Abstract
The self-assembly in aqueous solution of three novel telechelic conjugates comprising a central hydrophilic polymer and short (trimeric or pentameric) tyrosine end-caps has been investigated. Two of the conjugates have a central poly(oxyethylene) (polyethylene oxide, PEO) central block with different molar masses. The other conjugate has a central poly(L-alanine) (PAla) sequence in a purely amino-acid based conjugate. All three conjugates self-assemble into β-sheet based fibrillar structures, although the fibrillar morphology revealed by cryogenic-TEM is distinct for the three polymers--in particular the Tyr5-PEO6k-Tyr5 forms a population of short straight fibrils in contrast to the more diffuse fibril aggregates observed for Tyr5-PEO2k-Tyr5 and Tyr3-PAla-Tyr3. Hydrogel formation was not observed for these samples (in contrast to prior work on related systems) up to quite high concentrations, showing that it is possible to prepare solutions of peptide-polymer-peptide conjugates with hydrophobic end-caps without conformational constraints associated with hydrogelation. The Tyr5-PEO6k-Tyr5 shows significant PEO crystallization upon drying in contrast to the Tyr5-PEO2k-Tyr5 conjugate. Our findings point to the remarkable ability of short hydrophobic peptide end groups to modulate the self-assembly properties of polymers in solution in model peptide-capped "associative polymers". Retention of fluidity at high conjugate concentration may be valuable in potential future applications of these conjugates as bioresponsive or biocompatible materials, for example exploiting the enzyme-responsiveness of the tyrosine end-groups.
Highlights
Conjugation of polymers with peptides enables the unique properties of each component to be combined, leading to novel biomacromolecules with a diversity of potential applications.[1−8] In particular, polymers can be synthesized inexpensively and attached to peptides, modifying their solubility and aggregation properties
Hydrophilic polymers may be modified with hydrophobic end groups to create telechelic polymers, which among other properties resulting from the noncovalent association of the end groups, possess useful rheological behavior
We find using a comprehensive suite of characterization measurements that all three novel conjugates form β-sheet-based fibrillar structures via selfassembly in aqueous solution, the fibril morphology is distinct for the three conjugates
Summary
Conjugation of polymers with peptides enables the unique properties of each component to be combined, leading to novel biomacromolecules with a diversity of potential applications.[1−8] In particular, polymers can be synthesized inexpensively and attached to peptides, modifying their solubility and aggregation properties. Hydrophilic polymers may be modified with hydrophobic end groups to create telechelic polymers, which among other properties resulting from the noncovalent association of the end groups, possess useful rheological behavior. This is exploited in associative polymers[18−20] widely used as thickeners in a variety of applications, and in commercially available polyethylene glycol (PEG)-based polymers such as reverse Pluronics,[20] for example. In the field of purely peptide/protein systems (i.e., systems not incorporating synthetic polymers), the properties of gels of genetically expressed proteins comprising a hydrophilic random coil-like middle block and collagen-like end blocks have been investigated by the groups of Tirrell[12,23] and Cohen-Stuart.[24−26]
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