Department of Chemical Engineering and ‡Department of Engineering and System Science, National Tsing Hua University , Hsinchu 300, Taiwan, ROC.
Langmuir. 2013 Dec 23;29(51):15981-91. doi: 10.1021/la403331f. Epub 2013 Dec 13.
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-β-benzyl L-Asp)-poloxamer-oligo(β-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 α-helix conformation, whereas that in polymer 2 adopted predominantly a β-sheet conformation at room temperature. The segmental dynamics of the PEG in polymer 1 remained essentially unperturbed upon heating from 10 to 50 °C; by contrast, the PEG segmental motion in polymer 2 became more constrained above ca. 35 °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 β 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.
我们揭示了在聚氧丙烯醚中引入的寡肽嵌段的功能基团的微小变化导致了在水介质中截然不同的分级组装行为和流变性质。一种寡（L-丙氨酸-co-L-苯丙氨酸-co-β-苄基 L-天冬氨酸）-聚氧丙烯醚-寡（β-苄基 L-天冬氨酸-co-L-苯丙氨酸-co-L-丙氨酸）嵌段共聚物（OAF-（OAsp（Bzyl））-PLX-（OAsp（Bzyl））-OAF，简称聚合物 1），其肽段中天冬氨酸部分带有苄基基团，通过开环聚合合成。然后将天冬氨酸上的苄基转化为羧酸，得到寡（L-丙氨酸-co-L-苯丙氨酸-co-L-天冬氨酸）-聚氧丙烯醚-寡（L-天冬氨酸-co-L-苯丙氨酸-co-L-丙氨酸）（OAF-（OAsp）-PLX-（OAsp）-OAF，简称聚合物 2）。圆二色性分析表明，聚合物 1 中的寡肽段在水溶液中主要呈现α-螺旋构象，而聚合物 2 中的寡肽段在室温下主要呈现β-折叠构象。聚合物 1 中的 PEG 链段在 10-50°C 加热时基本没有受到干扰；相比之下，聚合物 2 中的 PEG 链段在高于约 35°C 时的运动受到更多限制，表明链段的化学环境发生了明显变化。同时，聚合物 2 溶液的储能模量在该温度下突然增加，溶液变成凝胶。湿细胞 TEM 观察表明，聚合物 1 自组装形成几百纳米大小的微凝胶颗粒。微凝胶颗粒作为特征形态实体保留下来，使得 PEG 链在加热时其化学环境没有发生显著变化。聚合物 2 形成的水凝胶中发现存在纳米纤维网络，表明在高温下，羧酸基团之间的氢键导致β 片层沿着纤维轴的广泛堆积。聚合物 2 水溶液的体外细胞毒性在人视网膜色素上皮细胞中较低。低细胞毒性与溶胶-凝胶转变相结合，使相应的水凝胶成为生物医学应用的良好候选物。
