Carbohydrates

CARBOHYDRATES Carbohydrates are organic compounds made up of carbon, hydrogen and oxygen. In nature carbohydrate has a general elemental composition, namely, Cx(H2O)y, which signifies molecules containing carbon atoms along with hydrogen and oxygen atoms in the same ratio as they occur in water. Therefore, they named as carbohydrate (Carbon + Hydrates) means “hydrates of carbon”. Carbohydrates are also called saccharides generally from Greek “sakcharon”, meaning sugar. In structural formula saccharides referred as CHO. The names of most CHO are characterized by ending “OSE” e.g. Glucose, Lactose. In animal organism the main sugar is glucose & storage CHO is glycogen, in milk the main sugar is the disaccharide lactose. In plant organism a wide variety of monosaccharides and oligosaccharides occur & the storage carbohydrate is starch. The structural polysaccharide of plant is cellulose. The gums are a group of polysaccharides obtained from plants, seaweeds, and microorganisms Classification Dietary carbohydrates on the basis of their ability to digest by enzymes present in saliva, stomach or intestine can be categorized as Digestible Carbohydrates: These are a diverse group of substances with the primary function of serving as an energy source for all body cell. Dietary CHO may be categorized as digestible by enzymes present in the saliva, stomach or intestine, absorbable without digestion. Indigestible Carbohydrates Dietary fibers are non-digestible carbohydrates mostly derived from plant sources that reach the colon nearly intact. Dietary fibers found in cereals, Vegetables & fruits & fructooligosaccharides such as inulin, present in certain vegetables & processed foods. Types of carbohydrates There are three types of carbohydrates Monosaccharides Oligosaccharides Polysaccharides A. Monosaccharides: Monosaccharides are simple carbohydrates containing three to eight carbon atoms, but only those with five or six carbon atoms are common. The general formula is (CH2O)n where n is 3 to 7. Monosaccharides are polyhydroxyaldehydes (aldoses), formally considered to be derived from glyceraldehyde, or polyhydroxyketones (ketoses), derived from dihydroxyacetone by inserting CHOH units into the carbon chains. These can’t be hydrolyzed into simpler forms. They serve as the building blocks of complex sugars and polysaccharides e.g. Galactose. In monosaccharides all the carbon atoms contain one hydroxyl (–OH) group and except one carbonyl oxygen (as in –CHO, or C=O). On the basis of their functional group monosaccharides are classified in two types Aldoses If the carbonyl oxygen group is present at the terminal position, the monosaccharide is an aldehyde derivative and the sugar is called aldose sugar. The aldoses contain an aldehyde ( ) group. E.g. Glucose. Ketoses If carbonyl oxygen group is present present in any other position except terminal position, the monosaccharide is a ketone derivative that is known as ketoses. Ketoses which contain a ketone ( )group. E.g. Fructose. Nomenclature: The chain of monosaccharides numbered from the nearest location of functional group. In usual cases C-1 is aldehyde functional group and C-2 is ketone functional group. On the basis of the number of carbon atoms in aldose or ketose the monosaccharides can be classified as Triose, Tetrose, Pentose, hexose. E.g. Glucose with six carbon atom is an aldohexose Fructose with six carbon atom is a ketohexose. Aldoses Ketoses Triose Tetrose Pentose Hexose Structure: Carbohydrates are represented by three types Fischer Projection: Two dimensional representation of three dimensional carbohydrates structure on paper via projection. It drawn by showing vertical and horizontal bonds via single line for single bond e.g. in above fig D-Glucose & D-Fructose. The Fischer projection don’t accurately describe the true shape of the cyclic hemiacetal forms of glucose. Haworth Cyclic Structure: W. N. Haworth suggested a new formulation, in which rings are written as flat or planar hexagons, is more correct. Aldohexose, Ketohexose & aldopentose in solution undergo cyclisation when treated with equivalent amounts of alcohol & form a Hemiacetal in the aldose & Ketose respectively. The glucose structure contains an aldehyde group & five hydroxyl group in the same molecule. Consequently, there can be intramolecular interaction between the carbonyl group & one of the hydroxyl group. The hydroxyl group on C5 is able to react with the carbonyl group on C1 to produce a closed pyranose ring. The reaction results in a ring i.e., the product is cyclic hemiacetal i.e., linking