Investigate The Effect Of Solvent And Side Chains On The Single-Chain Elasticity Of A Macromolecule By SMFS

The conformation and the interactions with solvent molecules of the polymer chain in solution is one of the most important components in polymer science. With the development of technology and the perseverance research, a series of results, which describe the conformation and the interactions with solvent molecules of the polymer chain, had been reported. However, the study of that on the single chain level is very rare. Recently, the appearance of single-molecule force spectroscopy (SMFS) technique provides a more direct means for study the polymer chain at the single-chain level.In the thesis, the atomic force microscope (AFM) based SMFS has been used to study the single-chain elasticity properties of several kinds of polymers under different conditions at the single-chain level, which includes the effects of water on the single-chain elasticity of poly(U) RNA, the effect of length and shape of side chains on the single-chain enthalpic elasticity of a macromolecule and the effect of the size of the solvent molecules on the single-chain entropic elasticity of a macromolecule. Based on these systemic investigations, the main conclutions can be summarized as follows:(1) Water, the dominant component in the physiological condition, is a complicated solvent, which greatly affects the properties of solute molecules. In this thesis, we utilize atomic force microscope-based single-molecule force spectroscopy to study the influence of water on the single-molecule elasticity of an unstructured single-stranded RNA (poly(U)). In nonpolar solvents, RNA presents its inherent elasticity, which is consistent with the theoretical single-chain elasticity calculated by quantum mechanics calculations. In aqueous buffers, however, an additional energy of 1.88 kJ/mol·base is needed for the stretching of the ssRNA chain. This energy is consumed by bound water rearrangement (Ew) during the chain elongation. Further experimental results indicate that the Ew value is uncorrelated to the salt concentrations and stretching velocity. The results obtained in an 8 M guanidine·HCl solution provide more evidence that the bound water molecules around RNA give rise to the observed deviation between aqueous and nonaqueous environments. Compared with synthetic water-soluble polymers, the value of Ew of RNA is much lower. The weak interference of water is supposed to be the precondition for the RNA secondary structure to exist in aqueous solution.(2) To investigate the effect of side chains on the single-chain enthalpic elasticity of polymers, several kinds of polymers with different length and shape of pendants or side chains (i.e., PG1?PG2?PVS?PBCPS?PGMA-g-PS44 and PGMA-g-PS105) have been studied by single-molecule AFM. We find that both length and shape of the side chains count:only the side chains that are both long and bulky (i.e., bulky dendrons of second or higher generation as side chains) affect the SCEE. Thus, only rare polymers have special SCEE. For vast majority of polymers, the SCEE is identical to that of PE, which means that the SCEE is determined by the nature of the C-C backbone. It is expected that this conclusion can also be popularized to all polymers with various backbones. This study is an important update to the understanding of polymers at the single-chain level.(3) Excluded-volume (EV) interaction, also known as EV effect, can drive the collapse of polymer chains in a polymer solution and promote crystallization of polymer chains. Herein we report, for the first time, the effect of EV interaction on the single-chain mechanics of a polymer, poly(ethylene glycol) (PEG). By using AFM-based single-molecule force spectroscopy, single-chain mechanics of a PEG chain has been detected in various nonpolar organic solvents with different molecule sizes. It is observed that the nonpolar solvents can be classified into two categories. In the small-sized organic solvents (e.g., tetrachloroethane and n-nonane), PEG presents its inherent elasticity, which is consistent with the theoretical single-chain elasticity from quantum mechanical calculations. However, in the middle-sized solvents (e.g., n-dodecane and n-hexadecane), the single-chain entropic elasticity of PEG has been influenced by EV interaction apparently, which indicates that the PEG chain tends to adopt a compact conformation in this condition. To stretch a PEG chain from a free state to a fully extended state, more energy (1.89 kBT/repeating unit) is needed in small-sized organic solvents than that in middle-sized organic solvents. It is expected that a partially stretched PEG chain would shrink to some extent when the solvent is changed from a middle-sized organic solvent to a small-sized one. Accordingly, a novel design of PEG-based single-molecule motor that works with solvent stimuli is proposed.