Extraction of sinensetin, isosinensetin and rosmarinic acid from orthosiphon stamineus leaves using ethanol assisted supercritical carbon dioxide

Abstract

Sinensetin, isosinensetin and rosmarinic acid are potent bioactive compounds for human health benefits that can be extracted from Orthosiphon stamineus (O. stamineus) leaves. Supercritical carbon dioxide (SC-CO2) extraction which has emerged as the preferable green solvent for natural products has been employed in the present study. The most important factor for designing SC-CO2 extraction process is to obtain the solubility data. The solubility of extract yield in SC-CO2 extraction has been widely reported, however, the extract yield consists of a mixture of interacting solutes which could affect the solubility. Thus, the solubility data of the solutes from O. stamineus leaves were determined in this study. The chemical parameters of temperature and pressure in the range of 40 - 80 °C and 10 - 30 MPa, respectively were investigated. Meanwhile, insignificant mechanical parameters of mean particle size (400 µm), total flow rate (5 mL/min), ethanol (5% v/v) and extraction time (4 hours) were kept constant. The concentration of solutes increased as temperature increased while the dual effect of pressure was observed. The solute vapor pressure prevailed over solvent power. The optimum SC-CO2 condition was obtained at 10 MPa and 80 °C with a high concentration of sinensetin, isosinensetin and rosmarinic acid of 440.3, 392.9 and 752.0 mg/kg sample, respectively by using response surface methodology with central composite design. Due to interaction of solutes, the solubility of sinensetin, isosinensetin and rosmarinic acid were varied from 0.617 - 17.179, 0.446 - 10.119 and 0.066 - 8.729 mg/kg solvent, respectively. Investigation on the effect of chemical parameters on the solute’s solubility showed similar behaviour as the solute’s concentration. However, rosmarinic acid with the highest concentration has the lowest solubility in SC-CO2. The presence of hydrogen bond acceptor and donor sites on the rosmarinic acid’s structure causes strong solute-solute interaction which contributes to the shortest intermolecular distance of 1.669 Å. On the other hand, the solubility of sinensetin was significantly higher than its isomer (isosinensetin) due to difference in melting point and dipole moment. Sinensetin has a lower melting point and dipole moment of 452.15 K and 3.36 D, respectively compared to isosinensetin with 479.15 K and 4.9 D. Thus, isosinensetin has stronger interaction between its molecules with the intermolecular distance of 2.395 Å compared to sinensetin with 2.454 Å. Then, the solubility data were correlated using two semi-empirical models namely Chrastil and del Valle-Aguilera. The obtained results revealed that both models were successfully correlated the experimental solubility data with low value of average absolute relative deviation percent in the range of 3.04% to 5.24%. Sinensetin (0.337) has the lowest value of coefficient of interaction, ?? followed by isosinensetin (0.676) and rosmarinic acid (1.012). These values indicates that sinensetin has the weakest solute-solute interaction but strong solute-solvent interaction which led to highest solubility in SC-CO2. Negative value of ?? was obtained due to negative effect of pressure on the solute solubility. Besides, the enthalpy of vaporization and solvation of solutes were obtained from the correlation models which vary from 4.19 to 13.38 kJ/mol and -1.59 to -0.41 kJ/mol, respectively. Hence, the present study has provided various data on sinensetin, isosinensetin and rosmarinic acid extracted from O. stamineus leaves in SC-CO2 which can be applied for further research.