Amino Acid Reactions

This unit covers the various reactions that can occur with amino acids.

Dipolar Ions

In the previous section you learned that amino acids are classified as zwitterions. Depending on the pH, amino acids can exist in a protonated (cation) form or deprotonated (anion) form. The following figure demonstrates this phenomenon:

Amino Acid Zwitterion
By Ben Mills (Own work) [Public domain], via Wikimedia Commons

Sulfur Linkage

Two cysteine molecules are able to link together via a sulfur linkage to form cystine. The following figure demonstrates this reaction:

Sulfur Linkage

Peptide Linkage

Two amino acids can link together via a peptide bond to create a dipeptide. This reaction is classified as dehydration synthesis/condensation in which the two molecules (amino acids) join together and create one molecule of water (H2O) as a byproduct. This anabolic reaction is an essential preliminary step to creating larger polypeptide chains.

The peptide linkage forms when the carboxyl group of one amino acid interacts with the amino group of another. The following figure demonstrates this key reaction.

Peptide Bond
By GYassineMrabetTalk This vector image was created with Inkscape. (Own work) [Public domain], via Wikimedia Commons


This catabolic reaction is the opposite of dehydration synthesis. With the addition of a water molecule, the peptide bond is broken and the amino acids are separated. In biological systems, this reaction is performed by peptidases and proteases. More information about these enzymes will be discussed in future units. The following figure depicts a hydrolysis reaction occurring on a dipeptide:

Peptide Hydrolysis
By Boumphreyfr (Own work) [CC BY-SA 3.0 ( or GFDL (], via Wikimedia Commons


  • Amino Acid Zwitterion
  • Sulfur Linkage
  • Peptide Bond
  • Peptide Hydrolysis
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