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Renin Angiotensin Mechanism.

Physical Properties of Carbohydrates.

 

  • Definition:

    • Carbohydrates are hydrates of carbon, which on hydrolysis produce polyhydroxy aldehydes or polyhydroxy ketones.

OR

  • Carbohydrates are the derivatives of the polyhydroxy aldehydes or ketones or the products derived from them.

Physical Properties of Carbohydrates:

Asymmetric carbon atom

  • A carbon atom to which four different atoms or groups of atoms are attached is said to be an asymmetric carbon atom.

  • Many biochemicals contain two or more asymmetric carbon atoms.

  • The presence of asymmetric carbon atom allows formation of isomers. 

  • The compounds which have the same structural formula, but differ only in spatial configuration are called stereo-isomers or geometric isomers. 

  • More the number of asymmetric carbon atoms, more are the number of isomers; 

  • e.g. glucose with four asymmetric carbon atoms has 2"= 2 = 16 isomers; n indicates the number of asymmetric carbon atoms.

  • If the molecule contains one double bond with two different atoms or groups of atoms linked with every carbon, then there exists cis-trans isomerism. 

  • Since the double bond is rigid, the atoms attached to it are not free to rotate like those attached to a single bond. 

  • The example of cis-maleic acid and trans-fumaric acid on a planar form are show here. These isomers have different chemical and physiological properties; e.g. fumaric acid is physiologically active.

 

Classification of Carbohydrates Isomers:

  • Isomers of carbohydrates are classified as follows:

    • D and L isomers.

    • Optical isomers.

    • Epimers.

    • Anomers.

  • D and L isomers: 

    • When the -OH group around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in case of glucose) is on the right side, the sugar belongs to the D-series. 

    • If the hydroxyl group is on the left side, it is a member of the L-series. 

    • Majority of monosaccharides in mammals are of D configuration.

  • Optical isomers: 

    • When a beam of polarized light is passed through a solution exhibiting optical activity, it will be rotated to the right or left in accordance with the presence of the optical isomer. 

    • A compound which causes rotation of polarized light to the right is said to be dextro-rotatory and is designated with a plus (+) sign. 

    • Rotation of the beam to the left is termed as laevo-rotatory and is designated by the minus (-) sign.

    • Stereoisomerism and optical isomerism are independent properties.

  • Epimers: 

    • Isomers formed as a result of interchange of -OH and -H on carbon atoms 2, 3 and 4 of glucose are known as epimers. 

    • In the body, epimerization takes place by the enzyme epimerase.

  • Anomers:

    • The cyclic structure of glucose is retained in solution, but isomerism takes place about position 1. 

    • This is accomplished by optical rotation (mutarotation) by which the positions of -H and -OH groups are changed around carbon 1. 

    • Inter-conversion of A and B glucose in solution with change of optical activity is called mutarotation. 

    • It is explained by the A and B form of glucose requiring the presence of asymmetric carbon C.

Commonly Asked Questions.

  1. Define carbohydrates and write a note on different Isomers of Carbohydrates.
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