Dynamic protein-water relationships during β-sheet formation
Dynamic protein-water relationships during β-sheet formation
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DOI：
10.1021/ma071551d
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发表时间：
2008-06-10
期刊：
影响因子：
5.5
通讯作者：
Cebe, Peggy
中科院分区：
文献类型：
--
作者：
Hu, Xiao;Kaplan, David;Cebe, Peggy
我们研究了一个模型纤维蛋白中的聚合物与水的相互作用。家蚕丝素膜是一种典型的生物可降解材料，本研究从水溶液中浇铸家蚕丝素膜并对其进行分析。首次采用差示扫描量热仪(DSC)及其调温变型(TMDSC)和时间分辨傅立叶变换红外光谱(FTIR)技术，对丝素蛋白在玻璃化转变温度(T-g)为451K(178℃)以上的升温和恒温结晶过程中的详细结构变化进行了监测。结果表明，分子间结合水分子作为增塑剂对丝素蛋白的二级结构有很大的影响。DSC研究表明，丝素蛋白在加热过程中最初在80℃左右表现出水诱导的玻璃化转变，这是由于临时结合的水-丝素结构造成的。对该体系在加热过程中的热容变化进行了定量的热分析，发现在T-g以下没有形成β-片状晶体。FTIR扫描还证实，在加热和去除T-g以下的结合水过程中没有形成β-薄层。在去水过程中，酰胺II区向低频率移动，1515 cm(-1)带略有增加，表明丝素链的微环境发生了变化。在T-g以上的恒温结晶过程中，丝素蛋白光谱的酰胺I区表现出从非晶态无规卷曲和α螺旋的二级结构向β折叠片晶的相变，而酰胺II区和1515 cm(-1)带在β片晶形成过程中保持稳定。根据Strobl的四态方案，将合成聚合物的结晶过程与丝素蛋白的结晶过程进行了类比，我们认为这一过程是通过与脱水相关的中间前体阶段进行的。这项研究对丝素蛋白结晶过程中β-褶皱片层的形成有了更深的理解，对于家蚕和蜘蛛等动物的天然丝素纤维的结晶具有重要意义。
We investigated the polymer-water interaction in a model fibrous protein. Bombyx mori silk fibroin film, a typical model of biodegradable material, was cast from aqueous solution and analyzed in this study. Differential scanning calorimetry (DSC), its temperature-modulated variant (TMDSC), and the time-resolved technique of Fourier transform infrared spectroscopy (FTIR) were used for the first time to monitor the detailed structural changes of silk fibroin during heating and during isothermal crystallization above the glass transition temperature, T-g similar to 451 K (178 degrees C). Results show that intermolecular bound water molecules, acting as a plasticizer, will strongly affect the secondary structure of silk fibroin. DSC study shows that silk fibroin initially displays a water-induced glass transition around 80 degrees C during heating, resulting from a temporary bound water-silk structure. Quantitative thermal analysis of the heat capacity changes of this system during heating revealed that no beta-sheet crystals were formed below T-g. FTIR scans also confirmed that no beta-sheets were formed during heating and removal of bound water below T-g. During water removal, the amide II region shifts to lower frequency, and the 1515 cm(-1) band increases slightly, indicating change of the microenvironment in the silk fibroin chains. During isothermal crystallization above T-g, the amide I region of silk fibroin spectra showed a phase transition from the secondary structures of noncrystalline random coils and alpha-helices to the beta-pleated-sheet crystals, while the amide II region and the 1515 cm(-1) band remain stable during beta-sheet formation. Analogy is made between the crystallization of synthetic polymers according to the four-state scheme of Strobl and the crystallization process of silk fibroin, which we suggest proceeds through an intermediate precursor stage associated with water removal. This study provides a deeper understanding of the formation of beta-pleated sheets during the crystallization process in silk fibroin, with implications for the crystallization of naturally occurring silk fibers from animals such as the silkworm and spider.