X-Ray Scattering of Synthetic Poly(α-amino acid)s in the Solid State. III. Temperature Dependence of the 1.5 Å-Meridional Reflection of the α-Helix

Spacing of the 1.5 Å-meridional reflection characteristic of the α-helical conformation and corresponding integrated intensity for poly(γ-benzyl D-glutamate), poly(γ-methyl D-glutamate), copoly(γ-benzyl L-glutamate, γ-methyl L-glutamate), and the racemic form of poly(γ-benzyl glutamate) were measured as a function of temperature from −90 to 120°C. Except for racemic poly(γ-benzyl glutamate), it was found that the spacing—temperature curve shows a discontinuity in slope in the vicinity of room temperature, which has been attributed to the onset of side-chain motion. This suggests that the transition is a crystalline phase transition of the second order. The linear thermal expansion coefficients of the 1.5 Å spacings for these polymers all were in order of magnitude of 10−5/°C. It has been suggested that anharmonic vibration is affected by the nature of the side chain and by the side-chain motion. The intensity was found to decrease with increasing temperature, which was interpreted in terms of a Debye—Waller temperature factor for helical backbone atoms. From the temperature dependence of the 1.5 Å-reflection intensity, Young’s modulus of the helix in the direction of the helical axis was estimated. Both the spacing and the intensity of racemic poly(γ-benzyl glutamate) were found to show a small but definite sudden change in the vicinity of 80°C. These indicate that the first-order phase transition at 80°C is accompanied by not only the breakdown of regular stacks of benzyl groups but also a sudden change in the mobility of the α-helical backbone.

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