Peptide backbone atoms. (a) Resonance of the peptide bond gives it a partial double-bond character. (b) The cis and trans isomers of a peptide bond. The bonds in most proteins are trans. The peptide backbone is shaded in orange and the peptide bond is in red. (c) The three bonds that separate sequential a carbons in a polypeptide chain lie in a plane. The N–C_α and C_α–C bonds can rotate, with torsion angles designated φ and ψ.