【作者】 莫春丽；

【导师】 邵正中；

【作者基本信息】 复旦大学 ， 高分子化学与物理， 2009， 博士

【摘要】 丝蛋白是一类具有广泛应用前景的生物大分子，丝蛋白材料的形貌、力学性能、降解速度及适用性等各种理化性质的控制，都会涉及到对丝蛋白二级结构的调控问题。本论文利用了傅立叶红外光谱在分子结构检测中的高灵敏性和特殊技术手段的有效适用性，对丝蛋白二级结构进行了详尽地表征，主要采用近红外、衰减全反射红外光谱方法研究丝蛋白膜在含水量、温度变化、化学试剂处理等过程中的结构变化；并将红外光谱与流变表征结合，从微观的丝蛋白分子链结构变化来解释其溶液的宏观流变行为；同时，通过二维相关光谱分析技术，对中红外及近红外动态光谱做了详细分析探讨，将光谱信息与相关的微观反应和机理联系了起来。丝蛋白溶液性质对于我们充分理解并改善再生丝蛋白材料结构和性能是至关重要的。利用流变测试技术考察了丝蛋白溶液性质，通过比较不同分子量范围和不同浓度的丝蛋白溶液，发现了丝蛋白溶液的浓度是其流变性质重要影响因素，丝蛋白溶液浓度在低于临界浓度C_g时会转变为弱凝胶状态。我们将丝蛋白／水相互作用、丝蛋白构象形态转变用于解释丝蛋白溶液／凝胶流变性质变化的原因。C_g以下，肽链主要以无规线团构象存在于水中，肽链与水形成的弱凝胶网络是由于疏水区域的减少和肽链的伸展，肽链与肽链间、肽链与水间的氢键成为物理凝胶点。C_g以上，肽链倾向于形成更稳定的分子内氢键结合，β-折叠含量增加。这种氢键的变化使丝蛋白溶液的流变行为近似于缠结高分子亚浓溶液粘度与浓度的标度关系。基于对丝蛋白溶液性质的认识，我们将近红外光谱技术用于表征各种溶液在不同条件下所制备的丝蛋白膜的二级结构。在采集不同构象丝蛋白膜的近红外谱线后，结合中红外光谱、核磁共振测试结果进行比较分析，得到了丝蛋白近红外光谱的峰位归属并发现了位于4530 cm^-1的峰是丝蛋白β-折叠构象的特征峰。由此证明，近红外光谱可作为一种简便有效的手段对丝蛋白构象进行定性和定量分析，其优势在于可对潮湿环境中相对较厚的膜作透射测试。这些结果为近红外光谱研究丝蛋白结构的动态变化过程奠定了基础。在近红外光谱监测丝蛋白二级结构动态变化过程中，考察了丝蛋白膜在水分、温度、化学试剂等因素诱导下近红外光谱的动态变化，重点研究并阐明了水在丝蛋白构象转变中的作用。动态光谱显示，湿膜中丝蛋白构象转变与水的变化之间是密切相关的。吸收峰变化规律说明水与肽链之间氢键解缔，取而代之，肽链与肽链之间形成具有强氢键的β-折叠。由光谱变化结合DSC结果，证明了可冻结水在热诱导丝蛋白构象变化过程中发挥着增塑剂的作用。一方面水与肽链间的弱氢键结合赋予了肽链运动的弹性，另一方面足够水分子存在为多肽链运动提供了运动的自由空间，使链段调整成为可能。二维相关分析方法帮助我们认识到，构象转变过程各种分子基团变化的顺序是：首先，水与多肽之间的氢键解缔，无规卷曲结构发生调整；其次，肽链与肽链之间重排形成更强的氢键，实现构象转变。高温环境下丝蛋白膜的动态近红外光谱研究表明，以无规线团和／或螺旋结构为主的丝蛋白膜，在空气氛围内升温至220℃会发生氧化分解，出现位于5050cm^-1处的氧化特征峰，同时，在此温度下也存在发生链段重排并形成β-折叠的可能性。此外，醇溶液诱导丝蛋白的构象转变的实验，证明了近红外光谱同样适用于动态监测丝蛋白／有机溶剂体系中丝蛋白的二级结构变化。最后，采用时间分辨衰减全反射红外光谱原位地监测丝蛋白膜在不同溶剂氛围内发生结构变化的动态过程，并结合二维相关分析来研究醋酸诱导丝蛋白构象转变的作用机理、室温下水分子在丝蛋白膜中的吸附和脱附过程。实验结果表明，醋酸能够诱导丝蛋白形成高含量的β-折叠结构，醋酸分子渗透到丝蛋白聚集体中破坏了其原有氢键是肽链构象转变的一个先决条件。同时，证实了丝蛋白水含量是影响醋酸的扩散速度以及丝蛋白多肽链重排速度的一个重要因素。通过对水在丝蛋白中脱附和吸附的时间分辨光谱研究，确认了在丝蛋白／水体系中存在三种不同状态的水分子：自由水、弱氢键结合水和强氢键结合水，分别对应于不同波数的吸收。二维相关分析的结果显示，水脱附过程中，自由水先于结合水去除，而吸附过程则是结合水先于自由水进入丝蛋白中，通过与肽链形成氢键来进行扩散。由此证明，时间分辨光谱可以作为蛋白质结构变化机理研究的一种有效方法。更多还原

