Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA.
Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA 19104, USA.
Soft Matter. 2016 Jul 13;12(28):6096-110. doi: 10.1039/c6sm00879h.
Hydrogels formed by polypeptides could be much-favored tools for drug delivery because their main ingredients are generally biodegradable. However, the gelation of peptides in aqueous solution generally requires a minimal length of the peptide as well as distinct sequences of hydrophilic and hydrophobic residues. The aggregation of short peptides like tripeptides, which are relatively cheap and offer a high degree of biodegradability, are generally thought to require a high hydrophobicity of their residues. We found that contrary to this expectation cationic glycylalanylglycine in 55 mol% ethanol/45 mol% water forms a gel below a melting temperature of ca. 36 °C. A pure hydrogel state can be obtained after allowing the ethanol component to evaporate. The gel phase consists of crystalline fibrils of several 100 μm, which form a sample-spanning network. Rheological data reveal a soft elastic solid gel. We investigated the kinetics of the various processes that lead to the final gel state of the ternary mixture by a unique combination of UV circular dichroism, infrared, vibrational circular dichroism (VCD) and rheological measurements. A mathematical analysis of our data show that gelation is preceded by the formation of peptide β-sheet like tapes or ribbons, which give rise to a significant enhancement of the amide I' VCD signal, and the subsequent formation of rather thick and long fibrils. The VCD signals indicate that the tapes exhibit a right-handed helicity at temperatures above 16 °C and a left-handed helicity below. The tapes'/ribbons' helicity change occurs at a temperature where the UVCD data reflect a relatively long nucleation process. The kinetics of gel formation probed by the storage and loss moduli are composed of a fast process that follows tape/ribbon/fibril formation and is clearly identifiable in a movie that shows the gelation process and a slow process that causes an additional gel stabilization. The rheological data indicate that left-handed fibrils observed at low temperatures form a more solid-like structure than their right-handed counterparts formed at higher temperatures. Taken together our data reveal GAG as an unexpected gelator, the formation of which is underlied by a set of distinguishable kinetic processes.
由多肽形成的水凝胶可以成为药物输送的非常有吸引力的工具，因为它们的主要成分通常是可生物降解的。然而，肽在水溶液中的凝胶化通常需要肽的最小长度以及亲水性和疏水性残基的特定序列。像三肽这样的短肽的聚集，相对便宜并且提供高度的生物降解性，通常被认为需要其残基的高疏水性。我们发现，与这种预期相反，在 55mol%乙醇/45mol%水的混合物中，阳离子甘氨酰丙氨酰甘氨酸在低于约 36°C 的熔融温度下形成凝胶。允许乙醇成分蒸发后，可以获得纯水凝胶状态。凝胶相由几种 100μm 的结晶纤维组成，这些纤维形成贯穿样品的网络。流变数据显示出柔软的弹性固体凝胶。通过独特的组合，即 UV 圆二色性、红外、振动圆二色性（VCD）和流变测量，我们研究了导致三元混合物最终凝胶状态的各种过程的动力学。我们的数据的数学分析表明，凝胶化之前是形成肽β-折叠样带或带，这导致酰胺 I'VCD 信号显著增强，随后形成相当厚和长的纤维。VCD 信号表明，在高于 16°C 的温度下，带子表现出右手螺旋，而在低于 16°C 的温度下，带子表现出左手螺旋。带子/带的螺旋变化发生在 UVCD 数据反映相对长的成核过程的温度下。由储能和损耗模量探测的凝胶形成动力学由快速过程组成，该快速过程紧随带/带/纤维形成，并且可以在显示凝胶化过程的电影中清楚地识别，并且由导致凝胶进一步稳定的缓慢过程组成。流变数据表明，在低温下观察到的左手纤维形成比在较高温度下形成的右手纤维更固体状的结构。总的来说，我们的数据揭示了 GAG 作为一种意想不到的凝胶剂，其形成由一系列可区分的动力学过程为基础。
