Single molecule mechanochemistry of macromolecules

Single molecule force spectroscopy (SMFS) allows investigation of intramolecular or intermolecular interactions of macromolecules, thus potentially opening a new horizon for single-molecule mechanochemistry. In this article, we highlight the nascent, but rapidly developing SMFS field, focusing our discussion on recent advancements in atomic force microscopy based SMFS on macromolecules, leading to a bridge between force patterns and molecular and supramolecular structures. First, we introduce the principle of the SMFS experiment and the empirical criteria for single-chain stretching, and then discuss optimization of the experimental conditions for realization of single-molecule detection. Second, we discuss the force fingerprints of typical polysaccharides. It is found that the linkage methods of pyranose rings determine the fingerprint spectrum of the corresponding polysaccharide. The effects of side groups and tacticity of the polymer on single chain elasticity are also reviewed. It seems that larger side groups make the polymer chain stiffer, and a cis-tactic polymer is more flexible than a trans-tactic one. Third, we discuss the effects of intra- and intermolecular suprastructure on the mechanochemistry of polymers. These effects involve the unfolding of proteins, the unzipping of DNA and RNA, the stretching of the suprastructure containing polymers, and the detection of small molecule mediated supramolecular structure and polymer micelle. Finally, we discuss the relation between the force pattern and the adsorptive conformation and strength. Detaching of polymer loops from a substrate can produce a saw-tooth pattern; the peeling off of train-like structures from the substrate may give a long plateau.