Theoretical conformation of proline oligomers

Minimum energy (vacuum) conformations of proline oligomers and poly-L-proline have been calculated. The left-hand helix of trans-polyproline II becomes stable at the tetramer, whereas the right-hand helix of cis-polyproline I is not established until at least the pentamer. The potential minima include values of ψ (ψ = 163° trans, ψ = 56° cis) which yield forms of the polymer that are virtually identical with polyproline I and II in the solid state. Calculated repeat distances and residues per turn for the two forms are 1.90 Å and 3.18 (pp I) and 3.11 Å and 3.00 (pp II), compared with experimental values of 1.90 Å and 3.33 (pp I) and 3.12 Å and 3.00 (pp II). A third form, not previously described, is found to be almost equally energetically favorable with the trans form of polyproline II. This form, herein called polyproline III, is a trans left-hand helix with alternating rotation angles of 325° and 170°, and 3.35 residues per turn, which forms a right-hand superhelix. Using collagen II proline coordinates, this superhelix is found to have a translation of 8.87 Å with 8.5 residues per turn.