L- amino acids are the building blocks of proteins. There are twenty different L- amino acids, but only one bond, the peptide bond, is used to join them together. Peptide bonds form in the process of translation when the -amino group of one amino acid residue forms a covalent bond with the -carboxyl group of another amino acid residue. Water is eliminated in the process, as shown in Figure 5.8. The structure of the peptide bond is shown in Figure 5.12. Note that the -C=O and the -N-H bonds are nearly parallel and that the C, O, N, and H atoms are usually coplanar. There is little twisting possible around the C-N bond because the peptide bond has a substantial fraction of double-bond character. In fact, the peptide bond can be considered a resonance hybrid of the forms

The group of atoms about the peptide bond can exist in either the trans or cis configurations:

It turns out, however, that the trans configuration is usually favored in order to minimize the steric interaction between bulky R groups on adjacent -carbon atoms. The major exception is bonds in the sequence X-Pro, where X is any amino acid and Pro is proline. In this case, the cis configuration may be favored at times.