Loading Rate Independence of Single Molecular Unbinding Forces at a Higher Force Regime Studied by AFM

Forced unbinding studies provide valuable information about the interactions between molecules; however, these studies tend to be limited to a low-force regime (tens of piconewtons scale). Using atomic force microscopy, forced unbinding events in the higher force regime (hundreds of piconewtons) were observed between a model enzyme/inhibitor system consisting of 5’-Methylthioadenosine/S-adenosylhomocysteine nucleosidase and its transition state analogue inhibitor Homocysteinyl Immucillin A. The enzyme and ligand molecules were tethered to the sample surface and tip, respectively, using a polyethylene glycol diglycidyl ether linker. The intermolecular force was measured as a function of the distance between the two surfaces and the data were analyzed to find the relationship between the unbinding forces and the rate of force applied per unit time, or loading rate. The number of the polyethylene crosslinker molecules involved was determined using a freely jointed chain and this model that allowed the unbinding force versus loading rate for the individual molecular unbinding events to be calculated. Interestingly, the unbinding forces for individual chains were not dependent on loading rate, in contrast to previous observations in the low-force regime. To further explore the unbinding mechanism further, the data in the high-force regime was converted into intermolecular force-distance curves. The result suggested that the observed loading rate independence originated from the elastic deformation characteristic of the unbinding process.