Amino Acids and Proteins

Peptide Bonds and Amino Acids

• Peptide bonds are formed through a condensation reaction, combining simple compounds to create a complex compound and releasing water.
• Amino acids are joined together, releasing water, to form polypeptides.
• Formation of polypeptides involves removing oxygen from the precedent amino acid and two hydrogens from the subsequent amino acid, then placing the groups next to each other.

Protein Tests

• Biuret Test: general test for proteins, indicating the presence of amide/peptide bonds, resulting in a purple/violet solution.
• Dipeptide and amino acid will have a negative test, while tripeptide, tetrapeptide, and pentapeptide will have a positive test.
• 40 amino acids or more are considered a protein.

Amino Acid Tests

• Ninhydrin Test: tests for the presence of α-amino acid in proteins, resulting in a blue-violet/purple-blue solution.
• Alanine is an amino acid and will have a positive test.
• Egg albumin contains 18 amino acids, except glutamine and asparagine.

Aromatic Amino Acids

• Xanthoproteic Test: tests for the presence of aromatic amino acids, resulting in a yellow/orange-yellow solution.
• Egg albumin contains all aromatic amino acids, including tyrosine and tryptophan.
• Phenylalanine is an aromatic amino acid but does not react with the reagent, resulting in a negative test.

Phenol and Tyrosine

• Millon’s Test: tests for phenolic compounds and detects the presence of tyrosine, the only amino acid containing a phenol group, resulting in a pink/red/brick-red precipitate.

Tryptophan

• Hopkins-Cole Test: tests for the presence of tryptophan, the only amino acid containing an indole group, resulting in a violet-ring at the junction.

Sulfhydryl-Containing Amino Acids

• Lead Sulfide Test: tests for the presence of sulfhydryl-containing amino acids.

Amino Acid Structure

• Amino acids are organic compounds containing an amino group and a carboxyl group.
• The 20 common amino acids used to build proteins are mostly α-amino acids.
• The sequence of the amino acid chain causes the polypeptide to fold into a shape that is biologically active.
• The amino acid sequences of proteins are encoded in the genes.

Protein Classification

• 2-39 amino acids are considered polypeptides.
• Exceeding 40 amino acids, it will be called a protein.
• Alpha amino acid: 19 out of 20 amino acids are alpha amino acids.
• Not an alpha amino acid is an Imino acid; proline.