Nanomechanical Properties of Proteins and Membranes Depend on Loading Rate and Electrostatic Interactions

Knowing the dynamic mechanical response of tissue, cells, membranes, proteins, nudeic acids, and carbohydrates to external perturbations is important to understand various biological and biotechnological problems. Atomic force microscopy (AFM)-based approaches are the most frequently used nanotechnologies to determine the mechanical properties of biological samples that range In size from microscopic to (sub)nanoscopic. However, the dynamic nature of biomechanical properties has barely been addressed by AFM imaging. In this work, we characterizethe viscoelastic properties of the native lightdriven proton pump bacteriorhodopsin of the purple membrane of Halobacterlum salMarum. Using force-distance curve (F-D)-based AFM we imaged purple membranes while force probing their mechanical response over a wide range of loading rates (from similar to 0.5 to 100 mu N/s). Our results show that the mechanical stiffness of protein and membrane increases with the loading rate up to a factor of 10 (from similar to 0.3 to 3.2 N/m). In addition, the electrostatic repulsion between AFM tip and sample can alter the mechanical stiffness measured by AFM up to similar to 40% (from similar to 0.8 to 1.3 Wm).These findings indicate that the mechanical response of membranes and proteins and probably of other blomolecular systems should be determined at different loading rates to fully understand their properties.