Elucidation of the structure of poly(γ-benzyl-l-glutamate) nanofibers and gel networks in a helicogenic solvent

Abstract

The synthesis, characterization, self-assembly, and gel formation of poly(γ-benzyl-l-glutamate) (PBLG) in a molecular weight range from ca. 7,000–100,000 g/mol and with narrow molecular weight distribution are described. The PBLG is synthesized by the nickel-mediated ring-opening polymerization and is characterized by size-exclusion chromatography coupled with multiple-angle laser light scattering, NMR, and Fourier transform infrared spectroscopy. The self-assembly and thermoreversible gel formation in the helicogenic solvent toluene is investigated by transmission electron microscopy, atomic force microscopy, small-angle X-ray scattering, and synchrotron powder X-ray diffraction. At concentrations significantly below the minimum gelation concentration, spherical aggregates are observed. At higher concentrations, gels are formed, which show a 3D network structure composed of nanofibers. The proposed self-assembly mechanism is based on a distorted hexagonal packing of PBLG helices parallel to the axis of the nanofiber. The gel network forms due to branching and rejoining of bundles of PBLG nanofibers. The network exhibits uniform domains with a length of 200 ± 42 nm composed of densely packed PBLG helices.Electronic supplementary materialThe online version of this article (doi:10.1007/s00396-012-2866-9) contains supplementary material, which is available to authorized users.

Fibrillar gels via the self-assembly of poly(l-glutamate)-based statistical copolymers

“…8). The first peak can be assigned to a d-spacing of 1.3-1.4 nm corresponding to a pseudo-hexagonal packing of helices, 50,63,64 while the second 'peak', or shoulder, corresponds to the 0.5 nm helical pitch; 50 this indicates that P(BLG x -co-AG 1−x ) n -dioxane and PBLG 51 -dioxane aggregates have a similar microstructure. This result suggests that homopolymer PBLG 51 and statistical copolymer P(BLG x -co-AG 1−x ) n can both self-assemble in a similar fashion, that is pseudo-hexagonal packing of α-helices.…”

“…45,46 Although contrasting mechanisms have been proposed for the gelation of PBLG, including (i) phase separation by spinodal decomposition, (ii) nucleation growth, and (iii) a combination of both mechanisms, the result is the formation of a percolated network of fibers. 44,47,48 The stability of PBLG helices and fibers is often attributed to a combination of the following features: intra-molecular hydrogen bonding (especially in aprotic solvents), 11 dipole-dipole interaction of the PBLG helices (especially in non-polar solvents), 43,49,50 and π-π stacking of the outward-pointing pendant benzyl groups. 51 Such organogels present attractive features, including a robust fibrous network and a high porosity, 46 which would be valuable in the context of biomedical hydrogels.…”

Bending and kinking in helical polymers

“…The buckling mechanism described in MD simulations for single AH filaments can with some modification also be relevant to AH nanofibers. Two types of AH nanofibers should be highlighted : (a) poly(γ‐benzyl‐ l ‐glutamate) (PBLG) nanofibers and (b) nanofibers involving coiled‐coil (CC) structural motif . PBGL is a synthetic polypeptide that adopts a α‐helical conformation in organic solvents.…”

“…The PBLG nanofibers with uniaxial orientation along the fiber axis were prepared by electrospinning from the biphasic solution and the filaments were found to be organized into a closed‐packed hexagonal lattice. Alternatively, self‐assembling of PBGL into nanofibers was achieved by thermoreversible gelation . Uniformly sized PBLG molecules in AH form were found to aggregate side by side during gel formation into nanofibers consisting of densely packed PBLG helices in a distorted hexagonal array.…”

“…Alternatively, self-assembling of PBGL into nanofibers was achieved by thermoreversible gelation. 40 Uniformly sized PBLG molecules in AH form were found to aggregate side by side during gel formation into nanofibers consisting of densely packed PBLG helices in a distorted hexagonal array. As regards the CC nanofibers, the CC structure, consisting of two AHs of seven residue length, is a common rope-like motif found in diverse proteins.…”