Characterization of the potential of mean force between single nonpolar molecules in water by dynamic force spectroscopy

PHYS 157

Boris B. Akhremitchev,, Department of Chemistry, Duke University, Box 90346, Durham, NC 27708

An experimental single-molecule force spectroscopy approach is developed to measure pairwise interactions between non-polar solutes in water.  In this approach the non-polar molecules are tethered to the solid substrate and to the sharp probe of the atomic force microscope. The use of long (~30 nm) water-soluble tethers facilitates formation of single “hydrophobic bonds” between tethered molecules.  In experiments the increasing load forces the molecular bond to rupture.  The rupture forces are measured as a function of the probe velocity.  These data are used to extract kinetic parameters characterizing the potential of mean force between non-polar solutes in water.  Experimental measurements of pairwise interactions between the symmetric pairs of fullerenes, alkanes of different sizes (from dodecane to octadecane) and hydrophobic 12-mer fragment of alpha-synuclein are demonstrated.  The systematic errors typical to force-spectroscopy measurements are considered in the data analysis.  Measurements with fullerene molecules indicate that the transition state is separated from the equilibrium state by a distance of ~0.4 nm that is larger than a size of water molecule.  Measurements with alkanes and peptides hint at possible conformational transitions in individual molecules upon dissociation from the dimeric state.