3 - THE TORSION ANGLES ψ AND ϕ

Abstract

This chapter discusses the polypeptide model construction of torsion angles. As the repeating unit of the polypeptide backbone is a three-atom unit, the torsion angles of the two connecting bonds determine the secondary structure. The peptide linkage is virtually planar in most polypeptides and thus, only two torsion angles, called psi (ψ) and phi (φ), are needed to define the secondary structure of polypeptides. Psi is the torsion angle between the α-carbon and the carbon atom of one of the peptide units. Phi is the torsion angle between the α-carbon and the nitrogen atom of the other peptide unit. A simple mnemonic is helpful in remembering which angle applies to which bond: psi is the angle for the bond between the same atoms Cα–C. Phi is the angle for the bond between the two hetero atoms Cα–N.