Page 59 - Biology_F5

Biology for Advanced Level Secondary Schools

Properties of amino acids ion and it becomes positively charged. Each
They are colourless, crystalline solids which amino acid has its own pH value at which it
are soluble in water but insoluble in organic exists in its neutral zwitterionic form. The
solvent such as ether, chloroform, and pH at which the amino acids are electrically
acetone. They are amphoteric compounds, neutral is termed as an Isoelectric point
as they have both acidic and basic properties. (I.E.P). In the alkaline medium, when the
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In neutral aqueous solutions, they exist as pH of the I.E.P increases, the amino group
dipolar or zwitterions. On one side, the dissociates, releasing hydrogen ions, (H ),
+
acidic carboxyl group has a tendency of thus, the amino acid becomes negatively
donating hydrogen ion (proton); therefore, charged. In the acidic medium, (when the
it dissociates to release hydrogen ion and pH is lowered), the carboxyl group accepts
becomes negatively charged. On the other hydrogen ions, and the whole structure
side, the basic amino group has a high affinity becomes positively charged (Figure 1.32).
to hydrogen; therefore, it accepts hydrogen

Figure 1.32: Structure of the zwitterion
Figure 1.33 Structure of the zwitterion

Formation of proteins Further combination of this type extends
All proteins are formed by condensation the length of the chain to form a polypeptide
of amino acids to give peptide chains. The chain, which usually contains hundreds of

condensation reaction occurs between amino acids. Polypeptides may be linked

the amino group of one amino acid and by other forces such as disulphide bridges,
Figure 1.33 Structure of the zwitterion
the carboxylic group of other amino acid hydrogen bond, hydrophobic interaction
to form a dipeptide molecule linked by and ionic bonds.
peptide bond (Figure 1.33).

Figure 1.34 Formation of dipeptide molecule linked by the peptide bond

Figure 1.34 Formation of dipeptide molecule linked by the peptide bond
Figure 1.33: Formation of dipeptide molecule linked by the peptide bond
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