Phytochemistry: Plant Natural Products PDF

Phytochemistry Plant natural products Dr. Iman Ezzat  Plant natural products ❖Carbohydrates ❖Glycosides ❖Tannins ❖Volatile Oil ❖Alkaloids ❖Marine Natural products  Carbohydrates Carbohydrates are the most widely distributed, naturally occurring org...

Phytochemistry Plant natural products Dr. Iman Ezzat  Plant natural products ❖Carbohydrates ❖Glycosides ❖Tannins ❖Volatile Oil ❖Alkaloids ❖Marine Natural products  Carbohydrates Carbohydrates are the most widely distributed, naturally occurring organic compounds on Earth. Carbohydrates are produced during the process of photosynthesis They were thought to be hydrates of carbon as the ratio of the number of atoms of hydrogen to those of oxygen in the molecule is the same as in the molecule of water “H2O, 2 : 1” and hence were called carbohydrates. Cn(H2O)n Cho ???  Modern definition of carbohydrates  Pharmaceutical and biological importance ❖stores of energy for plants and animals, e.g. Starch and glycogen. ❖As supporting tissue, e.g. Chitin in insects and cellulose in plants. ❖Basis of several industries including sugar, sugar products, starch products, paper, ❖textile, food processing and some pharmaceutical industries. ❖ Essential components of nucleic acids (2-deoxy-D-ribose) ❖Some important drugs in medicine are carbohydrate in nature.  Classification of Carbohydrate On the basis of their behavior on hydrolysis, CHO classified to: A) Monosaccharide: They are the simplest sugars, which cannot yield simpler sugars by hydrolysis. B) Oligosaccharides: contain 2 - 10 monosaccharide units, mostly of hexoses. C) Polysaccharides: more than 10 units  Monosaccharides They are crystalline water-soluble substances with sweet taste. Parent monosaccharides are Polyhydroxy aldehydes or Polyhydroxy ketones with three or more carbon atoms Monosaccharides could be classified based on: (a) Number of carbon atoms present in the molecule (b) Presence of aldehyde or keto group ❖ The suffix-ose indicates that a molecule is a carbohydrate  The simplest monosaccharides is trioses having 3 carbons. Only two trioses are present; glyceraldehyde and dihydroxyacetone.  Oligosaccharides Products of glycosidic linkages of 2-10 monosaccharide units. Based on the number of monosaccharide units formed on hydrolysis, they are divided into: 1) Disaccharide: 2) Trisaccharides: 3) Tetrasaccharide: gal(α1→6)gal(α1→6)glc(α1 2β)fru stachyose  Polysaccharides also known as non-sugar as they are not sweet in taste. Have high molecular weight. contain very long chains of hundreds or thousands of monosaccharide units, which may be either in straight or branched chains. Types: Polysaccharides Polysaccharides composed of only one kind of monosaccharide Homopolysaccharides (homoglycans). If two or more different kinds of monomeric unit are present, the class name Heteropolysaccharide (heteroglycans) If yield on hydrolysis monosaccharide units in addition to sugar derivatives as monosaccharide sulfate ester, amino-sugar and uronic acid or acids. Derived carbohydrates:  Drawing sugar molecules 1. Fischer projection (Linear structure=open-chain forms ) 2. Haworth projection (cyclic forms) 3. chair/boat conformational 1- Fischer projection: - Ball and stick models show the actual configuration of molecules. - Fischer projection formulas - Perspective formula solid wedge-shaped bonds point toward the reader, dashed wedges point away.  Chiral carbon atom One of the distinguishing features of carbohydrates is that they contain one or more chiral carbon atoms A chiral carbon atom has four different groups attaching to it  ISOMERISM Isomers Compounds having the same molecular formula but having different structures or different orientation.  Structure (constitutional) isomers The compounds possessing same molecular formula and different structure. = with different functional groups. i.e.: one has an aldehyde group, the other has a ketone group.  Stereoisomers Stereoisomers are isomers that have the same molecular and structural formulas but differ in the orientation of atoms in space (spatial arrangement= configuration). Number of stereoisomers =2n (n= no of chiral centers) Enantiomers (optical isomers) Types Diastereomers  Enantiomers (optical isomers) are stereoisomers whose molecules are mirror images of each other but non superimposable Designated as D and L isomers Number of enantiomers pairs =2n / 2 (n= no of chiral centers)  Configurational atom Monosaccharide assigned to D or L according to absolute configuration of the highest-numbered chiral centre, i.e. the “configurational atom”. In the Fischer projection: If hydroxy group projects to the right : D series with prefix D- If the hydroxy group projects to the left: L series with prefix L- NB: Almost all of the naturally occurring carbohydrates have the D-configuration.  Enantiomers (optical isomers) D and L isomers have the same chemical and physical properties, except for the way that they rotate plane polarized light.  