Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, via Risorgimento 35, 56126 Pisa, Italy.
J Phys Chem B. 2010 Mar 4;114(8):2586-92. doi: 10.1021/jp907843r.
The molecular dynamic behavior of silkworm cocoons produced by a single Bombyx mori strain was investigated by means of high- and low-resolution solid-state NMR experiments. Cocoons with different moisture content were prepared to study the effects of physisorbed water on their molecular dynamics in the MHz regime, which was probed through the measurement of (1)H T(1) relaxation times at 25 MHz in the 25-95 degrees C temperature range. The water content of the different samples was determined from the analysis of (1)H free-induction decays. In addition to the rotation of methyl groups, mostly from alanine, and to the reorientation of physisorbed water molecules, already identified in previous works as relaxation sinks, the reorientation of serine side-chains was here found to contribute to (1)H T(1) above room temperature. The analysis of the trends of (1)H T(1) versus temperature was carried out in terms of semiempirical models describing the three main motional processes, and indicated that methyl rotation, water reorientation and serine side-chain motions are the most efficient relaxation mechanisms below 0 degrees C, between 0 and 60 degrees C, and above 60 degrees C, respectively. The activation energies were found to decrease passing from serine to water to methyl motions.
采用高分辨和低分辨固态 NMR 实验研究了单一桑蚕品种所产蚕茧的分子动态行为。通过在 25-95°C 温度范围内、25MHz 下测量（1）H T1 弛豫时间，研究了不同含水量的蚕茧中物理吸附水对其在 MHz 区域内分子动力学的影响，由此制得不同的蚕茧样品。通过分析（1）H 自由感应衰减，确定了不同样品的含水量。除了已在先前研究中确定为弛豫源的甲硫氨酸基团的旋转和物理吸附水分子的重新取向之外，还发现丝氨酸侧链的重新取向也有助于室温以上的（1）H T1。（1）H T1 随温度的变化趋势分析是根据描述三种主要运动过程的半经验模型进行的，结果表明，在 0°C 以下、0-60°C 之间和 60°C 以上，甲硫氨酸旋转、水重排和丝氨酸侧链运动分别是最有效的弛豫机制。发现激活能随着丝氨酸向水再向甲硫氨酸运动而降低。
