Abstract:SynopsisThe aggregation of poly(y-benzyl-a,I,-glutamate) and its enantiomer in toluene has been investigated by following the viscosity as a function of temperature, concentration, molecular weight, molecular-weight distribution, helix chirality, and shear rate. The temperature and concentration data for a 138,000-molecular-weight sample was fitted to an open, reversible end-to-end aggregation model. The aggregation numbers resulting from this fit were consistent with the sudden onset in non-Newtonian flow res… Show more
“…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.…”
Polypeptides having secondary structures often undergo self-assembly which can extend over multiple length scales. Poly(γ-benzyl-L-glutamate) (PBLG), for example, folds into α-helices and forms physical organogels, whereas poly(L-glutamic acid) (PLGA at acidic pH) or poly(L-glutamate) (PLG at neutral/basic pH) do not form hydrogels. We explored the gelation of modified PBLG and investigated the deprotection of the carboxylic acid moieties in such gels to yield unique hydrogels. This was accomplished through photo-crosslinking gelation of poly(γ-benzyl-L-glutamate-co-allylglycine) statistical copolymers in toluene, tetrahydrofuran, and 1,4-dioxane. Unlike most polymer-based chemical gels, our gels were prepared from dilute solutions (<20 g L −1 , i.e., <2% w/v) of low molar mass polymers. Despite such low concentrations and molar masses, our dioxane gels showed high mechanical stability and little shrinkage; remarkably, they also exhibited a porous fibrillar network. Deprotection of the carboxylic acid moieties in dioxane gels yielded pH responsive and highly absorbent PLGA/PLG-based hydrogels (swelling ratio of up to 87), while preserving the network structure, which is an unprecedented feature in the context of crosslinked PLGA gels. These outstanding properties are highly attractive for biomedical materials.
“…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.…”
Polypeptides having secondary structures often undergo self-assembly which can extend over multiple length scales. Poly(γ-benzyl-L-glutamate) (PBLG), for example, folds into α-helices and forms physical organogels, whereas poly(L-glutamic acid) (PLGA at acidic pH) or poly(L-glutamate) (PLG at neutral/basic pH) do not form hydrogels. We explored the gelation of modified PBLG and investigated the deprotection of the carboxylic acid moieties in such gels to yield unique hydrogels. This was accomplished through photo-crosslinking gelation of poly(γ-benzyl-L-glutamate-co-allylglycine) statistical copolymers in toluene, tetrahydrofuran, and 1,4-dioxane. Unlike most polymer-based chemical gels, our gels were prepared from dilute solutions (<20 g L −1 , i.e., <2% w/v) of low molar mass polymers. Despite such low concentrations and molar masses, our dioxane gels showed high mechanical stability and little shrinkage; remarkably, they also exhibited a porous fibrillar network. Deprotection of the carboxylic acid moieties in dioxane gels yielded pH responsive and highly absorbent PLGA/PLG-based hydrogels (swelling ratio of up to 87), while preserving the network structure, which is an unprecedented feature in the context of crosslinked PLGA gels. These outstanding properties are highly attractive for biomedical materials.
“…The question arises why these nanofibers are formed when a hot solution of PBLG in toluene is allowed to cool down. Miller and coworkers [32] discovered that PBLG aggregates end to end even in hot toluene. Figure 5 shows a schematic illustration of the proposed transition of dissolved PBLG in hot toluene to PBLG spheres with tentacles at PBLG concentrations below C gel and self-assembled PBLG networks at PBLG concentrations at or above C gel .Fig.…”
Section: Resultsmentioning
confidence: 99%
“…However, for an infinite network to be formed, these fibers need to be cross-linked. The formation of cross-links takes places by the branching and rejoining of different sheaf-like aggregates throughout the solution, resulting in an interconnected fibrillar morphology [32]. The same mechanism of branching and rejoining has been proposed by Tadmor et al from investigation of the gelation kinetics by small-angle neutron scattering [10].…”
Section: Introductionmentioning
confidence: 97%
“…Shukla does not take into account the end-to-end aggregation of PBLG chains observed by Chakrabarti et al [32] even in hot toluene solutions. This kind of aggregation phenomenon may occur in benzyl alcohol as well.…”
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.
“…33, 34 PBLG gels have been shown to consist of fibrillar networks with fibril diameters ranging from tens to hundreds of nanometers. 35–38 Larger aggregates (micron size) have also been observed by light scattering and optical microscopy. 39 The detailed structure of these gels depends on the quenching temperature, quenching rate, and polymer molecular weight.…”
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