Amino Acids Chemistry Tutorial

Key Concepts

• 20 amino acids are commonly found in proteins.

• The 11 amino acids that the human body makes are called non-essential amino acids.
• The 9 amino acids that the human body needs but cannot make are called essential amino acids. These must be eaten.
• Amino acids contain both an amine (-NH₂) and a carboxylic acid (-COOH) functional group.
• α-amino acids, or 2-amino acids, are amino acids in which both functional groups are attached to the same carbon atom.
• The general formula for a 2-amino acid (α-amino acid) is:

  H2N -

  R | C | H

  - COOH

  
  Where R is a carbon containing side chain or branch.
  This carbon side chain may also contain sulfur, nitrogen or oxygen.
• Amine groups are basic (proton acceptor), carboxylic acid groups are acidic (proton donator).
• Amino acids are both amphiprotic (can accept or donate protons) and amphoteric (can react with acids or bases).
• A zwitterion is the representation of the dipolar amino acid.
• For each amino acid there is a particular pH, the isoelectric point, at which the amino acid exists as the neutral zwitter ion.

  ⚛ Below this pH the amino acid exists as a positively charged ion (cation).
  ⚛ Above this pH the amino acid exists as a negatively charged ion (anion).

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Structure of an Amino Acid

If R = CH₃ (methyl group) the amino acid is alanine (symbol Ala)

Serine (Ser) is an amino acid in which R contains an alcohol (hydroxyl or OH) functional group.

Aspartic acid (Asp) contains a carboxylic acid functional group.

Cysteine (Cys) contains sulfur.

Zwitterions

Amino acids in solution exist as an equilibrium mixture of neutral molecules and dipolar ions called zwitterions:

H2N - R | C | H - COOH neutral molecule

+H3N - R | C | H - COO- zwitterion

In acid solution the zwitterion converts to a positively charged ion:

+H3N - R | C | H - COO- zwitterion

+ H3O+

+H3N - R | C | H - COOH positive ion

+ H2O

By Le Chatelier's Principle, the more acidic the solution, the more positive ions will be formed.

In alkaline solution the zwitterion converts to a negatively charged ion:

+H3N - R | C | H - COO- zwitterion

+ OH-

H2N - R | C | H - COO- negative ion

+ H2O

By Le Chatelier's Principle, the more alkaline the solution, the more negative ions will be formed.