Bridging the Gap Between Single-Molecule Unbinding Properties and Macromolecular Rheology

Conformation and interactions between biological macromolecules are crucial for the mechanical properties of biological soft matter. In this chapter, the method and applications of the mechanical characteristics at the single-molecule level, from a fundamental point of view, are described as basis for understanding aspects of rheology. Atomic force microscope (AFM) and optical tweezers can be applied to investigate mechanical properties and interactions of molecules in the single molecular level. The force between two molecules as a result of specific and/or non-specific interactions can be determined as a function of distance between two molecules. Selected examples for interactions in macromolecules were highlighted based on observations by AFM-based force spectroscopy. This includes polysaccharide pairs such as interactions among hydrophobically modified hydroxyethyl cellulose (HMHEC), between protein polysaccharides and mucin–alginate. The mechanism of physically cross-linked hydrogel formation, HMHEC–amylose gel and alginate gels was also discussed based on single molecular pair interactions. For slower bond formation systems, which may not be capable with normal dynamic force spectroscopy, slide contact force spectroscopy can be applied. For slower dissociation rate, Dudko–Hummer–Szabo model and Friddle–Noy–De Yoreo model can be used for the analysis as an extension of the Bell–Evans model. The relation between characteristic timescale of interaction estimated in the single molecular study and relaxation spectra in the mechanical properties obtained at the macroscopic scale is presented as a possible way forward in understanding the gap between the mechanical properties in macroscopic and microscopic scale.