Dynamic force spectroscopy on soft molecular systems: improved analysis of unbinding spectra with varying linker compliance.

Dynamic force spectroscopy makes it possible to measure the breaking of single molecular bonds or the unfolding of single proteins subjected to a time-dependent pulling force. The force needed to break a single bond or to unfold a domain in a protein depends critically on the time dependence of the applied force. In this way the elastic response couples to the unbinding force. We have performed an experimental and theoretical examination of this coupling by studying the well-known biotin-streptavidin bond in systems incorporating two common types of linkers. In the first case biotin is linked by bovine serum albumin (BSA) and it is observed that this linker has a linear elastic response. More surprisingly we find that its force constant varies significantly between repeated force curves. It is demonstrated that by sorting the force curves according to the force constant of the linker we can improve the data analysis and obtain a better agreement between experimental data and theory. In the second case biotin is linked by poly(ethylene glycol) (PEG), which has a soft nonlinear elastic response. A numerical calculation of the unbinding statistics for the polymer system agrees quantitatively with experiments. It demonstrates a clear decrease in unbinding forces resulting from the polymer linker.