Although π–π interactions have been studied for several decades, the quantification of the strength of π–π interactions in a macromolecule remains a big challenge. Herein, we utilize single-molecule atomic force microscopy and steered molecular dynamics simulations to study the π–π interactions in polystyrene (PS). It is found that in high vacuum, the single-chain mechanics of PS differs largely from that of polyethylene (PE). Accordingly, the strength of intrachain π–π interactions in PS is estimated to be 0.7 kcal/(mol stack), which is much lower than that in a small-molecule system (benzene dimer, 2–3 kcal/(mol stack)). Further study shows that in high vacuum, there are two types of π–π stacking in the single PS chain, i.e., the every-other-moiety (E) type and the adjacent-moiety (A) type. Upon force stretching, a transition from E-type to A-type π–π stacking can be observed.

It is generally accepted that water is deeply involved in the structures and functions of DNA and proteins. For polysaccharides, however, the role of water remains poorly understood. Amylose and natural cellulose (NC) are two polysaccharides with similar molecular structures but different linkages (α or β) between the pyranose rings. In this study, the effects of H-bonds on the single-molecule mechanics and affinity for water of amylose and NC are explored by single-molecule atomic force microscopy (AFM) and molecular dynamics (MD) simulations, respectively. The experimental results show that the single-molecule mechanics of both amylose and NC are dependent on the solvent polarity. Accordingly, the status of H-bonds of each polysaccharide can be inferred. We find that the two polysaccharides present the same status of H-bonds in a given organic solvent: the intrachain H-bonds can be formed in a nonpolar solvent (nonane), while they are completely prohibited in a highly polar solvent (dimethyl sulfoxide, DMSO). However, the statuses of H-bonds differ largely in water, where NC can form more intrachain H-bonds than amylose. This finding, which is supported by MD simulations, indicates that NC is more hydrophobic than amylose at the single-molecule level. These results reveal that water is a special solvent for these two polysaccharides: Both mechanics and affinity for water of them can be effectively affected by water through regulation of the H-bonds. The present study provides new insight into the role of water (the key environment of organisms) in the structures and functions of polysaccharides.

On the basis of a study of a simplified model system and Set Theory, a novel pathway on the formation of polydopamine is proposed.

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