Nature’s exquisite designs are a constant source of inspiration for materials scientists, yet our ability to replicate biological interactions in engineered systems is often limited. Biological systems exist in perfectly balanced environmental conditions that regulate their functionality. A major obstacle in exploiting biological interactions in macroscale materials is a lack of understanding of how environmental changes affect the intrinsic molecular functionality of the biological unit, ultimately impacting macroscale behaviour.<br/>From a material design standpoint bacterial adhesins, cell surface proteins allowing bacteria to bind to, and colonise, a host, are particularly interesting. Many of these adhesins exhibit so-called ‘catch bond’ behaviour. Essentially, the adhesive force of their receptor-ligand complexes increases at high shear, in contrast to traditional ‘slip’ bonds, which rupture under high shear. Transplanting catch bond-forming protein complexes onto polysaccharide chains opens up the possibility of creating bio-based hydrogel networks with unique tensile properties. Specifically, networks that display both shear-thinning and shear-thickening properties depending on the applied shear.<br/>Taking the example of serine-aspartate repeat protein G (sdrG) from Staphylococcus epidermidis and its ligand fibrinogen beta strand (FgB), we explore how the receptor-ligand complex behaves as we move from the molecular to the macroscale. First, sdrG-FgB interactions were probed using single molecule force spectroscopy to understand fundamental molecular behaviour. The mesoscale was emulated by expressing sdrG on yeast cells and probing interactions with FgB coated surfaces using spinning disk microscopy. Finally, the sdrG-FgB complexes were embedded into hydrogel networks and their macroscale behaviour examined by rheology, allowing us to build an understanding of the impact of protein orientation, chemical environment, cooperativity and stress dissipation in sdrG-FgB complexes across multiple lengthscales.<br/>Examining how protein behaviour and macroscale materials properties change as a function of receptor-ligand complex density and environmental complexity allows more accurate development and manipulation of adhesin-derived responsive biomimetic materials. Thus, leading to a greater understanding of the nano-macroscale relationships in complex multiphase systems and enhanced strategies for the translation of biological interactions into macroscale materials.

CatchGel: A molecular to macroscale investigation of catch bond-crosslinked polysaccharide hydrogels

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ACS Spring 2023

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