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, %
Determination of the content and state of water bound in medicinal phytopreparations is necessary for establishing a scientifically justified methodological approach to standardization of these substances and the regime of their drying, packaging, and storage. The accuracy and reliability of the evaluation methods can be significantly increased by using standard substances. One of these standards is rutin, or 3-{[6-O-(cc-L-rhamnopyranosyl)-13-D-glucopyranosyl]oxy}-5,7,3',4'-tetrahydroxyflavone, used for determination of the content of vitamin P in drugs and raw plant materials and their classification with respect to the total content offlavonoids [1].As is known, rutin contains three water molecules [2]. Crystal hydrates are the most difficult objects for investigation: despite a huge number of works in this field, the results are frequently ambiguous, especially when the state of water is considered. It was demonstrated that the state of water in hydrated compounds is affected by a number of factors, ineluding the presence of polar groups and their interaction with water molecules, incorporation of water into the crystal lattice, donor-acceptor interactions, etc. [3]. Crystal hydrates are stable within certain intervals of the temperature and the pressure of water vapor in the environment. These regions of parameters are individual for each crystal hydrate. The amount of crystallization water entering into a particular crystal hydrate may vary within wide limits, depending on the external conditions.The purpose of this work was to study the content and state of water in rutin in order to elaborate a correct methodological approach to standardization of this substance and its use for the analysis of raw plant materials and medicinal preparations containing phenolic compounds such as flavonoids. E~ERIMENT~P~TThe amount of bound water and the character of its bonding in rutin were determined by methods of thermal analysis in combination with potentiometric titration and determination of the weight loss upon drying to a constant weight in a dry box. This approach, combining physical and chemical methods, is widely employed in the practice of pharmaceutical analysis [4][5][6].The thermal analysis was performed on a Perkin-Elmer DSC-2 differential scanning calorimeter. Rutin samples weighing 5 -8 mg were placed into standard aluminum cells and hermetically sealed so as to avoid drying. The thermograms were measured in the temperature range from 20 to 220~ scanned at a heating rate of 10K/rain. The total water content in the rutin preparations was determined either by the method ofpotentiometric titration using the Fischer reagent or by the thermoanalytical method using the DSC-2 instrument. The hygroscopicity ofrutin was studied by the conventional technique at a relative humidity of 90% [7]. The isotherms of water vapor sorption by rutin were obtained under static conditions at a relative humidity of air varying within 2 -98%. RESULTS AND DISCUSSIONAs is seen from the thermoanalytical curves depicted in Fig. 1, the...
A reliable way to increase the level of standardization of phytochemical preparations consists in establishing analytical procedures based on officially certified standards represented by compounds of the corresponding chemical nature. In application to flavonoid-eontaining phytoprepamtions, the role of these reference compounds is frequently performed by readily available standard samples of rutin [1] or quereetin [2] belonghag to the group of flavonols. Recently, a new standard sample-dihydroquercetin [3], representing the group of hydroxyflavonols, was created in connection with the development of a new antioxidant and capillary-protector phytopreparation diquertin, in which dihydroquercetin is the active substance [4,5].The standard compounds must satisfy high requirements with respect to the degree of purity. Standard substances of the flavonoid nature have to meet another important requiremerit, reflecting the amount and state of water in the compound. This circumstance is explained by the fact that molecules of the flavonoid compounds contain polar functional groups (carbonyl, hydroxyl) and are capable of interacting with other polar molecules and creating various structural formations. Study of the standard turin and quercetin samples showed that they may contain, depending on the hydrotherreal parameters of the environment, water molecules of the adsorption, "pseudo-crystal-hydrate" and crystal-hydrate nature [6,7].The purpose of this work was to study the amount and character of bound water in the standard dihydroquercetin sample, in the context of characterization of the new phytopreparation diquertin. MATERIALS AND METHODSExperiments were performed on samples ofdihydroquercetin standard and diquertin synthesized at the Irkutsk state University from crushed wood of larch tree [8]. The content of dihydroquercetin in the diquertin sample was 95.4%, and dihydrokaempferol and naringenin accounted for 4.5%. A standard dihydroquercetin sample was obtained by methods of preparative I-IPLC and represented a high-purity compound cor~mlnirtg foreign impurities [9].Thermoanalytical investigations included differential scanning calorimetry (DSC) and thermogravimetric (TG) measurements. To this end, samples weighing 4-5 mg were placed into standard aluminum boats and closed. The desired temperature interval was scanned at a heating rate of 10 K / rain. The total water content was determined by potentiometric titration using the Fischer reagent and by gravimettic determination of the weight loss upon drying to constant weight in a dry box at 105 +__ 2.5~ The sorption of water vapors was studied by exposure of dihydroquercetin and diquertin samples in desiccators above saturated solutions of standard salts, followed by measuring the isotherms of sorption in the static regime and the sorption dynamics in time. RESULTS AND DISCUSSION 94Study of the initial samples showed that the water content in diquertin was several times that in dihydroquercetin (Table 1), albeit still not exceeding the level (not above 7%) stip...
Methods of thermal analysis, including differential scanning calorimetry (DSC) and thermogravimetry (TG), increasingly are being applied in pharmaceutics for the evaluation and development of properties of medicinal phytopreparations and standard substances. The melting temperature and water content are among the most important parameters characterizing the quality of these materials, and can be determined by DSC and TG techniques [ 1 -31.The use of thermoanalytical methods is especially helpful when it is necessary to obtain information on the state of water in various preparations. Other analytical techniques, based on the use of the Fischer reagent and the drying weight loss (DWL) measurements, are capable of determining (in contrast to the thermal analysis) only the quantitative characteristics of preparations. The thermal analysis is aIso advantageous in the case when the Fischer reagent may interact with some chemical groups in the drugs to be analyzed, which excludes its use for the water content determination [4].The purpose of this work was to study the nature of interaction between water and 7-(p-D-glucopyranosyloxy)-8-(3-methylbut-2-enyl)-3,4',5&hydroxyflavanone. EXPERIMENTAL PARTWe have studied the State Standard Sample (SSS) of flacoside used for the certification of plant raw materials, such as the leaves of Phellodendron Amurense and Phellodendron LavaiUeus, and the analysis of flacoside drug in substance and tablets. The phase transitions, connected with evaporation, melting, or decomposition of the objects, were determined by calorimetric measurements on a Perkin-Elmer DSC-2 differential scanning calorimeter. The water content was determined by thermogravimetric measurements on a Perkin-Elmer TGA-7 instrument and by the DWL method. The thermoanalytical measurements were performed in the temperature range from -30 to + 235"C, by heating samples
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