Amino acids are obtained by hydrolysis of proteins. The amino acids can be
represented by:
The simplest amino acids are:
2-Aminopropanoic acid and all
amino acids except
aminoethanoic acid contain an asymmetric carbon atom .
Since the amino acids contain amino ( -NH2 )and carboxyl groups( -COOH ), they show the
properties of both groups. Amino acids have high melhng points and are usually water-soluble.
Amino acids usually exist as zwitterions (i.e. a species containing both
positive and negative charges):
Amino acids are linked together by peptide linkages:
Amino acid polymers whose relative molecular masses are 10 000 or less are called
polypeptides.Those with larger molecular masses ( 40 000 ) are called proteins, e.g. egg albumen,.
Protein molecules, it is now believed, are constructed from up to 20 different amino acids joined
together by peptide linkages. X-ray analysis has shown that protein chains have a
zig-zag structure with R groups sticking out alternately in different directions.
Different chains are linked by:
 disulphide bridges between two reactive R groups:
 hydrogen bonding.
Proteins can be denatured by heating or altering the pH. If egg white (containing the protein of
albumin) is heated to 60°C, the protein starts to separate out as a white solid. If the pH of milk is
lowered below 4.6, the protein caseinogen starts to precipitate out as a solid.
Denaturing involves the permanent breakdown of the crosslinks between the protein molecules and a
resulting change in physical, chemical and biological properties.
The presence of protein can be shown by warming the suspected protein with dilute sodium
hydroxide solution and copper sulphate solution. A violet coloration confirms the presence of protein.
The particular amino acids present in a protein can be identified by hydrolysis with dilute acid
followed by paper chromatography.
Ninhydrin produces pink/purple spots on the chromatogram, showing where the amino acids are.
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