| Abstract: | 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 ?, namely, ? = 15°–30° for χ1 = 300° and for ? = 225°-230° for ? = 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 ? = 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. |
