Thermosensitive Hydrogel from Oligopeptide-Containing Amphiphilic Block Copolymer: Effect of Peptide Functional Group on Self-Assembly and Gelation Behavior

We reveal that a slight change in the functional group of the oligopeptide block incorporated into the poloxamer led to drastically different hierarchical assembly behavior and rheological properties in aqueous media. An oligo (L-Ala-co-L-Phe-co-beta-benzyl L-Asp)-poloxamer-oligo(beta-benzyl-L-Asp-co-L-Phe-co-L-Ala) block copolymer (OAF(OAsp(Bzyl))-PLX-(OAsp(Bzyl))-OAF, denoted as polymer 1), which possessed benzyl group on the aspartate moiety of the peptide block, was synthesized through ring-opening polymerization. The benzyl group on aspartate was then converted to carboxylic acid to yield oligo(L-Ala-co-L-Phe-co-L-Asp)-poloxamer-oligo(L-Asp-co-L-Phe-co-L-Ala) (OAF(OAsp)-PLX-(OAsp)-OAF, denoted as polymer 2). Characterization of the peptide secondary structure in aqueous media by circular dichroism revealed that the oligopeptide block in polymer 1 exhibited mainly an alpha-helix conformation, whereas that in polymer 2 adopted predominantly a beta-sheet conformation at room temperature. The segmental dynamics of the PEG in polymer 1 remained essentially unperturbed upon heating from 10 to 50 degrees C; by contrast, the PEG segmental motion in polymer 2 became more constrained above ca. 35 degrees C, indicating an obvious change in the chemical environment of the block chains. Meanwhile, the storage modulus of the polymer 2 solution underwent an abrupt increase across this temperature, and the solution turned into a gel. Wet-cell TEM observation revealed that polymer 1 self-organized to form microgel particles of several hundred nanometers in size. The microgel particle was retained as the characteristic morphological entity such that the PEG chains did not experience a significant change of their chemical environment upon heating. The hydrogel formed by polymer 2 was found to contain networks of nanofibrils, suggesting that the hydrogen bonding between the carboxylic acid groups led to an extensive stacking of the 13 sheets along the fibril axis at elevated temperature. The in vitro cytotoxicity of the polymer 2 aqueous solution was found to be low in human retinal pigment epithelial cells. The low cytotoxicity coupled with the sol gel transition makes the corresponding hydrogel a good candidate for biomedical applications.