## 4 Conclusions  
A new set of deep eutectic solvents, with glycerol as a hydrogen bond donor and lysine, proline, and arginine as hydrogen bond acceptors were synthesized and tested for their extraction capacity for antioxidant polyphenic compounds from olive leaves (_Olea europaea L_). To our knowledge, this is the first time these novel DES solvents are tested on olive leaves. Box-Behnken design was applied to optimize the extraction conditions related to liquid/solid ratio, the concentration of DESs, and extraction temperature (\(R_{L\text{S}}\), \(C_{\text{DES}}\), and \(T\), respectively). The optimal conditions for the best extraction of total phenols were predicted to be \(R_{L\text{S}}\), 150 mL g\({}^{-1}\), \(C_{\text{DES}}\) 90 (%, _w/o_) and 7 80 \({}^{\circ}\)C. The predicted total phenol yield, under optimal conditions (188.39, 100.01, and 95.97 mg g\({}^{-1}\) GAE dm, corresponding to glycerol-lysine, glycerol-arginine, and glycerol-proline solvent systems, respectively), were matched quite closely by the experimental values obtained. Glycerol-lysine DES was found to be the most effective in extracting phenolic compounds from olive leaf compared with glycerol-arginine DES and glycerol-proline and even to conventional solvents--namely, ethanol (70% _v/v_), methanol (70% _v/v_), and water. The results showed that the extraction with glycerol-lysine permitted 40.32%, 54.83%, 64%, 66.01%, and 71.09% higher total phenol yield (\(Y_{TP}\)), compared with that reached with glycerol-arginine, glycerol-proline, 70% ethanol, 70% methanol, and water solvents, respectively. Moreover, liquid chromatography-tandem-mass spectrometry analyses proved the high extraction capacity of glycerol-lysine DES and showed a selective behavior toward the quantified phenolic compounds. Overall, the DESs used in this study revealed a favorable extraction for tyrosol with excessive amounts in comparison with previous investigations, but more or less important differences were observed concerning hydroxytyrosol, oleuropein, luteolin-7-O-glucose, and rutin. The new DESs, designed, synthesized, and tested in this study, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Solvent** & \multicolumn{4}{c}{**Phenolic Compounds Concentrations \(\mu\)g g\({}^{-1}\))**} \\ \hline  & **Tyrosol** & **Hydroxytyrosol** & **Oleuropein** & **Luteolin-7-O-Glucoside** & **Rutin** \\ \hline
70\% aq.EOH & 19398.64 \(\pm\) 362.12 & 205.78 \(\pm\) 9.11 & 7381.03 \(\pm\) 52.14 & 1418.64 \(\pm\) 28.74 & 714.63 \(\pm\) 18.25 \\ GL–Lys & 53420.23 \(\pm\) 1135.12 & 1687.95 \(\pm\) 2.4 & n.d & 1377.14 \(\pm\) 17.25 & 286.69 \(\pm\) 0.69 \\ GL–Pro & 27358.48 \(\pm\) 175.71 & 231.95 \(\pm\) 9.8 & 7908.10 \(\pm\) 59.45 & 997.84 \(\pm\) 80.51 & 463.06 \(\pm\) 4.92 \\ GL–Arg & 28312.65 \(\pm\) 445.30 & 2686.44 \(\pm\) 7.9 & n.d & 1564.97 \(\pm\) 63.98 & 268.59 \(\pm\) 33.82 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Quantification of phenolic compounds in dry olive leaf extracts (\(\mu\)g g\({}^{-1}\)). Figure 4: Relationship between total phenolic content and tyrosol concentration. were proven to be successful in general on the extraction of antioxidant polyphenics from olive leaves, a by-product of a very important industry--olive oil production. Conceptualization, N.L.-S., S.G. and D.P.M. ; methodology, N.L.-S., S.G., D.P.M. and H.A. ; software, H.A. and S.G.; validation, N.L.-S., S.G., D.P.M., A.C. and A.M.; formal analysis, H.A., S.L. and A.K. ; investigation, H.A., S.G. and S.L. ; resources, N.L.-S., S.G. and D.P.M. ; data curation, H.A. and S.G.; writing-original draft preparation, H.A., S.G. and N.L.-S.; writing-review and editing, H.A., S.G., D.P.M., N.L.-S., A.C. and A.M.; visualization, H.A. ; supervision, S.G., D.P.M. and N.L.-S.; project administration, S.G.; funding acquisition, S.G. All authors have read and agreed to the published version of the manuscript. This research received no external funding. * [PERSON] et al.