Deuterium NMR study of the structure and dynamics of the side chains of several solid polyglutamates

A ²H NMR study of polycrystalline powders and oriented films of several selectively deuterium-labeled poly(γ-benzyl L-glutamate) (PBLG) and poly(γ-ethyl L-glutamate) (PELG) homopolymers is reported. The deuterium labels on the ester side chain of PBLG indicate the presence of two polymer species below 330 K, one with benzene rings flipping rapidly about their para axis and the other with benzene rings static on the ²H NMR time scale. The relative contribution of the "mobile" population increases from about half (55%) at 240 K to nearly total (96%) at 314 K. A phase transition is detected at 330 K. Above this transition temperature, we detect an additional motion consisting of three site jumps of the benzyl moiety among unevenly populated sites about the O^ε-C^ζ bond. Thus the methylene deuterons experience, on the whole, nonaxial symmetry about this bond. At 395 K this motion attains its motional-narrowing limit for all the deuterons that it affects, whereas at 340 K, only 25% of the benzyl moieties are rapidly reorienting, with 75% reorienting at a slower rate. With powders of PBLG labeled selectively at the γ-carbon position, onset of motion at the phase transition is observed. The spectral line shape indicates that, above 330 K, the γ deuteriums experience moderate (at 340 K) to rapid (at 395 K) jumps about the C^β-C^γ bond among three unequally populated sites. The phase transition observed at 330 K, associated with the side-chain melting phenomenon, is likely to be accompanied by rotational or translational diffusion of the helices while they maintain their axial orientation. Furthermore, a comparison with our conformational analysis suggests a high degree of cooperativity between the adjacent helices. At the phase transition, these nearest-neighbor interactions would be reduced, and the NMR results are consistent with computed 3-fold rotational potentials for side-chain motion around the C^β-C^γ and O^ε-C^ζ bonds, predicted from the conformational analysis of the side chains attached to the α-helix. With powders and films of PELG, the dominant motion at low temperatures (besides rapid methyl rotation) can be interpreted either as a discrete jump rotation with 3-fold symmetry but about a diffusion axis that does not coincide with the O^ε-C^ζ bond, or in terms of a three-site jump motion about this bond among unevenly populated sites. Motional averaging becomes very effective and isotropic in nature at about room temperature. This abrupt change from anisotropic to isotropic motion is indicative of a phase transition. Results obtained with oriented films indicate that the PBLG benzene rings orient preferentially parallel to the axes of the polymer helices, consistent with theoretical predictions, whereas the PELG ethyl groups are distributed randomly in space.