the carbonyl carbon atom with the other carbon atom by a C-O-C linkage. As a result of cyclisation, carbon one becomes asymmetric resulting in the formation of two isomers. The isomers having the hydroxyl group to the right of the C-1 is designated as α- D glucose and the one having the hydroxyl group on the left of C-1 is designated as β-D Glucose. The α & β sugars are known as anomers i.e., anomers differ only in the configuration around C-1 & this carbon is referred to as anomeric carbon. The six membered ring shown for α & β- D glucose is known as pyran ring because pyran is the name of a heterocyclic compound whose ring consists of five carbons atoms & one oxygen atom. Any CHO containing a six membered ring is called pyranose & its glycosides are called pyranosides. The Haworth formula for α– D glucose shows the C1 -OH group below the plane of the pyranose ring, for β- D glucose C1 alcohol pointing above the plane of pyranose ring. i.e., the α- form has the – OH group on the opposite side from the – CH2OH & β- form has the –OH group on the same side as the CH2OH group. A carbohydrate containing a five membered ring is called furanose because furan is the name of a heterocyclic compound whose ring consists of four carbon & one oxygen atom. The glycosides of furanose are called furanosides. Conformational formula: The sugar ring in the Haworth structure is not planar & therefore it does not represent the correct conformation of monosaccharide. So, conformational formulas are used. There are two types of conformational formula, Chair Conformer: Boat Conformer: The chair conformer is more stable than boat form The stable chair & boat conformational formula of α-D glucose & β- D glucose is ISOMERISM OF MONOSACCHARIDES Existence of different compounds having same molecular formula but different structural arrangement are isomers. Monosaccharides exhibit a variety of isomerism such as Aldose- Ketose isomerism In monosaccharides if compound having group aldehyde called aldose & if having ketone then called ketose. Glucose & fructose both have a formula C6H12O6 but glucose is an aldohexose (aldehyde bearing hexose) & fructose is a Ketohexose (Ketone bearing hexose), so they are isomers to one another (As shown P-6 Fig). Optical isomerism A compound which can rotates the plane polarized light passing through it is said to be optically active. When an optically active substance rotates the plane of polarized light in a clockwise direction, it is called as dextrorotatory or ‘d- isomer’ & when it rotates the plane polarized light in the anticlockwise direction, it is levorotatory or ‘l- isomer’ of the substance. Two compounds having similar formula may have different optical activity. The d & l isomers are also expressed as (+) & (-) respectively. The two forms of glyceraldehyde (d & l forms) rotate the plane of polarized light in the opposite direction by the same amount. As it turns out D- Glyceraldehyde rotates the plane of polarized light to the right & is therefore dextrorotatory & rotates the plane of polarized light to the left & is therefore levorotatory. (pg;2 fig) Stereo isomerism It occurs when the same compound due to different spatial arrangement of the groups attached to its asymmetric carbon atom/chiral carbon (When any carbon atom of a compound is attached with four different groups or atoms, it is called an asymmetric/chiral carbon) exists in more than one form e.g. in glyceraldehyde carbon no. 2 is chiral carbon in both d & l form depending on location of OH group, the D- form will have the –OH group on the right side of the penultimate C- atom (C- atom away from functional group & near to terminal C-atom) , while the L- form will have it on the left side of the penultimate C- atom. Enantiomers: The non-super impossible mirror images of stereo isomers are known as enantiomers e.g. d & l form of glyceraldehyde. Epimers: Two sugars which differ from one another only in the configuration around a single C- atom is said to be epimers e.g. Glucose & Galactose differ in the configuration of a single carbon atom i.e., C-atom 4, while glucose & mannose differ in a C-atom 2(pg;3 fig). Diastereomers: Optical isomers that are not mirror images are called diastereomers. These are structural isomers i.e. They have same molecular formula of C6H12O6 but different structural formula & consequently they differ in their physical & chemical properties. DERIVED MONOSACCHARIDES This group includes compound which are structurally similar to the monosaccharides but deviating from them in some regard. There are two important derivatives 1- Deoxy Sugar: Sugars in which one of the hydroxyl group is replaced by a hydrogen atom are known as deoxy sugars. The common example is deoxy ribose found in nucleosides, nucleotides & nucleic acids (DNA). 