pharmacognosy_General_methods_associated_with_the_phytochemical.pdf

pharmacognosy General methods associated with the phytochemical investigation of herbal products General methods Contents : General methods 3 The phytochemical investigation of a plant 6 Extraction of Plant Material 8 Spouted bed extraction 12 Supercritical fluid extraction 13 Solid phase microextra...

pharmacognosy General methods associated with the phytochemical investigation of herbal products General methods Contents : General methods 3 The phytochemical investigation of a plant 6 Extraction of Plant Material 8 Spouted bed extraction 12 Supercritical fluid extraction 13 Solid phase microextraction 18 General methods General methods associated with the phytochemical investigation of herbal products EXTRACTION OF PLANT MATERIALs. SEPARATION AND ISOLATION OF CONSTITUENTS. CHARACTERIZATION OF ISOLATED COMPOUNDS. General methods Not all the chemical compounds produced by plants are of equal interest to the pharmacognosist. Until relatively recently the so-called ‘active’ principles were frequently alkaloids or specific glycosides usually with pronounced pharmacological properties; these therefore received special attention, and in large measure constituted the principal plant drugs of the allopathic system of medicine. It is now realized that many other constituents of plants, particularly those associated with herbal medicine, have medicinal properties which manifest themselves in more subtle and less dramatic ways than the obviously poisonous plants. General methods This has considerably widened the scope of plant metabolites considered worthy of more detailed investigation. Other groups such as carbohydrates, fats and proteins are of dietetic importance, and many such as starches and gums are used in pharmacy but lack any marked pharmacological action. Substances, such as calcium oxalate, silica, lignin and colouring matters, may be of assistance in the identification of drugs and the detection of adulteration. General methods As a result of the recent interest in the plant kingdom as a potential source of new drugs, strategies for the fractionation of plant extracts based on biological activity rather than on a particular class of compound, have been developed. The phytochemical investigation of a plant may thus involve the following: 1. authentication and extraction of the plant material. 2. separation and isolation of the constituents of interest. 3. characterization of the isolated compounds. General methods 4. investigation of the biosynthetic pathways to particular compounds; and quantitative evaluations. 5. Parallel to this may be the pharmacological assessment of the separated components, which may, in some investigations, precede the characterization. General methods Extraction of Plant Material : All plant material used should be properly authenticated, as much time and money can be wasted on the examination of material of doubtful origin. The choice of extraction procedure depends on the nature of the plant material and the components to be isolated. General methods Dried materials are usually powdered before extraction, whereas fresh plants (leaves, etc.) can be homogenized or macerated with a solvent such as alcohol. The latter is also particularly useful for stabilizing fresh leaves by dropping them into the boiling solvent. General methods Alcohol is a general solvent for many plant constituents (most fixed oils excepted) and as such may give problems in the subsequent elimination of pigments, resins, etc. Waterimmiscible solvents are widely used—light petroleum (essential and fixed oils, steroids), ether and chloroform (alkaloids, quinones). General methods The extraction of organic bases (e.g. alkaloids) usually necessitates basification of the plant material if a waterimmiscible solvent is to be used; for aromatic acids and phenols acidification may be required. Extraction itself may be performed by repeated maceration with agitation, percolation or by continuous extraction (e.g. in a Soxhlet extractor). Ultrasound may enhance the extraction process for some plant materials. General methods Spouted bed extraction : In certain instances the physical removal of the pigment layer of the seed-coat can yield a less impaired product than that produced by solvent extraction. Such methods can involve the use of a ball mill or a spouted bed unit. Basically it consists of a cylinder tapered at both ends and containing the seeds at the lower end through which a jet of hot air is forced. General methods Supercritical fluid extraction : The use of supercritical fluids for the extraction of a range of materials including plant products of medicinal, flavouring and cosmetic interest has become of increasing economic and research interest. In 1822, Cagniard de la Tour reported that above a certain temperature, and pressure, single substances do not condense or evaporate but exist as a fluid. General methods Under these conditions the gas and liquid phases both possess the same density and no division exists between the two phases. This is the critical state. For water, the critical conditions for temperature (tc) and pressure (pc) are 374°C and 220 atmospheres respectively. For carbon dioxide tc = 31°C and pc = 74 atm. In practice conditions somewhat above the critical temperature and pressure for a particular substance are usually used and these supercritical fluids exhibit properties intermediate between those of the liquid and gaseous phases. General methods In phytochemistry these properties can be exploited to maximize the extraction of plant constituents. For industrial purposes supercritical fluid carbon dioxide has an environmental advantage over many common organic solvents and leaves no solvent residues in the product. It also allows a low temperature process and has proved of value for the extraction of labile expensive fragrances and medicinal phytochemicals. General methods To render it more polar a small amount of modifier, e.g. methanol, may be added to the carbon dioxide. The high pressures, and for some substances the high temperatures, involved in supercritical fluid extraction are the principal disadvantages of the technique. General methods General methods Solid phase microextraction : The method is suitable for some volatile oilcontaining drugs using methyl polysiloxane solid phase microextraction fibres. Example is the extraction of volatile oils from eucalyptus leaves (37° for 10 min.). General methods The fibres were then desorbed at 200° for capillary gas chromatography of the oil. Not surprisingly, the oil composition differs from that of steam-distilled