Figure 4.9 Rotation about bonds in a polypeptide. The structure of each amino acid in a polypeptide can be adjusted by rotation about two single bonds. (A) Phi (ϕ) is the angle of rotation about the bond between the nitrogen and the α-carbon atoms, whereas psi (c) is the angle of rotation about the bond between the carbonyl carbon and the α-carbon atoms. (B) A view down the bond between the nitrogen and the α-carbon atoms. The angle ϕ is measured as the rotation of the carbonyl carbon attached to the α-carbon atom: positive if to the right, negative if to the left. (C) The angle ϕ is measured by the rotation of the amino group as viewed down the bond from the carbonyl carbon to the α-carbon atom: positive if to the right, negative if to the left. Note that the view shown is the reverse of how the rotation is measured and consequently the angle has a negative value.