POSSIBLE FORMATION OF LEFT-HANDED ALPHA-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. Copyright © 1968 John Wiley & Sons, Inc.