Carbohydrates structure

This ppt explains the structure of carbohydrates and its occurrence. It explains the linear chain structure, haworth projection, fischer projection and hemiacetal structure of carbohydrates.

1. Carbohydrates – Structure and Occurrence Dr.N.C.J.Packia Lekshmi ALLIED HEALTH SCIENCES NOORUL ISLAM CENTRE FOR HIGHER EDUCATION

2. Definition A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula Cm(H2O)n. Saccharide , a group that includes sugars, starch, and cellulose. Carbohydrates are the sugars, starches and fibers found in fruits, grains, vegetables and milk products. carbohydrates — one of the basic food groups — are important to a healthy diet.

3. STRUCTURE AND OCCURRENCE Carbohydrates consist of carbon, hydrogen, and oxygen. The general empirical structure for carbohydrates is (CH2O)n. They are organic compounds organized in the form of aldehydes or ketones with multiple hydroxyl groups coming off the carbon chain. The building blocks of all carbohydrates are simple sugars called monosaccharides . A monosaccharide can be a polyhydroxy aldehyde ( aldose ) or a polyhydroxy ketone ( ketose ).

4. The carbohydrates can be structurally represented in any of the three forms: Open chain structure/ fischer projection - It is the long straight-chain form of carbohydrates The L and D confirmations represent the absolute configuration of the asymmetric carbon farthest away from the ketone or aldehyde group on the monosaccharide. On the Fischer projection, if the farthest hydroxyl(-OH) group is on the right, then it is classified as D sugar, if the hydroxyl group is on the left, then it is a L sugar. Haworth structure - It is the presence of the pyranose ring structure Hemi- acetal structure - Here the 1st carbon of the glucose condenses with the -OH group of the 5th carbon to form a ring structure

5. Fischer Projection German chemist Emil Fischer in 1891 In a Fischer projection the carbohydrate is shown in its open chain form, rather than a cyclical one. Carbon atoms in the main chain of the carbohydrate molecule are connected vertically, whilst hydrogen atoms and hydroxyl groups are bonded horizontally. D-Glucose

6. Fischer projections have one main advantage: it is easy to visually identify the stereochemical properties of a carbohydrate and compare the difference between two carbohydrates quickly and easily. For example, it is simple to tell the difference between two enantiomers

7. Carbohydrates are classified as diastereomers if their chiral carbons are connected to the exactly the same substrates but connected at differing configurations (R or S). Unlike an enantiomer , diastereomers are NOT object and mirror image. An example of two carbohydrates that are diastereoisomers are D-Glucose and D- Altrose as seen in the figure to the left.

8. Epimers are two diastereomers that differ only at one stereocenter . As shown in the figure, D-Glucose and D-Mannose are an example of an epimer .

9. Haworth Projection A Haworth projection differs from a Fischer projection in that it is used to represent the carbohydrate in its cyclical form. This is especially useful for sugars which have a ring structure. It was devised by the English chemist Sir Norman Haworth who expanded on the work of Fischer, characterizing many more carbohydrates.

10. A Haworth projection is a simple way to show cyclic sugars and their glycosidic linkages. If drawn from a Fischer projection with the carbonyl on top, the groups on the right side become the groups on the bottom of the ring and the groups on the left become the groups on the top. The carbon at the very bottom of the Fischer projection is placed on top of the ring by default (if it is not a part of the ring itself). An α-linkage occurs when the hydroxyl of the hemiacetal is on the bottom of the right and a β-linkage occurs when the hydroxyl is on top.

11. Hemi- acetal structure In hetercyclic compounds, five- membered and six- membered rings are favored over others due less steric strain of the cyclic structure. Thus, furanoses ( pentoses ) and pyranoses ( hexoses ) rings are the predominant cyclic structures in monosaccharides . The rings of monosaccarides resemble the molecules furan and pyran , thus the names furanoses and pyranoses

12. Straight-chain monosaccharide, aldehyde or ketone group react with other hydroxyl group, for which oxygen is bonded between the two carbon atom, to form a hemiacetal or hemiketal , which forms a heterocyclic ring; the five-atom furanose and six-atom pyranose forms are in equilibrium with their straight-chain structures.

13. The pyranose ring can take on different conformations. Either of the chair or boat form. The chair form allows for substituents to be in two positions, either axial or equatorial. Substituents in the axial position create steric hindrance, thus larger substituents normally are in the equatorial position, due to less crowding. The boat form is disfavored due to steric hindrances. The furanose ring can take on the form of a envelope. It has four nearly coplanar atoms with a puckered fifth atom.

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