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

PHYS 157

Boris B. Akhremitchev, boris.a@duke.edu, 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.