Pharmacognosy Alkaloids PDF

pharmacognosy Alkaloids Alkaloids contents : Overview 3 Classification 7 Possible function of alkaloids in plants 14 Properities 15 Tests for alkaloids 21 Biosynthesis of alkaloids 27 The major amino acids Extraction 29 32 Method A 33 Method B 35 Alkaloids Overview : Many of these metabolic by-produ...

pharmacognosy Alkaloids Alkaloids contents : Overview 3 Classification 7 Possible function of alkaloids in plants 14 Properities 15 Tests for alkaloids 21 Biosynthesis of alkaloids 27 The major amino acids Extraction 29 32 Method A 33 Method B 35 Alkaloids Overview : Many of these metabolic by-products are derived from amino acids and include an enormous number of bitter, nitrogenous compounds. More than 10,000 different alkaloids have been discovered in species from over 300 plant families. Alkaloids Alkaloids often contain one or more rings of carbon atoms, usually with a nitrogen atom in the ring. The position of the nitrogen atom in the carbon ring varies with different alkaloids and with different plant families Alkaloids The Names of The Alkaloids Are Obtained in Various Ways: 1. From The Generic Name Of The Plant Yielding Them (Hydrastine, Atropine). 2. From The Specific Name Of The Plant Yielding Them (Cocaine, Belladonnine). 3. From The Common Name Of The Drug Yielding Them (Ergotamine). Alkaloids 4. From Their Physiologic Activity (Emetine, Morphine). 5. From The Discoverer (Pelletierine). By agreement, chemical rules designate that the names of all alkaloids should end in “ine”. Alkaloids Classification : Alkaloids are generally classified by their common molecular precursors, based on the biological pathway used to construct the molecule. 1. True Alkaloids : True alkaloids derive from amino acid and they share a heterocyclic ring with nitrogen. These alkaloids occur in a limited number of species and families. Morphine The primary precursors of true alkaloids are such amino acids as Lornithine, L-lysine, L-phenylalanine/Ltyrosine, L-tryptophan and Lhistidine. Alkaloids Examples of true alkaloids include such biologically active alkaloids as cocaine, quinine and morphine. Alkaloids 2. Protoalkaloids : Protoalkaloids are compounds, in which the N atom derived from an amino acid is not a part of the heterocyclic. Such kinds of alkaloid include compounds derived from L-tyrosine and L-tryptophan. mescaline and yohimbine are good examples of these kinds of alkaloid. mescaline Alkaloids 3. Pseudoalkaloids : These alkaloids can also be derived from nonaminoacid precursors. Pseudoalkaloids can be acetate and phenylalanine derived or terpenoid, as well as steroidal alkaloids. Caffeine Examples of pseudoalkaloids include such compounds as coniine, capsaicin, ephedrine, solanidine, caffeine and theobromine. Alkaloids NOTE: Alkaloids are mainly divided into two categories on the basis of their chemical structure, that is, heterocyclic rings. Alkaloids Atypical alkaloids Typical alkaloids These are also known as nonheterocyclic alkaloids and contain nitrogen in aliphatic chain. These are also known as heterocyclic alkaloids and contain nitrogen in heterocyclic ring system. Alkaloids Alkaloids Alkaloids The possible function of alkaloids in plants as: Poisonous agents protecting the plant against insects and herbivores. End products of detoxification reactions representing a metabolic locking-up of compounds otherwise harmful to the plant. Regulatory growth factors. Reserve substances capable of supplying nitrogen or other necessary elements to the plant’s economy. Alkaloids Properties : In general, they are colourless, crystalline solids which are basic, have a ring structure, and have definite melting points. They are also derived from plants and have a bitter taste. However, some exceptions are known. For instance, some alkaloids are not basic and others are brightly coloured (betanidine, beriberine) or liquid (nicotine). Other alkaloids are produced synthetically Alkaloids Most alkaloids are also chiral molecules which mean they have nonsuperimposable mirror images. This results in isomers that have different chemical properties. For example, one isomer may have a physiological function while the other does not. Alkaloids Generally free bases of alkaloids are soluble in organic solvents and insoluble