JEE Syllabus
Amino acids and peptides:
General structure (only primary structure for peptides) and
physical properties.
-----------------
Main Topics Covered in the TMH Book
AMINO ACIDS
PEPTIDE LINKAGE
---------------
amino acids are organic compounds containing both an amino group (NH2) and carboxylic group (COOH). They are represented by the general formula:
R
|
C-COOH
|
NH2
Essential amino acids
These amino acids are required by the body but are not made by our bodies. hence they must be supplied in diets.
Valine
Leucine
Isoleucine
Phenylalanine
Methionine
Trptophan
threonine
Lysine
Agrinine
L-Family of Amino Acids
With the exception of glycine, all other α amino acids have chiral carbon atom and have two optically active isomers.
However all naturally occurring amino acids belong to L series which have -NH2 group on the left.
Structure of Amino acids
amino acids exist as dipolar ion called a zwitter ion.
i) when an acid is added to an amino acid-COO- accepts this proton and therefore the basic character is due to -COO-group.
ii) When an alkali is added to amino acid -NH3+ group releases the proton and therefore acidic character is due to -NH3+ group.
Peptide
peptides are compounds formed by the condensation of two or more same or different α amino acids. The condensation occurs between amino acids with the elimination of water. In this case,the carboxyl group of one amino acid and amine group of another amino acid gets condensed with the elimination of water molecule. The resulting C-N linkage is called peptide linkage. The bond of C with O becomes a double bond.
The formation of peptide can be between two amino acid molecules or the number can go on until a single molecule containing several hundred thousands of aminoacids is formed.
Proteins
Structurally, proteins are long polymers of amino acids linked by peptide bonds
There is a question is about optical activity of amino acids in TMH JEE book. Which amino acid does not exhibity optical activity?
I did a google search on optical activity of organic chemical compounds and I got the following entries.
http://en.wikibooks.org/wiki/Organic_Chemistry/Chirality/Optical_activity
http://www.chemguide.co.uk/organicprops/aminoacids/background.html
Optical activity
If you look yet again at the general formula for an amino acid, you will see that (apart from glycine, 2-aminoethanoic acid) the carbon at the centre of the structure has four different groups attached. In glycine, the "R" group is another hydrogen atom.
This is equally true if you draw the structure of the zwitterion instead of this simpler structure.
Because of these four different groups attached to the same carbon atom, amino acids (apart from glycine) are chiral.
Hence the amino acid Glycine does not exhibit optical acivity. This is the answer to the question.
------------------
A set of questions for practice on this chapter posted in
www.iit-jee-chemistry-ps.blogspot.com
No comments:
Post a Comment