【Abstract】 Silk fibroin based materials possess many attractive properties that make them assuitable candidates for various potential applications.Understanding the secondarystructure of silk fibroin is vital if we are ever to truly capitalize upon its superiormechanical properties.Fourier transform infrared spectroscopy is very sensitive tomolecular structure and has been extensively used in the study of protein structure aswell as conformation transition.Hereby near-infrared （NIR） and attenuated totalreflectance （ATR） spectroscopy were employed in our study to investigate the effectof external disturbance,namely water content,thermal and chemical treatment,on thestructural modification of silk fibroin.Additionally,spectral results revealed that theconformation of peptide chains might be contributive to the rheological characteristicof silk solution.The dynamic spectra were analyzed by 2D correlation method whichcan not only provide better resolution,but also distinguish the motion sequential orderof different functional groups of materials.Thus progress had been made regardingthe relationship between molecular mechanisms and spectral signals.The ability to control the processing of silk protein based materials is the key tosuccessful applications of this important and high performance biopolymer.Rheological measurements were applied to regenerated silk fibroin solution withdifferent molecular weight cut-offs and concentrations,observations revealed theexistence of critical concentration Cg below which an extended gel network can form.This concentration dependence was explained as the preponderance for reconstitutedsilk molecules to form either inter- or intra- molecular bonds depending on theirconcentration.It showed that a change in rheological properties can be linked to achange in fibroin structures present in solution.The results may have importantimplications for future successful artificial processing of silk.Based on our comprehension on silk fibroin solution,NIR spectra of various silkfibroin films cast from its aqueous solution were collected for the characterization ontheir secondary structure.Clear band assignment was made and the peak at 4530 cm^-1 was assigned to the combination mode of amide A and amideⅢcharacteristic ofβ-sheet.In the NIR spectrum of silk fibroin,analysis on the combination modes ofamide groups could lead to a profile of conformation ratio,which was confirmed bythe ATR and NMR observation.Therefore,NIR spectroscopy was proved to be arapid and effective tool to investigate qualitatively or even quantitatively theconformation of the RSF （regenerated silk fibroin） film with different thicknessesregardless of the disturbance effect of water.Furthermore,NIR spectroscopy was used to monitor dynamic changes in thehydration and secondary structure of silk fibroin induced by thermal and chemicaltreatment.Inspired by differential scanning calorimetry （DSC） observations wesuggested that the formation ofβ-sheets in wet film should be strongly related to thedissociation of hydrogen bonds between water and peptide residues,and the freezingwater might facilitate the movement of peptide chains and thereby contribute to theconformational transition at 60℃.In addition,structural changes of fibroin inducedby higher temperature and ethanol aqueous solution were also analyzed respectively.By monitoring the dynamic variations of the spectral components,an NIR procedurefor tracking the conformational transition of silk fibroin was established.At last,time-resolved ATR infrared spectroscopy was used to monitor thestructural change of silk fibroin in water and chemical vapor respectively in situ,thereafter the mechanism of the conformation transition as well as water absorptionand de-association in silk fibroin at room temperature was discussed.The ATRobservations showed that acetic acid was contributive to the highly dominatingcontent ofβ-sheet conformation in silk fibroin film.The kinetic curve of solventdiffusion was similar to theβ-sheet growth,which indicated that the access of aceticacid into silk fibroin architecture was a necessary step to break the original hydrogenbond of silk fibroin and to force the re-alignment of peptide chains.Meanwhile,threetypes of water were distinguished in this dynamic system,whose motion sequentialorder was illustrated by the two-dimensional correlation analysis.更多还原