Let's see how Organic Chemistry in A-Level Chemistry can help us understand amino acids in biology!
STRUCTURE OF AMINO ACIDS
Amino acids are molecules containing anamine group and a carboxylic acid group. Naturally occurring amino acids have the following general formula:
There is a central carbon atom attached to a hydrogen, a carboxylic acid group, an amine group and an alkyl group. Amino acids are thus all chiral except for glycine, in which the R is another H atom.
Amino acids occur widely in nature and have a number of uses in the human body. Some simple examples are:
ACID-BASE PROPERTIES OF AMINO ACIDS
Carboxylic acids have acidic properties and react with bases. Amines have basic properties and react with acids.
It therefore follows that amino acids have both acidic and basic properties.
They make very effective buffer solutions.
i) reaction with bases
Amino acids react with strong bases such as sodium hydroxide:
In high pH, therefore, amino acids exist in anionic form:
ii) reaction with acids
Amino acids react with strong acids such as hydrochloric acid:
In low pH, therefore, amino acids exist in cationic form:
iii) reaction with itself
Since amino acids have a proton donating group and a proton accepting group on the same molecule, it follows that each molecule can undergo an acid-base reaction with itself:
The double ion that is formed as a result of this reaction is called a Zwitterion.This reaction happens in the solid state. In the solid state, therefore, amino acids are ionic. This explains why they are solids with a high melting point.
CONDENSATION REACTIONS OF AMINO ACIDS
Amines undergo condensation reactions with acyl chlorides and acid anhydrides.
Carboxylic acids are less reactive than acyl chlorides and acid anhydrides but can still undergo a similar reaction with amines under certain conditions inside living organisms.
Since amino acids have both acid and amine groups, two molecules can therefore react with each other to form a molecule containing a peptide or amide link:
This reaction is called a condensation reaction because two amino acid molecules join together and one molecule of water is eliminated.
The resulting molecule is called a dipeptide.
Dipeptides can also be formed by the condensation of two different amino acids. In this case two different molecules can be formed:
Since the resulting dipeptides also have both amine groups and carboxylic acid groups, they can undergo further condensation reactions, eventually forming polymers:
This reaction is carried out alongside DNA molecules in the bodies of living organisms. The DNA lines up the amino acid molecules in the correct order so that the condensation reactions can takeplace. The resulting polymer is called a protein, and is an essential component of living organisms.
Drafted by Eunice (Chemistry)