The 20 amino acids specified by the genetic code are linked into proteins by the peptide bond, as illustrated here by the dipeptide, alanylserine. Complicated names are avoided by using the three-letter (Ala-Ser) or one-letter (A-S) abbreviations, starting with the N-terminal residue, which is at the left.
the peptide bond. The partial double bond of the peptide bond prevents free rotation around the bond. The atoms are now colored by their formal charge to represent where the charge is delocalized. As a result, the six atoms that make up the peptide group lie in the same plane.
Ball-and-stick model of Ala-Ser. The α carbons are colored dark gray. Can you identify the six atoms in the peptide group?
atoms of the peptide group green. Take a moment to convince yourself that the peptide group in Ala-Ser is planar by looking at the molecule from different directions.
A tripeptide has two peptide bonds: and .
The polypeptide chain of this tripeptide is almost fully extended, which places the side chains on alternate sides of the backbone.
entire peptide backbone. The α carbons are dark gray.
the β carbons, which are carbon atoms that belong to the R-groups and are connected to the backbone via the α carbons. The β carbons are shown here in green.