(4) Molecular interactions measured by nanometer-sized probes using subpiconewton intermolecular force microscopy

Michio HIROSHIMA¹ and Makio TOKUNAGA (¹Department of Biophysical Engineering, Osaka University)

　Interaction between biological macromolecules involves several kinds of forces such as electrostatic forces, hydrophobic attractive forces, and so on. For a detailed understanding of the interactions between biological macromolecules, it is required to obtain information about the properties of these forces at the molecular level. We have refined atomic force microscopy (AFM) to improve the sensitivity of force detection and control of probe position. Force resolution of subpiconewton has been achieved, which is over 100-fold more sensitive than that of conventional AFM. In this study, we have measured the forces exerted on a nanometer-sized probe using this novel microscopy. Forces between hydrophobic surfaces and between weakly charged surfaces were measured in aqueous solutions.

　A long-range attractive force was observed between hydrophobic surfaces out to 150 nm with a decay length of 30 nm in water. Both the range and the decay length of the force were decreased with the increase of KCl concentrations or with the decrease of hydrophobicity of the surfaces. A linear relation was found between the decay length and the contact angle between the hydrophobic surfaces and drops of electrolyte solutions. This result suggests that the long-range attractive force results from the interaction between the hydrophobic surfaces and water.