硫酸钠诱导单链聚(N-异丙基丙烯酰胺)相变的单分子力谱研究

doi:10.6023/A13121281

摘要/Abstract

聚（N-异丙基丙烯酰胺）（PNIPAM）具有独特的相变行为，已成为人们研究蛋白质折叠等生命过程发生机理的模型体系. 我们利用基于原子力显微镜（AFM）的单分子力谱技术（SMFS）研究了单链PNIPAM在硫酸钠诱导下的相转变过程，并定量化了相变后所形成塌缩结构的稳定性. 通过对单链PNIPAM的单分子力谱实验得知：在相变前，得到单调上升的力曲线，对应着PNIPAM无规线团结构的形变过程；相变后，得到的锯齿型力曲线，对应着PNIPAM塌缩结构在外力诱导下的解折叠过程. 首次从单分子水平观察到在外加盐的作用下，单链PNIPAM低温相转变和高温相转变的差异：相比于低温相转变，高温相转变生成的塌缩结构更加稳定.

关键词: 原子力显微镜, 单分子力谱, 聚(N-异丙基丙烯酰胺), 低临界溶解温度, 盐效应

In this study, the sodium sulfate (Na₂SO₄)-induced conformational transition of a single PNIPAM chain, a model system for the investigation of protein folding, was studied by using atomic force microscopy (AFM) based single-molecule force spectroscopy (SMFS). Before the SMFS experiments, thermal-responsive poly(N-isopropylacrylamide) (PNIPAM) molecules were attached individually onto gold surfaces for SMFS experiments. By covalently grafting sulfydryl- functionalized PNIPAM into the isolated defects of freshly prepared self-assembled monolayers (SAMs) of dithiothreitol (DTT) on gold substrates, the grafting density of PNIPAM molecules could be conveniently controlled and the substrate-induced conformational changes have also been prevented effectively. During the SMFS experiments, the AFM tip was gently brought in contact with the sample surfaces during which part of the polymer chain was adsorbed onto the tip surface, forming a connective bridge in-between. And then, the grabbed polymer chain has been manipulated repeatedly under different extension length until the molecule detached from the AFM tip. Characteristic smooth-and sawtooth-pattern force curves were obtained before and after the conformational transition of a single PNIPAM chain, respectively. It was demonstrated that the smooth-pattern force curves corresponded to the pulling of the random coiled structure of single PNIPAM chains, while the sawtooth-pattern force curves corresponded to the unraveling of the collapsed globule structure. Besides, a consecutive two-step collapsing process (low-and high-temperature collapsing step) of the conformational transition of a single PNIPAM chain was detected, for the first time, in our SMFS experiments. It was found that more compact collapsed structures formed in the high-temperature collapsing step, which may due to the cooperative effect of the weakening of hydrogen bonding (between PNIPAM chains and water molecules) and increasing of hydrophobic interaction of the polymer chains. Our results show that AFM-based SMFS is a powerful method that complementary to other ensemble measurements in the determination of conformation transition of thermal-responsive polymers.

Key words: atomic force microscopy, single molecule force spectroscopy, poly(N-isopropylacrylamide) (PNIPAM), Lower Critical Solution Temperature (LCST), salt effect

引用此文

薛玉瑞, 张文科. 硫酸钠诱导单链聚(N-异丙基丙烯酰胺)相变的单分子力谱研究[J]. 化学学报, 2014, 72(4): 481-486.

Xue Yurui, Zhang Wenke. Single Molecule Force Spectroscopy Investigation on Na₂SO₄-inducedConformational Transition of Single PNIPAM Chains[J]. Acta Chimica Sinica, 2014, 72(4): 481-486.

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