Conformation of poly(glutamic acid) and of poly(aspartic acid) in the solid state. X-ray diffraction, infra-red and 13C cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopic study

In order to assess the manifestation of possible conformations of poly(amino acids) in 13C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS n.m.r.) spectra, the conformational structures of poly(α-l-glutamic acid) (poly(Glu)), of poly(α-L-aspartic acid) (poly(Asp)) and of irregular poly(α,β,-dl-aspartic acid) in the solid state were studied by X-ray diffraction and infra-red spectroscopy and compared with the results of 13C CP/MAS n.m.r. spectroscopy. None of the methods alone can fully characterize the conformation; by a combination of the results it could be determined that poly(Glu) and poly(Asp) in the free acid form exhibit both α-helical and β-structure, whereas the random-coil structure predominates in their sodium salts. Both the free acid and the sodium salt of poly(α,β-dl-Asp) exist predominantly in the random-coil form. It turns out that the sodium salts are especially suitable for determining the n.m.r. characteristics of the random-coil conformation in the solid state. The dynamic behaviour studied by the spin-lock cross-polarization experiment indicates that the studied poly(amino acids) in the free acid form are much more rigid than, for example, the poly(alkyl methacrylates).