Self-assembly and dynamics of poly(γ-benzyl-L-glutamate) peptides
The structure and the associated dynamics have been investigated in a series of oligopeptides of γ-benzyl-L-glutamate using DSC, WAXS, FTIR, NMR and dielectric spectroscopy, and rheology, respectively. The peptides with degrees of polymerization below 18 are mixtures of a lamellar assembly of β sheets and of columnar hexagonal arrangement of α helices, whereas for longer chains, the intramolecular hydrogen bonds stabilize only the α-helical conformations. Multiple dielectrically active processes were found. Starting from low temperatures, the two Arrhenius processes (γ and β), with apparent activation energies of 20.6 and 50.2 kJ/mol, respectively, associate with the local relaxation of the side-chain methylene units (γ process) and with more cooperative motions of the side chain dipoles sensitive to the 7/2 helical packing (β process). The glass transition is manifested in the thermal properties with a step in the heat capacity and with an intense dielectric process bearing characteristics (molecular weight dependence, temperature dependence of relaxation times) known from amorphous polymers. Based on these findings, the α process is attributed to the relaxation of amorphous segments located between and at the end of helically ordered segments. Two slower processes were identified with opposite molecular weight dependence. The weak intermediate mode with an M2 molecular weight dependence of the characteristic relaxation times suggests amorphous-like chains, whereas the strong slower process originates from the loss of dipole orientational capacity caused by structural defects and reflects the migration of helical sequences along the chains. This identifies the helices as structures extending over rather short fragments of chains (i.e., of low persistence length). The viscoelastic response indicated that the structural defects arise from locally aggregated chains that inhibit the flow of oligopeptides.