Amino Acids

• Alpha-Amino acids contain both the NH₂ (amino) group and the COOH (carboxylic acid) group attached to the same carbon atom.
• Proteins are made from amino acids and so they are very important in living organisms.
• In addition to forming proteins, amino acids also have many other uses, for example, the neurotransmitter GABA (gamma-aminobutanoic acid or 4-aminobutanoic acid).
  H₂N-CH₂-CH₂-CH₂-COOH
  
• Amino acids have two functional groups; they are therefore examples of bifunctional compounds.
• Bifunctional compounds often share the same properties of the two functional groups.
• However, this is not the case with amino acids, as both of their functional groups interact.
• The COOH group acts as an acid (donating protons) and the NH₂ group acts as a base (accepting protons); therefore these two groups can react with each other forming zwitterions (compounds containing both anionic and cationic groups):
• An aqueous solution of an amino acid is formed mainly of zwitterions (solution is neutral, unless there is an extra COOH or NH₂ on the R group); this gives the solution certain characteristics that are common of ionic compounds:
  • They are soluble in water.
  • They have a higher melting and boiling point than expected.
• When they dissolve in water, the NH₂ group accepts a proton and the COOH group donates one to another water molecule.
  H₂O + H₂N—CH(CH₃)—COOH + H₂O HO^- + ⁺H₃N—CH(CH₂)—COO^- + H₃O^-
  The OH^- and H₃O⁺ ions then go on to react with each other, to restore the equilibrium within water:
  OH^-_(aq) + H₃O⁺_(aq) 2H₂O_(l)
  
• This maintains that the overall solution is neutral.
• Zwitterions act as good buffers; addition of small quantities of acid or alkali to the solution has little affect upon the pH of the solution:
  OH^-_(aq) + ⁺H₃N—CH(R)—COO^- _(aq) H₂O_(l)+ H₂N—CH(R)-COO^-_(aq)
  
  H₃O⁺_(aq) + ⁺H₃N—CH(R)—COO^-_(aq) H₂O_(l) + ⁺H₃N—CH(R)-COOH_(aq)
  

Naming an Amino Acid Sequence

• Amino acid sequences are named in the direction with the free NH₂ group on the left; for example:

Useful books for revision