Study of bound water in quercetin

Abstract

Study of bound water in medicinal preparations has both great practical value and independent theoretical and experimental significance [1]. Various substances, including the quercetin standard, are frequently obtained in the high-purity form by the method of recrystallization from 70% aqueous ethanol. As is known, this procedure may lead to the formation of crystal hydrates whose stability depends on the temperature and water content in the ambient medium [2].Because quercetin standard is used for the certification of drugs and raw plant materials [3,4], it is necessary to know precisely the content of water in the standard preparation. Taking into account that quercetin is dried at a temperature of 105~ prior to the analysis, after which it may be exposed to various temperature and humidity conditions, k was of interest to study the amount and state of water in quercetin depending on the hydrothermal parameters of the surrounding. EXPERIMENTAL PARTThe amount of bound water and the character of its binding in quercetin were determined by thermoanalytical methods, including differential scanning calorimetry (DSC) and thermogravimetry (TG), in combination with potentiometric titration using the Fischer reagent, gravirnetric determination of the weig~ht loss upon drying to constant weig~at in a dry box at 105 + 2.5~ measurement of the sorption isotherms in the static re,me, and study of the dynamics of sorption with time. The sorption of water vapor was studied by exposure of quercetin samples in desiccators above saturated solutions of standard salts [5] capable of creating relative humidity in the air ranging from 2 to 98 %. RESULTS AND DISCUSSIONAs is seen from the data presented in Table I, the initial qucrcetin samples contain about I% water. The therrnoanalyrical curves of this dry sample (Fig. Ib) reveal no thermal effects in the temperature range from -30~ to + 300~ suggesting that water contained in quercetin is subjected to neither freezing nor melting in this interval. These data are indicative of sufficiently strong binding between water molecules and hydrophilic groups of quercetin in the initial sampie. Subsequent scanning in the temperature interval 314-332~ reveals sharp endothcrmal peaks. As is seen from data presented in Table 1, this phase transition has a rather high enthalpy, approaching the energy of bond breaking. These results lead to a conclusion that this temperature interval features the melting of quercetin accompanied by the removal of water. Thus, water contained in the initial quercetin represents a strongly bound crystallization water.Because the initial quercetin samples were obtained by reerystallization, followed by drying in a thermal box at 105~ it was also of interest to study the stability of the crystal hydrate of quercetin under these conditions. As is seen from Table 1, the wei~ht loss of the sample upon drying at 105~ for 3 h amounts to + 0.8%. Study of the dynamics of water vapor sorption by the dried quercetin showed (Fig. 2) 91 Wei~t, %

Solubility and solution thermodynamic properties of quercetin and quercetin dihydrate in subcritical water

“…Independent studies have indicated a 10% loss in weight of quercetin when heated to temperatures as high as 103-342°C (Noubigh et al, 2008). This loss in mass at temperatures greater than 100°C is mainly attributed to the loss of water molecules in quercetin (Vyaznikova et al, 1997). Since, anhydrous quercetin does not have any extra water molecules, we can assume that they degrade faster than quercetin dihydrate at higher temperatures.…”

“…Since, anhydrous quercetin does not have any extra water molecules, we can assume that they degrade faster than quercetin dihydrate at higher temperatures. However, in presence of water as solvent, the effect of heat is also predicted to aid in the rapid crystallization of quercetin, thereby reducing the loss in mass due to degradation (Vyaznikova et al, 1997). Such an effect can be studied by analyzing the crystal structure of quercetin by a scanning electron microscope (SEM).…”

“…8d). This aggregation of crystals can be attributed to the dehydration of quercetin at temperatures greater than 100°C (Vyaznikova et al, 1997;Salameh and Taylor, 2006). At such temperatures and low flow rates, water would not effectively contact the entire surface area of quercetin in the saturation cell due to a channeling of water flow through the cell.…”