低温诱导桑蚕体内腺体相行为的高分辨^13C固体核磁共振研究

为什么蚕在常温常压水溶液条件下即能纺出力学性能优异的蚕丝纤维, 一直是科学家们感兴趣的问题. 在过去的几十年, 人们曾用多种表征手段, 如双折射、扫描电镜、原子力显微镜和电子散射等, 在界观尺度下对蚕在吐丝过程中腺体的相行为进行研究. 发现腺体在靠近吐丝口时呈液晶态, 并认为这是导致蚕丝力学性能的重要因素. 本文则在分子水平尺度下利用核磁共振方法, 对五龄蚕活体在常温和6 ℃下存储数天后解剖的腺体进行研究. 经对化学位移及其线型的各向异性分析发现, 当将体内腺体沿吐丝方向分为3部, 即后部、中部及靠近吐丝口的前部时, 常温下, 腺体后部和中后部分子呈无规线团, 而腺体中中部、中前部和前部分子呈液晶态. 6 ℃时, 中后部分子亦呈液晶态, 前部分子排列则各向异性更大, 说明更为有序. 这种液晶态呈分形结构, 在小于纳米尺度下为无规线团, 大于纳米尺度呈有序排列. 这表明, 降温过程可使呈无规线团的丝素蛋白分子转变为液晶态, 其效果如同蚕在吐丝过程中对其腺体施加的剪切应力. 该结果对于人们探索人工合成高性能类丝素纤维的纺丝工艺和条件将有启发和指导作用.

Temperature-induced Phase Transitions in the Silk Gland of Bombyx mori Silkworm Lumen: A High-resolution Solid-state 13C NMR study

In this contribution, the temperature-induced phase changes of the silk gland of Bombyx mori silkworm were studied using high-resolution 13C solid-state NMR spectroscopy. The silk gland samples were obtained from alive silkworms which were stored under either normal temperature or 6 ℃ for 2 weeks. The results showed that storing the alive silkworms at 6 ℃ could induce a phase transition in the fibroin gland in the middle and anterior parts of the middle division of the lumen, from an isotropic phase to an anisotropic liquid crystalline phase. Such phase transition was not observed in the silkworms stored under normal temperature. Previous studies have demonstrated, both experimentally and theoretically, lowering temperature can induce an action equivalent to a shearing stress in many elastic polymers (including proteins), and shearing stress may play an important role in the spinning of the silkworm. The results of this study suggest that lowering temperature or applying strong shearing may be used as ways to control the process to fabricate silk-like fibers of high performance.