\pi\) * electron transition in ring A [39]. The intensity of the bands in the UV/Vis spectra of Co(II)-quercetin complex dropped significantly, and a slight shift of band II towards longer wavelengths and a shift of band I towards shorter wavelengths was observed. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Assignment**} & \multicolumn{2}{c}{**Quercetin**} & \multicolumn{2}{c}{**Co(II)-Quercetin**} \\ \cline{2-5}  & **IR** & **Int.** & **IR** & **Int.** \\ \hline \(\
u\)(OH) * & 3426–3291 & s & 3397–3266 & s \\ \(\
u\)(C=O) & 1672 & s & 1663, 1655 & s \\ \(\
u\) ring & 1614 & vs & 1611 & vs \\ \(\
u\) ring & 1514 & vs & 1522 & vs \\ \(\
u\)(C-O) catechol group & - & & 1460 & m \\ \(\
u\) ring & 1429 & s & 1449 & s \\ \(\beta\)(C-OH) & 1362 & vs & - & \\ \(\beta\)(CH) & 1317 & s & 1317 & s \\ \(\
u\)(C-O-C) & 1244 & vs & 1261 & vs \\ \(\beta\)(OH) & 1213 & vs & 1210 & m \\ \(\
u\)(C-CO-C) + \(\beta\)(C-CO-C) & 1165 & vs & 1167 & s \\ \(\
u\)(Co(II)-O) & & & 604 & w \\ \hline \hline \end{tabular}  
* symbols denote: \(\
u\)–stretching vibrations, \(\beta\)–deforming in-plane vibrations; int-intensity, m–medium, s–strong, vs–very strong.  
\end{table} 
Table 1: The wavenumbers and intensity of selected bands from the FT-IR and FT-Raman spectra of studied compounds. In order to establish the metal-ion-ligand ratio in the solution, the molar ratio method was applied. Figure 4 shows the dependence of the absorbance maxima (for the I band; 375 nm), on the composition of the Co(II)-quercetin complex. The analysis showed that in the solution, at pH = 7.4, the complex is present in the composition at a ratio of 1:2 (metal:ligand). Similarly in the solid state, the complex shows a molar ratio of 1:2 (metal:ligand), as was shown by an elemental analysis. Figure 3: UV spectra for quercetin (Q) and cobalt(II) complex of quercetin(Co-Q) at the concentration C = 5\(\cdot\)10\({}^{-5}\) M registered in Tris-HCl for (pH = 7.4). Figure 2: The FT-IR spectra of Co(II) complex of quercetin and quercetin registered in the range 4000–400 cm\({}^{-1}\). The analysis of infrared spectra of the cobalt with quercetin complex indicates the participation of the carbonyl group and the hydroxyl group, from the B ring, in the formation of the complex.