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Chemistry LibreTexts

Structure of Amino Acids

Key structural features

  • Central carbon (known as the "alpha carbon") is tetrahedral and chiral (i.e. each of the four functional groups are different)
  • The "R" functional group is known as the amino acid "side chain". There are 20 common different amino acid side chains. This is what distinguishes one amino acid from another. Amino acids are also known as "residues".
  • There is an amino group and carboxylic acid group. At pH 7.0 these are both (oppositely) charged. Amino acids are therefore also zwitterions



Amino acid monomers are chemically linked to form linear polymers known as proteins:


Note: this is drawn "flat" for clarity, but the Ca are still tetrahedral (the H atom on the Ca is also not shown in this diagram)

Key structural features:

  • A "peptide bond" is formed by a condensation reaction between the carboxylic acid of one amino acid with the amino group of the next amino acid
  • The amino acid R1, at the "amino terminus" of the polymer is the "first" amino acid. The residue (R3 in the above diagram) at the carboxyl terminal is known as the "last" amino acid. These termini define the directionality of the protein.