The Interaction Between Macromolecules And Small Molecules Studied By Single Molecule Force Spectroscopy

Both polymer science and supramolecular science include one important research subject---inter- and intramolecular interaction. The investigation of inter- and intermolecular interactions at molecular lever will provide useful information for the molecular design of high performance polymer materials and supramolecular system. In the recent years, atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS) has proven to be a versatile platform for studying the minute force in polymers as well as in supramolecular systems. In chapter 1, the basic principle of AFM-based SMFS are specially introduced, including the immobilization method of the samples to be investigated, the experimental control of single molecule detection and the standard for the distinguishment of single molecule detection. Lastly, some recent progresses in SMFS on polymers are briefly summarized. Utilizing SMFS, this dissertation attempted to detect directly the interactions between macromolecules and small molecules at single molecular level.In chapter 2, we have investigated the interaction between poly(N-vinyl-2-pyrrolidone) (PVPr) small molecules, such as water and iodine using SMFS. The force-extension curve (force curve) of PVPr in water is markedly deviated from that obtained in ethanol or tetrahydrofuran, which is resulted from the water bridge between pyrrolidone rings. Moreover, we have comparatively studied the force signals of PVPr-I2 and PVPr in an aqueous solution of KI or KI3, and found that only KI3 influences the elastic property of PVPr dramatically. These experimental results indicate that there exists a specific interaction between PVPr and KI3, which is also supported by Fourier transform infrared data. By the integration of the deviated area between the force curve and the modified freely jointed chain fitting curve, we estimate that the energy needed to destroy the interaction between PVPr and water is 5.3 kBT and between PVPr and KI3 is 3.6 kBT per repeating unit, respectively.In chapter 3, we investigated the force spectroscopy of single amylose chain in a media of crowded environment by using surfactant molecules as crowders. The plateau feature, which is characteristic of the force spectroscopy of amylose in water, disappears in the media of high concentration of CTAB or SDS. The SMFS experimental results indicate that the large number of micelles affects the force-induced conformational transition of pyranose rings of amylose chain and results in the change of force spectroscopy of amylose chain in high concentration of surfactants.In chapter 4, we investigated the unwinding process of the surfactant-induced helical structure of carboxymethyl amylose (CM-amylose) at the single molecular level using SMFS. In doing so, the nanomechanical fingerprint for the conformational transition of CM-amylose at about 270 pN is used as an indicator for identifying the single chain elongation. The flat plateau is attributed to the unwinding process of CTAB-induced helical structure of CM-amylose. By Gaussian fitting, the center value for plateau height histogram is 17 pN, which reflects the force needed to unwind the CTAB-induced helical structure of CM-amylose.In chapter 5, we have directly measured the intercalation interaction between acridine and dsDNA using SMFS. To implement the single molecule experiment, we immobilized dsDNA onto gold substrate through a thiol-Au bond, and attached acridine on AFM tip through a flexible poly(ethylene oxide) chain. The interaction between acridine and dsDNA is broken by force of 36 pN at a loading rate of 5.0 nN/s. The most probable rupture force between acridine and dsDNA is dependent on the loading rate. The combination of experimental data with the theoretical model clearly indicates that the intercalation interaction betweenacridine and dsDNA originates from the combination of different forces with longand short range.