Measurements of single molecular affinity interactions between carbohydrate-binding modules and crystalline cellulose fibrils.

Combining atomic force microscopy (AFM) recognition imaging and single molecule dynamic force spectroscopy (SMDFS), we studied the single molecule affinity interactions between the carbohydrate-binding module (CBM) and plant cell wall cellulose using the CBM3a (from Clostridium thermocellum) and CBM2a (from Cellvibrio japonicus) functionalized AFM tips. The binding efficiencies of the CBMs to the cellulose were determined by the binding areas on the crystalline cellulose fibrils surface using the recognition imaging. Several dynamic and kinetic parameters, such as the reconstructed free energy change, energy barrier and bond lifetime constant, were also obtained based on the measured single molecule unbinding forces, which are used to illuminate the affinity of the CBMs binding to the natural and single cellulose surface from a totally different aspect. It was found that CBM3a has a little higher binding efficiency and affinity than CBM2a to both natural and extracted cellulose surfaces and both the CBMs have higher affinities to the natural cell wall cellulose compared to the extracted single cellulose. The in-depth understanding of the binding mechanisms of the CBM-cellulose interactions of this study may pave the way for more efficient plant cell wall degradation and eventually facilitate biofuel production.