Lu Zhenyu, Nowak Wieslaw, Lee Gwangrog, Marszalek Piotr E, Yang Weitao
Department of Chemistry, Duke University, Durham, North Carolina 27708, USA.
J Am Chem Soc. 2004 Jul 28;126(29):9033-41. doi: 10.1021/ja031940x.
Recent single-molecule atomic force microscopy (AFM) experiments have revealed that some polysaccharides display large deviations from force-extension relationships of other polymers which typically behave as simple entropic springs. However, the mechanism of these deviations has not been fully elucidated. Here we report the use of novel quantum mechanical methodologies, the divide-and-conquer linear scaling approach and the self-consistent charge density functional-based tight binding (SCC-DFTB) method, to unravel the mechanism of the extensibility of the polysaccharide amylose, which in water displays particularly large deviations from the simple entropic elasticity. We studied the deformations of maltose, a building block of amylose, both in a vacuum and in solution. To simulate the deformations in solution, the TIP3P molecular mechanical model is used to model the solvent water, and the SCC-DFTB method is used to model the solute. The interactions between the solute and water are treated by the combined quantum mechanical and molecular mechanical approach. We find that water significantly affects the mechanical properties of maltose. Furthermore, we performed two nanosecond-scale steered molecular dynamics simulations for single amylose chains composed of 10 glucopyranose rings in solution. Our SCC-DFTB/MM simulations reproduce the experimentally measured force-extension curve, and we find that the force-induced chair-to-boat transitions of glucopyranose rings are responsible for the characteristic plateau in the force-extension curve of amylose. In addition, we performed single-molecule AFM measurements on carboxymethyl amylose, and we found that, in contrast to the results of an earlier work by others, these side groups do not significantly affect amylose elasticity. By combining our experimental and modeling results, we conclude that the nonentropic elastic behavior of amylose is governed by the mechanics of pyranose rings themselves and their force-induced conformational transitions.
最近的单分子原子力显微镜(AFM)实验表明,一些多糖与其他通常表现为简单熵弹性弹簧的聚合物的力-伸长关系存在很大偏差。然而,这些偏差的机制尚未完全阐明。在此,我们报告使用新颖的量子力学方法,即分治线性缩放方法和基于自洽电荷密度泛函的紧束缚(SCC-DFTB)方法,来揭示多糖直链淀粉的伸展性机制,直链淀粉在水中与简单熵弹性表现出特别大的偏差。我们研究了直链淀粉的组成单元麦芽糖在真空和溶液中的变形。为了模拟溶液中的变形,使用TIP3P分子力学模型对溶剂水进行建模,使用SCC-DFTB方法对溶质进行建模。溶质与水之间的相互作用采用量子力学和分子力学相结合的方法处理。我们发现水显著影响麦芽糖的力学性能。此外,我们对溶液中由10个吡喃葡萄糖环组成的单个直链淀粉链进行了两次纳秒级的定向分子动力学模拟。我们的SCC-DFTB/MM模拟重现了实验测量的力-伸长曲线,并且我们发现吡喃葡萄糖环的力诱导椅式到船式转变是直链淀粉力-伸长曲线中特征平台的原因。此外,我们对羧甲基直链淀粉进行了单分子AFM测量,并且我们发现,与其他人早期工作的结果相反,这些侧基不会显著影响直链淀粉的弹性。通过结合我们的实验和建模结果,我们得出结论,直链淀粉的非熵弹性行为由吡喃糖环本身的力学及其力诱导的构象转变所控制。