The polyproline II (PPII) regions of proteins are important for
protein-protein and protein-nucleic acid interactions. In addition, the
individual strands of collagen, the most abundant protein in mammals, exist in
the PPII helical conformation. However, unlike α-helices and β-sheets, the
origin of the PPII helical stability is not clearly understood. Herein, we
discovered several backbone noncovalent interactions that can explain the
stabilization of the PPII helical conformation. At the centre of the cover
art, these interactions are highlighted in a PPII helix. One of these
interactions is the well-studied carbonyl-carbonyl (CO CO) interaction. In the
PPII helices, the CO··· CO interactions form a unique pattern with an almost
linear Oi···Ci+1···Oi+2 angle among three
neighboring CO groups, which could be an important structural parameter when
considered together with the φ and ψ torsion angles to identify and assign the
PPII helical regions of proteins.
