The Investigation Of Interactions Between Ions And Several Typical Polymers By Single Molecule Force Spectroscopy

The interactions between small ions and macromolecules are important in chemical and biological systems. It is well known that there are various inorganic ions in a cell. The ions coexist with the macromolecules, such as DNA, RNA, and proteins, in vivo. They can also interact with those macromolecules to implement different functions. The most famous study on the interactions between ions and macromolecules is the Hofmeister effects, which is also called specific ion effects. Recently, scientists have studied the nature of specific ion effects with many methods, and proposed variety of theoretical models. However, there are still many disputes on the specific ion effects. That is to say, the true answer of problem have not been found yet.With the progress of science and technology, AFM has become a powerful tool in nanoscale measurements. The AFM-based single molecule force spectroscopy (SMFS) plays an important role in the study of the single-chain elasticity of macromolecules and the intramolecular and intermolecular interactions of macromolecule. In this thesis, AFM-based SMFS and the theoretical models are used to study the effects of the ion species and concentration on the single-chain elasticity of different macromolecules. The principle and applications of AFM-based SMFS, specific ion effects and the theories of dilute polyelectrolyte solution were introduced in details firstly. Then, the sensitivity of different polyelectrolytes in DI water and 1 M KC1 had been studied, which can be affected by the groups, hydrophilicity and the length of the side chain in aqueous solution. Whereafter, we studied the effects of ion species on the single-chain elasticity of PSSNa and then proposed the specific ion series at the single-molecule level. Finally, a typical neutral polymer PEO had been studied, which showed a similar single-chain elasticity behavior as a polyelectrolyte. Based on these investigations, the main conclutions can be summarized as follows:(1) In a nonpolar organic solvent, a single PSSNa chain shows the same inherent elasticity to the uncharged polymer with the same C-C backbone (PS), even though the side chains are different. By means of single-molecule AFM, the single-chain mechanics of a strong polyelectrolyte (PSSNa) in KCl aqueous solutions over almost whole concentration range have been studied. The M-FJC model has been used to describe the single-chain elasticity of PSSNa in KCl solutions with a parameter of single-chain modulus (K0). Along with the increase of the concentration of KCl from zero to almost the saturation concentration, a reentrant variation of K0 of single PSSNa chain can be observed. When [K+]is between 0.01 M to 3 M, the charges on the PSSNa backbone are almost completely screened, i.e., the PSSNa chain is virtually neutral in this case. Because K0 has a positive correlation with the net charge of the polymer chain, the increased K0 at very high KC1 concentrations (? 3.5 M) indicates that the chain is charged again. Due to the negative charges on the backbone of PSSNa, only the positively charged counterions (K+) can be adsorbed on the chain. Thus, the PSSNa chain should be positively charged when KCl concentrations > 3.5 M. That is, the charge inversion occurs in this case, which is induced by a monovalent salt. This finding may lay the foundation for the future applications of drug delivery and gene therapy.(2) PSSNa showed a more stable conformation in 0.01M?3M KCl, which means the suitable conformation for studing the specific ion effects is in this region (1M in this thesis).By the means of SMFS, we studied three polyelectrolytes (PAMPS, PSSNa and PVSK) with the same backbone in DI water and 1M KCl. The results indicate that the external salt will reduce the rigidity of the single polyelectrolyte chain by screening the electrostatic force between repeating units. The sensitivities of the polyelectrolyte in salt aqueous solution are different: PAMPS > PVSK > PSSNa. In aqueous solution, the rigidity of polyelectrolyte will be influenced by their side chains (special groups, hydrophilicity and the length).Furthermore, due to the similar aromatic structure with the ion channels, PSSNa has been chosen to study the specific ion effects on the single-chain elasticity.(3) Comparing with the single-chain elasticities of PSSNa in different ion species solutions, we found that ions can affect the single-chain elasticity of PSSNa through the influence on the hydrogen-bond network, which leads to the different energy consumption in water rearrangement. We proposed the first specific ion series at the single-molecule level:Na+ > K+ > Rb+ ? Cs+. Na+ is a structure maker ion, which can promote the formation of hydrogen bond and lead to the increase of bound water of polyelectrolyte. The energy consumption of water rearrangement is larger than that in K+. Whereas, Rb+ and Cs+ are structure breaker ions, which will break the hydrogen bond and lead to the decrease of bound water of polyelectrolyte. Therefore, the energy consumption of water rearrangement is smaller than that in K+. This is consistent with the classic theory of specific ion effects.(4) We have measured the single-chain mechanics of a neutral polymer, PEO, by SMFS in aqueous solution, aiming at studying the interactions among water, polymer and ions.When dissolved in water, the strong electronegativity of O atoms of PEO makes the backbone "charged". It can be inferred that water might induce a neutral polymer with a special structure to be charged in aqueous solution as a revulsive. Water molecules and ions are adsorbed on the backbone by electrostatic interactions. A significant conformational transitions, which is known as the fingerprint plateau of PEO, can be observed when the surrounding environment changes from organic solvents to water. We find the direct evidence, for the first time, that when salt presents in the solution, PEO behaves like a polyelectrolyte instead of a traditional neutral polymer at the single-molecule level.Furthermore, like a typical polyelectrolyte, the single-chain enthalpic elasticity of PEO is sensitive to the external salt concentration.