Hidden variety of biotin-streptavidin/avidin local interactions revealed by site-selective dynamic force spectroscopy.

By site-selective dynamic force spectroscopy realized with the combination of cross-linkers and anatomic force microscope with a force feedback system, we have revealed, for the first time, that the slight difference between the local structures of amino acid residues at the middle sites, SER45 and THR35 for streptavidin and avidin, respectively, strongly affects the microscopic reaction processes, i.e., the variation governs the type of bond as well as the fine structure of the potential landscape. For streptavidin, a bridged or direct hydrogen bond is induced depending on the molecular structure in the buffer solution. For avidin, in contrast, only a direct hydrogen bond is observed for all the buffer solutions used in the experiment. Since final functions in a system are realized through the assembly of local effects, the obtained results indicate the importance of analyzing the reaction processes with respect to the local structures of molecules, for further development of nanoscale functional devices.