| Abstract: | A unique crystal structure of the α-helical form of poly(γ-methyl L -glutamate) appearing above 170°C is analyzed by x-ray diffraction. At room temperature, the unit cell determined for a well-annealed sample is hexagonal with dimension a = 11.82 Å and contains one α helix with pitch P = 5.42 Å and unit height p = 1.493 Å. This form is reversibly transformed at about 170°C into a modification characterized by a trigonal unit cell (a = 21.27 Å) having three times the cross section of the single-chain unit cell and containing three α helices with P = 5.50 Å and p = 1.504 Å. In a narrow temperature range below 180°C, the three chains in the unit cell are not interrelated by any crystallographic symmetry element, but they are reasonably associated with one another so that all helices are surrounded by other helices in the same way. The axial and azimuthal displacements between neighboring chains are expressed by z = ±4p/3 and ? = ± (2π/3)(4p/P ? 1), respectively. The segmental main-chain motion, which becomes appreciable above 150°C, is composed of screw-type motion along the α-helix fold and correlated P/3 axial jumps. The relation between the structural transition and thermal motion is discussed. |
