Exerting pulling forces in fluids by directional disassembly of microcrystalline fibres

Abstract

Bio-molecular polymerization motors are molecular systems that use supramolecular (de-)polymerization to mediate the conversion of chemical potential into useful mechanical work. Microcrystallization is ideally suited to reverse-engineer this strategy in artificial systems since self-organization of molecules in a crystal lattice resembles the process of polymer growth/disassembly but with increased predictability of molecular events in a highly ordered microscopic structure. With the intent to explore new chemo-mechanical transduction strategies, we developed a synthetic mechanically-active molecular system that can generate forces via controlled disassembly while being freely suspended in fluid. The amphiphilic monomer self-assembles into rigid, high-aspect ratio microcrystalline fibres with assembly regulated by a coumarin-based pH switching motif. In the structures formed, the crystal morphology determines the monomer reactivity at the interface, resulting in anisotropic etching of the fibres. This effect is used to exert directional pulling forces on microscopic beads adsorbed on the crystal surface through weak multivalent interactions. Mechanistic insights were provided by optical tweezers force spectroscopy, which quantified a stall force of 2.3 pN (+/- 0.1 pN) exerted by this ratcheting mechanism and the mechanical work produced by the disassembly of the fibres.