Possible formation of the left-handed α-helix of poly-L-serine

A conformational study of poly-L-serine has shown that it can exist in the left-handed α-helical form. A study of a pair of peptide units with the serine sidegroup attached to the α carbon atom linking the two units showed that OH ⃛O hydrogen bonds between the OH group of the side chain and a carbonyl oxygen of the first peptide group in the backbone can occur in two regions of ϕ, namely, ϕ = 15°-30° for χ₁ = 300° and for ϕ = 225°-230° for ϕ = 60°. The latter is close to a possible left-handed helix of poly-L-serine, stabilized by NH ⃛O hydrogen bonds. From a study of contact criteria, the best conformation for this helix is found to be ϕ = 227°, Ψ = 238°, χ₁ = 65° which has n = 3.65, h = 1.51 A. The NH ⃛O hydrogen bond has a length of 2.90 A. (6°) and the OH ⃛O hydrogen bond is of length 2.60 A. (0°). There are no other bad short contacts in the structure. The cylindrical coordinates of the atoms, as well as a perspective view of the structure arc given in this paper.