Molecular Tensile Machines: Anti-Arrhenius Cleavage of Disulfide Bonds

Molecular tensile machines are bottlebrush molecules where tension along the backbone is self-generated due to steric repulsion between the densely grafted side chains. Upon adsorption onto a substrate, this intrinsic tension is amplified to the nanonewton range depending on the side chain length, grafting density, and interaction with the substrate. In this paper, bottlebrushes with a disulfide linker in the middle of the backbone were designed to study the effect of force and temperature on the scission of an individual disulfide bond. The Scission process was monitored on molecular length scales by atomic force microscopy. The scission rate constant has been shown to increase exponentially with bond tension but decrease with temperature. This anti-Arrhenius behavior is ascribed to the decrease of substrate surface energy upon heating, which overpowers the corresponding effects of thermal energy and temperature dependent pre-exponential factor. Quantitative analysis using the force-modified Arrhenius and transition state theory (TST) equations, respectively, was conducted to determine the dissociation energy, maximum rupture force, and activation barrier of a disulfide bond under tension.