CHAPTER 19: Unit 7. Amino Acids and Proteins Chemical Reactions

The main chemical reactions of the amino acids:

1. Zwitterion formation:

Amino acids can donate its proton (acidic) of the carboxylic group (negatively charged) internally to amino group (basic) (positively charged). The formed compound is neutral.

Reference:https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Proteins/Peptides_and_Proteins/Proteins_and_Amino_Acids

Two forms of the zwitterion are present depending on the pH of the solution.

At low pH (acidic medium), a positive cation is formed.At higher pH (basic medium) a negative anion is formed.

Reference: https://www.aqion.de/site/zwitterions

The Isoelectric Point pI is the pH of an aqueous solution of an amino acid at which the molecules on average have no net charge. All positively charged groups will equal all negatively charged groups.

2. Amino Acids Esterification:

Reference:https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Proteins/Peptides_and_Proteins/Proteins_and_Amino_Acids

3. Amine Acetylation (Reaction with Phenylacetyl chloride):

Amino acid + phenylacetyl chloride (in basic medium)  -> Amide

Reference:https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Proteins/Peptides_and_Proteins/Proteins_and_Amino_Acids

4. Ninhydrin Reaction:

Amino acid + ninhydrin   ->  imino derivative of amino acid

This reaction is used to positively identifying amino acidsNinhydrin is a compound used in the thin layer chromatography (TLC) to identify organic compounds.The IUPAC name of ninhydrin is triketohydrindene.

Reference:https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Proteins/Peptides_and_Proteins/Proteins_and_Amino_Acids

5.  Oxidation:

Amino acid  +   [O]  [in the presence of S-S bond]  -> amino acid with S-S bond introduced

Reference:https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Proteins/Peptides_and_Proteins/Proteins_and_Amino_Acids

6. Protein Hydrolysis

Amino acids with several peptide bonds + H2O  (in acidic medium HCl)  -> amino acids (the number of amino acids equal to the number of peptide bonds)

Reference: https://www.chemguide.co.uk/organicprops/aminoacids/proteinhydrolysis.html

The video below illustrates the hydrolysis of protein: