Figure 1

Proteins are chains of amino acids. A) Structure of a typical amino acid, showing the central C_α atom with a hydrogen atom attached to it, an amino group (NH₂), a carboxyl group (COOH, carbon atom named C') and the side chain (R) that varies among the different amino acids. B) Schematic representation of a polypeptide chain with two amino acids, showing average distances of bonds. The main chain atoms between two C_α (dark gray) atoms are generally fixed in a plane, with the C'-N peptide bond angle (ω) being 0º or 180º (trans, most common, or cis forms), thus only the C_α-C' and N-C_α bonds exhibit rotational mobility. C) Polypeptide chain formed by three glycines showing the bond angles. C atoms in green, O in red, N in blue and H in white. As the C_α is always saturated, the two H linked to it are omitted.