Conformational and packing modeling of optically active polyesters. 2. Helical structure of an isotactic polylactone

Optically active poly(alpha-methyl-alpha-n-propyl-beta-propiolactone) (PMPP L) crystallizes in a 2(1) helical conformation, but its conformational stru cture and packing symmetry have not been solved and refined yet. On the bas is of the rotational isomeric state approximation and the conformational al gorithm for polymer helices, optimum conformational models derived from sin gle helices were used as a starting point in building crystal structures. B oth intra- and intermolecular interactions were simultaneously optimized. S emiempirical molecular mechanics calculations of an isotactic single chain having fixed experimental helical parameters revealed that the preferred co nformation for PMPPL entails the ttg(-)g(-) backbone chain conformation wit h a g-t side-chain orientation. Crystal models compatible with the observed orthorhombic lattice dimensions (a = 10.6, b = 11.1 Angstrom) were built a nd refined against electron diffraction data, X-ray powder diffraction spec tra, and structure factor calculations. The favored structure contains two 2-fold screw helices packed antiparallel in an orthorhombic lattice with sp ace group P2(1)2(1)2(1)-D-2(4). With the ttg(-)g(-) main chain conformation , both the g-t and tt conformations are possible for the side chain accordi ng to the refinement of X-ray structure factors. However, the g-t side-chai n conformation shows a better fit than that of the tt conformation with the electron diffraction patterns. The flexible procedure of energy minimizati on yields distinct values of fiber period for the g-t and tt conformations, e.g., 6.31 and 6.14 Angstrom, respectively. The final discrepancy R factor s are 0.20 for the g-t and 0.17 for the tt side-chain models when compared to X-ray data and 0.17 for the g-t and 0.27 for the tt side-chain models wh en compared to electron diffraction data.