2- Amino Sugar: Sugars in which hydroxyl group has been replaced by an amino group are known as amino sugars for example D- Glucosamine, D- galactosamine, Dmannosamine, all of these are found in a wide variety of biological materials PROPERTIES OF MONOSACCHARIDES These are colorless, sweet in taste, solid at room temperature & crystalline compound. They are soluble in water because of hydroxyl group. They are optically active and exhibit mutarotation. D- glucose can be obtained in two forms - α form & β form α form has a melting point 146° C & a specific rotation of +113º β form has a melting point of 150° C & a specific rotation of +19º If either form of these crystalline form is dissolved in water & allowed to stand, the specific rotation of the solution so prepared changes gradually until a final value of +53° is obtained. This type of change in rotation called mutarotation. Ability to exists either a linear or ring structure. In these 5 or 6 carbon compound usually exist in cyclic form, 6 membered ring is called pyranose ring & five membered rings are called furanose ring. These sugars are readily fermented by microorganisms. They prevent the growth of microorganisms in high concentration so they may be used as preservative. As monosaccharides possess hydroxyl groups (-OH), they form esters with acids. E.g., esterification of glucose to Glucose - 6-phosphate. When monosaccharide is added with concentrated alkali, it is burnt and this process is called caramelization. It produces a series of decomposition products. Oligosaccharides: The reducing group of one monosaccharide can connect to one of the hydroxyl groups on another through glycosidic bond, to form disaccharides. More connections of glycosidic bonds will give rise to trisaccharides, tetrasaccharides, depending on numbers of subunits. They can be hydrolyzed by acids or enzymes into their subunits. Disaccharides: These are formed when 2 monosaccharide molecules joined together with the elimination of one molecule of water i.e., these consists of two monosaccharide units/ molecules joined by glycosidic linkage They have the general formula C12H22O11, upon hydrolysis which yield two monosaccharide molecules Ex. of disaccharides are Sucrose, Lactose, Maltose. Trisaccharides: Some trisaccharides are freely available in nature. Trisaccharides are oligosaccharides composed of 3 monosaccharides with two glycosidic bonds connecting them. They are not common in food but are formed during breakdown of starch into simpler sugars. One important member of this group is raffinose, which is made up of the derivative of galactose, glucose & fructose. It is not hydrolyzed by enzymes of digestive tract of human beings, it passes to the colon where it is fermented by bacterial enzymes producing gas. Raffinose occur in small amounts in many foods and in fair amount in beetroot & cotton seed meal. Tetrasaccharides: Stachyose is a tetrasaccharides. It is made up of 2 molecules of galactose, 1 molecule of glucose, 1 molecule of fructose i.e., α- D galactopyranose (1,6), α D-galactopyranose (1,6), α- D glucopyranose (1,2) β D- fructofuranose. i.e., Raffinose & Stachyose are formed by the addition of one & two galactose residue respectively to a sucrose molecule. Polysaccharides: When oligosaccharides have more than ten monosaccharides the unit is known as polysaccharide. Polysaccharides consist of monosaccharide units bound to each other by glycosidic linkages. Their complete acidic hydrolysis yields monosaccharides. Polysaccharides (glycans) can have only a type of sugar structural unit (homoglycans) or several types of sugar units (heteroglycans). Polysaccharides may have a linear pattern (as in cellulose and amylose) or a branched fashion (amylopectin, glycogen, guaran) of monosaccharide unit linkage. They show following properties: They don’t have sweet taste. They are amorphous substances Most of them are insoluble in water but some are soluble like glycogen. They dissolved in boiling water forming collides Starch is insoluble in cold water but in warm water it swells until its gelatinization temperature begins to lose its structure and leaches out its constituents. This act of converting into a substance like jelly is called gelatinization. Glycogen is non-reducing, readily soluble in water & gives red color with iodine. Carbohydrates Carbohydrates Shafqat Rasool Shahzad 7