Direct measurement of the viscoelasticity of adsorbed protein layers using atomic force microscopy

Thick layers of the protein lysozyme have been deposited on mica, and their force-distance hysteresis measured using atomic force microscopy in the presence of different salts. Sodium thiocyanate, which is known to lower the melting temperature of proteins and increase their solubility, increases lysozyme deformability and lowers the viscosity of the protein layer, compared with sodium chloride. Sodium phosphate, known to raise the melting temperature and lower the solubility, decreases deformability and increases the viscosity.