Acta Chimica Sinica ›› 2014, Vol. 72 ›› Issue (4): 481-486.DOI: 10.6023/A13121281 Previous Articles     Next Articles



薛玉瑞, 张文科   

  1. 吉林大学化学学院 超分子结构与材料国家重点实验室 长春 130012
  • 投稿日期:2013-12-28 发布日期:2014-01-22
  • 通讯作者: 张文科; Tel:0431-85159203; Fax:0431-85193421
  • 基金资助:


Single Molecule Force Spectroscopy Investigation on Na2SO4-inducedConformational Transition of Single PNIPAM Chains

Xue Yurui, Zhang Wenke   

  1. State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012
  • Received:2013-12-28 Published:2014-01-22
  • Supported by:

    Project supported by the National Natural Science Foundation of China (Nos. 20640420622, 91127031, 21221063).

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