in water, whereas alkaloidal salts are soluble in water and partially soluble in organic solvents. For example, strychnine hydrochloride is much more soluble in water than strychnine as a base. Alkaloids Alkaloids usually contain one nitrogen atom, although some like ergotamine may contain up to five. The nitrogen may exist as a primary amine (RNH2), as a secondary amine (R2NH), or as a tertiary amine (R3N). Since the nitrogen atom bears an unshared pair of electrons, such compounds are basic and resemble ammonia in chemical properties. Alkaloids The degree of basicity varies greatly depending upon the structure of the molecule and the presence and location of other functional groups. Like ammonia, the alkaloids are converted into their salts by aqueous mineral acids, and when the salt of an alkaloid is treated with hydroxide ion, nitrogen gives up a hydrogen ion and the free amine is liberated. Alkaloids Quaternary ammonium compounds [R4N+ X-] such as tubocurarine chloride or muscarine chloride have four organic groups covalently bonded to nitrogen, and the positive charge of this ion is balanced by some negative ion. The quaternary ammonium ion, having no proton to give up, is not affected by hydroxide ion; consequently, quaternary ammonium compounds have chemical properties quite different from those of the amines Alkaloids Tests for alkaloids : Most alkaloids are precipitated from neutral or slightly acid solution by : Mayer's reagent (potassium mercuric iodide solution). Wagner's reagent (solution of iodine in potassium iodide), - Solution of tannic acid. Alkaloids Alkaloids Hager's reagent (a saturated solution of picric acid). Dragendorff's reagent (solution of potassium bismuth iodide) Alkaloids These precipitates may be amorphous or crystalline and are of various colors: cream (Mayer's), yellow (Hager's). reddish-brown (Wagner's and Dragendorff's) Alkaloids NOTES: 1- the reagents also give precipitates with proteins. During the extraction of alkaloids from the plant and subsequent evaporation. some proteins will not be extracted and others will be made insoluble (denatured) by the evaporation process and may be filtered out. Alkaloids NOTES: 2- caffeine, and other purine derivatives, does not precipitate like most alkaloids. It is usually detected by mixing with a very small amount of potassium chlorate and a drop of hydrochloric acid, evaporating to dryness and exposing the residue to ammonia vapor. A purple color is produced ,this is known as the murexide test. Alkaloids Biosynthesis of alkaloids The biosynthesis of many alkaloids’ structure can be rationalized through simple chemical reactions that involve amino acids. The general reactions that are of particular importance include the decarboxylation & transamination of the amino acids to yield the corresponding amine or aldehyde respectively. These can react to form a Schiff base which in turn, can react with a carbanion in a Mannich type of condensation. Alkaloids General reactions in alkaloid biosynthesis. Alkaloids The major amino acids involved in the biosynthesis of alkaloids are: 1. Ornithine ti 2. Lysine Alkaloids 3- Phenylalanine 4. Tyrosine Alkaloids 5. Anthranilic acid It differs from other amino acids in that the COOH & NH2 groups are not on the same carbon atom. 6. Tryptophan Extraction Alkaloids Extraction : Method A: The powdered material that contains alkaloidal salts is moistened with alkaline substances like sodium bicarbonate, ammonia, calcium hydroxide, etc., which combines with acids, tannins and other phenolic substances and sets free the alkaloids bases. Extraction is then carried out with organic solvents such as ether or petroleum spirit. Alkaloids The concentrated organic liquid is then shaken with aqueous acid and allowed to separate. Alkaloid salts will be present in aqueous liquid, while many impurities remain behind in the organic liquid Alkaloids Method B: The collected powdered material is extracted with water or aqueous alcohol containing dilute acid. Chloroform or other organic solvents are added and shaken to remove the pigments and other unwanted materials. The free alkaloids are then precipitated by the addition of excess alkalis like, sodium bicarbonate or ammonia and separated by filtration or by extraction with organic solvents