X-Ray Scattering of Synthetic Poly(α-amino acid)s in the Solid State. IV. Transition of Poly(γ-methyl D-glutamate) in the β-Form

Abstract

ABSTRACT:The temperature dependence of two Bragg reflections was measured for solid poly(r-methyl o-glutamate) in the p-form in the temperature range from -80 to 120°C. The spacing-temperature curve has a break point in the vicinity of 20°C. From the temperature dependence of integrated intensity, the break point observed is considered to be related to the onset of side-chain motions and to be a transition analogous to the glass transition.KEY WORDS X-Ray / Poly(r-methyl o-glutamate) / p-Form / Transition / Structures and properties of poly(a-amino acid)s in the solid state have been studied extensively. While most previous efforts have been focussed on the a-helical form, little work has been done with the p-form.Molecular motions of a-helical poly(a-amino acid)s in the solid state have been accounted for wholly in terms of side-chain motions. It was suggested that poly(r-benzyl L-glutamate) (PBLG) and poly(r-methyl L-glutamate) (PMLG), whose side chains are rather flexible, manifest a glass transition at room temperature, due to the onset of the side-chain motions. 1 -5 We also obtained the same conclusion from X-ray studies. 6 The molecular motions of ,8-forming poly(aamino acid)s in the solid state have been investigated on poly(S-benzyloxycarbonyl-L-cysteine), poly(S-benzyloxycarbonylethyl-L-cysteine), and poly(S-methoxycarbonylethyl-L-cysteine) using NMR and dielectric methods. 7 All results obtained were interpreted in terms of the rotation of the side chains.It is, of interest therefore to examine whether a transition related to the onset of the side-chain motions occurs even in the p-form, and what that transition is, if it occurs. The purpose of * Address of person to whom correspondence should be directed. this work is to obtain a more detailed understanding about the relation between the structure and the thermal motion of solid poly(a-amino acid) in the ,8-form. EXPERIMENT ALPoly(r-methyl o-glutamate) (PBDG) used in this work was kindly supplied Ajinomoto Co., Ltd. The film was obtained by casting trifluoroacetic acid solution of the polymer on a glass plate. The film in the p-form was prepared by allowing it to swell in formic acid at room temperature for two weeks and then annealing it in vacuo at a temperature of 160°C for an hour. 8 X-ray diffractometric measurements were performed using a Rigaku Denki diffractometer equipped with a scintillation counter and a pulse height analyzer. Cu-Ka radiation at 40 kV and 20 mA was used. A symmetrical reflection (SR) technique and a symmetrical transmission (ST) technique were employed to measure two strong Bragg reflections of ( 100) and (020). 9 Intensities were counted in the step-scan mode for 40 sec. at intervals of 0.05° in Bragg angle. The peak position was defined as the center of the halfwidth. Only the relative change in the peak position with temperature was measured. The 391