They represent the union of two or more amino acids through a connection between the original and the peptides. In products and in proteins, these bonds are related to peptide bonds and the result of the reaction of the carboxyl group, with the amino group of another, with the elimination of a water molecule.
The peptide bond (-CO-NH-) is usually represented as a single bond. Without however, has a number of characteristics that approximate as a double bond. Since nitrogen is less electronegative than oxygen, the CO bond has 60% double bond character while the CN bond is 40%. Therefore, the CO and NC bonds of the peptide bond have intermediate characteristics between the single bond and the double bond. In fact, the interatomic distances measured in the CO and CN bonds are intermediaries between the single connections and the double bond. This atomic arrangement is stabilized by resonance, so that the six atoms involved in the formation of the peptide bond are contained in the same plane.
Another important consequence of resonance is that it increases the polarity of the peptide bond and establishes a dipole moment (right figure in the table above). For this reason, each peptide bond can participate in two hydrogen bonds. In one of them, the -NH- group acts as a hydrogen donor and in the other, the -CO- group acts as a hydrogen receptor. This property contributes significantly to the three-dimensional folding of proteins, as we will see later.
The partial nature of the double bond prevents free rotation of the bond that joins the C and N atoms in the peptide bond. This stiffness of the double bond limits the conformational possibilities of the peptides. There are two possible configurations:
- Cis configuration: the two Ca's are on the same side of the double bond.
- Trans configuration: the two Ca's are on a different side of the double bond.
It should be noted that, in general, the peptide bond is represented as a single bond. In any case, it has several characteristics that make it close to a double bond. That is why specialists generally mention that a peptide bond has properties that place it halfway between a single bond and a double bond.