optical activity Ordinary Light: Move in all directions Plane polarized light move only in one direction Plane polarized light is rotated clockwise (to right) or counterclockwise (to left) when passed through enantiomers Direction and extent of rotation (angle) will depend upon the enantiomer Polarimeter  optical activity Compound with optical activity means it has chiral carbon and it can rotate the plane of polarized light as it travels through it to either right (clockwise) or to the left (anticlockwise). (optically active compound) Classification of the molecule based on the rotation of plane polarized light: Dextrorotatory - rotate clockwise and labelled (+) or (d) e.g., d-glucose Levorotatory - rotate anti-clockwise and labelled (-) or (l) e.g., l-fructose  Diastereomers These are the stereoisomers that differ in one or more chiral centers but are not mirror images of each other. do have different physical and chemical characteristics, and thus, have different names  Epimers Special subclass of diastereomers that differ only in the stereochemistry at one chiral center  Sugar Cyclization 5 and 6 carbon sugars tend to cyclize SPONTANEOUSLY in solution forming either a furan ring or a pyran ring structure Furan Ring Pyran Ring Basics of Sugar Cyclization hemiacetals hemiaketals Spontaneous and Reversible hemiacetals Hemiketal  2. A Haworth projection It is a way to keep track of relative stereochemistry of carbohydrates in cyclic forms. ❖Rules for converting a Fischer structure to a Haworth structure. Fischer Haworth OH to right OH down (below ring) OH to left OH up (above ring) Naming cyclized sugars  Anomers The two new sugar stereoisomers created by ring closure (α- and β- anomers) Anomeric carbon: the new chiral center created when the sugar ring is formed. Anomeric carbon??? optical activity of the anomers Same or different??? Mutarotation is the interconversion of α- and β- anomers in an aqueous solution of the monosaccharide to reach equilibrium leading to change in the value of optical rotation with time Pure α-o-glucopyranose ([α]= + 112°) How Pure β -o-g1ucopyranose ([α]= + 19°) When any of the two forms is dissolved in water, the specific optical rotation of the solution changes overtime until it reaches a constant value of +52.7°  Physical properties of monosaccharides 1. They are solids at room temperature. (crystalline) 2. They are extremely soluble in cold water?? Despite their high molecular weights, the presence of large numbers of OH groups make the monosaccharides much more water soluble than most molecules of similar MW. 3. Most monosaccharides have a sweet taste.  Physical properties of monosaccharides 4. Optical activity Monosaccharides and water-soluble oligosaccharides are optically active and determination of their specific rotation is useful for their identification 5. Many display the phenomenon of mutarotation  Enantiomers (optical isomers) D and L isomers have the same chemical and physical properties, except for the way that they rotate plane polarized light. Enantiomer pairs are very difficult to separate from one another and are given the same name. However, their biological activities are often dramatically different, as enzymes require the correct structure to bind with a molecule. The wrong stereoisomer will not bind an enzyme of interest.  Reactions of monosaccharides Esterification (production of phosphate esters) Oxidation-reduction Amino derivatives Glycoside formation (linkage of monosaccharides to form oligo- and polysaccharides)  1. Esterification The most important biological esters of carbohydrates are phosphate esters. e.g. glucose 6-phosphate found in the sugar-phosphate backbone of DNA and RNA, and as intermediates in the metabolism of carbohydrates in the body.  2. Glycoside formation Glycoside- compound formed when a sugar in the cyclic form is bonded to another sugar molecule through a glycosidic bond Mutarotation ??? Glycosidic linkage is formed between the anomeric carbon of a carbohydrate and the oxygen, nitrogen or phosphorous of other compound.  3. Reduction to sugar alcohols The carbonyl group in a monosaccharide (either an aldose or a ketose) is reduced to a hydroxyl group using hydrogen as the reducing agent or specific enzymes. The product is the corresponding polyhydroxy alcohol =sugar alcohol=alditols. e.g. Sorbitol e.g. Erythritol (60-70% as sweet as sucrose, But contributes for fewer calories when eaten) Alditols are named by changing the suffix -ose of corresponding aldose into -itol  4. Oxidation Reducing sugars- a sugar which can be oxidized to an acid. Oxidation can yield three different types of acidic sugars depending on the type of oxidizing agent used: 1-Aldonic acid 2-aldaric acid=Saccharic acid 3-Uronic acid  Enzyme catalyzed HNO3 Br2/H2O  Reaction with oxidising cations All monosaccharides and reducing disaccharides reduce mild oxidizing agent such as ferric (Fe+3) or cupric (Cu+2) ion or (Ag+) and the carbonyl carbon is oxidized to a carboxylic group plus ferrous (Fe+2) or cuprous (Cu+) or ( Ag0) (Fehlings’ reagent, Benedicts’ reagent, Tollens’ reagent). ❖These reactions are used for identification and quantification of reducing sugars.  The action of alkalis The reaction with alkali produces aldose-ketose isomerization and epimerization of aldoses. e.g. At pH (11-13), alkali catalyzes the transformation of D-glucose into D- fructose and D-mannose.  Amino derivatives Formed by the replacement of a hydroxyl group on a carbohydrate with amino group results in an amino sugar. It’s always at C2 The amino group of glucosamine may be acetylated. A component of glycoproteins and glycolipids.