Self-assembly and molecular dynamics of copolymers of γ-methyl-L-glutamate and stearyl-L-glutamate

We report on the self-assembly and molecular dynamics in a series of poly(gamma-methyl-L-glutamate-co-gamma-stearyl-L-glutamate) [poly(MLG-co-SLG)] using thermodynamic (differential scanning calorimetry), structural (wide-angle X-ray scattering and C-13 solid-state NMR), as well as dynamic (dielectric spectroscopy and site-specific solid-state NMR) techniques. Stearyl crystallization takes place in the copolymers with W-PSLG >= 18% which reflects changes of the hydrocarbon side chains conformation. Extruded fibers of the copolymers are composed of both alpha-helical and beta-sheet secondary structures oriented along the fiber axis with a bottleneck structure with MLG/SLG sequences forming the neck and more distant SLG sequences forming the bottle. This supramolecular organization has direct consequences on the dynamics. Two dielectrically active processes with strong temperature dependences were found. Site-specific NMR allowed the identification of the geometry of motions. One is associated with small-amplitude motions of amorphous segments along the polypeptide backbone and at the chain ends that freeze at the liquid-to-glass transition. The slower process is associated with the relaxation of the alpha-helical peptide secondary structure. The analysis of the dielectric strength associated with the slower process revealed that helices are objects of low persistence.