Lec. 6 Tannins PDF

LEC.6 Tannins Tannins comprise a large group of complex substances that are widely distributed in the plant kingdom; almost every plant family embodies species that contain tannins. When tannins occur in appreciable quantities, they are usually localized in specific plant parts, such as leaves, fr...

LEC.6 Tannins Tannins comprise a large group of complex substances that are widely distributed in the plant kingdom; almost every plant family embodies species that contain tannins. When tannins occur in appreciable quantities, they are usually localized in specific plant parts, such as leaves, fruits, barks, or stems. Chemically, tannins are complex substances; they usually occur as mixtures of polyphenols. Tannins are customarily divided two classes; hydrolysable tannins and non-hydrolyzable tannins (condensed tannins). 1-Hydrolyzable tannins: these may be hydrolysed by acids or enzymes such as tannase. They are formed from several molecules of phenolic acids such as gallic acid and hexahydroxy-diphenic acid which are united by ester linkages to a glucose molecule. Two principal types of hydrolysable tannins are gallitannins and ellagitannins which are, respectively, composed of gallic acid and hexahydroxy-diphenic acid. 1 Examples of plants containing hydrolysable tannins are: Gallitannins: rhubarb ‫راوند‬, hamamelis ‫ بندق الساحرة‬, cloves ‫قرنفل‬, and chestnut ‫كستناء‬. Ellagitannins: pomegranate rind, pomegranate bark, and oak bark ‫لحاء‬ ‫السنديان‬. 2-Non-hydrolysable tannins (condensed tannins, (proanthocyanidins): Unlike hydrolysable tannins, these are not readily hydrolysed to simpler molecules and they do not contain a sugar moiety. They are related to the flavonoid pigments and have polymeric flavan-3-ol structures. On treatment with acids or enzymes condensed tannins are converted into red insoluble compounds known as phlobaphenes. Phlobaphenes give the characteristic red colour to many drugs such as red cinchona bark, which contains these phlobaphenes and their decomposition products. 2 The condensed tannins are rich in: barks of cinchona, oak, and cinnamon; roots and rhizomes of krameria ‫ ;الرطن او الكراميريا‬flowers of lime and hawthorn‫ ; الزعرور‬seeds of cocoa and areca; fruits of grape; leaves of hamamelis and hawthorn; extracts and dried juice of catechu ‫ الكاتشو‬and acacia ‫االكاسيا‬. Both classes of tannins are widely distributed in nature. It must be emphasized that, in many species, both types are present, although one type generally tends to predominate in any particular plant part. Tannins are non-crystallizable compounds that, with water, form colloidal solutions possessing, an acid reaction and sharp puckering taste. Tannins cause precipitation of solutions of gelatine as well as alkaloids. They form a deep red colour or greenish black soluble compounds with ferric salts, they produce a deep red colour with potassium ferricyanide and ammonia. They are precipitated by salts of copper, lead, tin and by strong aqueous potassium dichromate solution. In alkaline solutions, many of their derivatives readily absorb oxygen. Tannins precipitate proteins from solution and can combine with proteins, rendering them resistant to proteolytic enzymes. Tannins when applied to living tissues, this action known as an astringent action and forms the basis for therapeutic application of tannins. Tannin-bearing plants, such as hamamelis and nutgall, as well as tannic acid and acetyltannic acid are employed in medicine as astringent in the gastrointestinal tract and on skin abasions. In the treatment of burns, the proteins of the exposed tissues are precipitated 3 and form a mildly antiseptic, protective coat under which the regeneration of new tissues may take place. The ability of tannins to precipitate proteins is also utilized in the process of vegetable tanning, which converts animal hides to leather. The tannin not only affects the pliancy and toughness of the leather but also acts as preservative because of its antiseptic quality. The deeply coloured compounds obtained with iron salts have been used on a commercial scale in the manufacturing of inks. Because of their precipitating action, solutions of tannins are utilized in the laboratory as reagents for the detection of gelatine, proteins, and alkaloids. In the antidotally treatment of alkaloidal poisoning, tannin solutions are extremely valuable for inactivating the alkaloid by the formation of insoluble tannate. Tannin-containing plant materials Hamamelis leaf ‫ بندق الساحرة‬Hamamelis leaf or witch hazel leaves is the dried leaf of Hamamelis virginiana L. (Fam: Hamamelidaceae). Hamamelis leaf contains hamamelitannin and a second tannin that appear to be derived from gallic acid, a hexose sugar, a volatile oil, a bitter principle, and calcium oxalate. Hamamelis leaf possesses astringent and hemostatic properties. Hamamelis water or Distilled Witch Hazel Extract which is a hydro- alcoholic solution of the volatile oil of the hamamelis is used for its astringent properties. It is incorporated in hemorrhoidal products, preparations for treating insect bites and stings, and even teething 4 preparations. The contained volatile oil does not confer a pleasant fragrance to such products. Nutgall ‫البلوط‬ Nutgall is the excrescence obtained from the young twigs of Quercus infectoria ‫ (البلوط الحلبي‬Fam: Fagaceae). The excrescence (gall) caused by the puncture of a hymenopterous insect, Cynips tinctoria, and the presence of deposited ovum. Several stages of in the development of the gall correspond to the development of the insect. 1-When the larva ‫ اليرقة‬begins to develop and the gall begins to enlarge, the cells of the outer and central zones contain numerous starch grains. 2-When the chrysalis stage ‫ الخادرة‬is reached, the starch near the middle of the gall is replaced in part by gallic acid, but the peripheral and central cells contain masses of tannic acid. 3-As the winged insect develops, nearly all the cells contain masses of tannic acid with a slight amount of adhering gallic acid. 4-When the insect emerges from the gall, a hole to the central cavity is formed. Thus, the tannic acid, owing to the presence of moisture and air, may be oxidized in part into an insoluble product, and the gall becomes more porous, thereby constituting the so-called white gall of commerce. 5 Its principal constituents is tannic acid, which is found to the extent of 50-70%: the drug also contains gallic acid, ellagic acid, starch, and resin. Nutgall, is the chief source of tannic acid, is used in the tanning and dyeing industry, and formerly, in the manufacture of ink. Medicinally, it has astringent properties. Tannic acid Tannic acid or tannin is usually obtained from nutgall. The powdered galls are extracted with a mixture of ether, alcohol, and water and the liquid separates into two layers. The aqueous layer contains gallotannin and the ethereal layer contains the free gallic acid present in the gall. After separation, the solution of gallotannin is evaporated and the tannin is purified in various ways. 6 Composition of tannic acid is not a single homogeneous compound but is a mixture of esters of gallic acid with glucose, whose exact composition varies according to the source. Tannic acid is an astringent. It was formerly used in the treatment of burns, but this application has been discontinued. Its topical use is now restricted to the treatment of bed sores, minor ulcerations. As an alkaloidal precipitant, it has been employed in cases of alkaloidal poisoning. Gallic acid is 3,4,5-trihydroxybenzoic acid that crystallizes with one molecule of water. It occurs in nutgall and be prepared from tannic acid by hydrolysis with dilute acids. Bismuth subgallate is used by ostomates to help control odours. 7