Artificial Molecular Ratchets: Tools Enabling Endergonic Processes

Abstract

Non‐equilibrium chemical systems underpin multiple domains of contemporary interest, including supramolecular chemistry, molecular machines, systems chemistry, prebiotic chemistry, and energy transduction. Experimental chemists are now pioneering the realization of artificial systems that can harvest energy away from equilibrium. In this tutorial review, we provide an overview of artificial molecular ratchets: the chemical mechanisms enabling energy absorption from the environment. By focusing on the mechanism type – rather than the application domain or energy source – we offer a unifying picture of seemingly disparate phenomena, which we hope will foster progress in this fascinating domain of science.

Kinetic Asymmetry and Directionality of Nonequilibrium Molecular Systems

“…Recently rhodopsin molecules were shown to undergo repeated light-driven rotation. [3] Feringa and colleagues reported the first synthetic molecular rotor capable of performing repeated 360°rotations [4] in 1999, and progress in designing different molecular machines in the intervening years has been remarkable, [5][6][7] with several excellent review [8][9][10][11][12][13][14][15][16][17][18] articles describing this progress. Outstanding recent examples of molecular rotors include ones driven by light, [19] by electricity or modulation of the redox potential, [20] and by chemical catalysis.…”

Stepwise Operation of a Molecular Rotary Motor Driven by an Appel Reaction

“…Artificial molecular motors have been developed that transduce energy from various sources (including chemical reagents, , light, and electricity) into directionally biased dynamics. The earliest synthetic chemically driven motor molecules required up to 8-step reaction sequences for their operation. , Simpler “pulsed fuel” systems , (one fuel pulse required for each cycle of the motor) were subsequently developed, and the first autonomous artificial chemically fueled motor molecules based, like biomolecular motors, on the motor’s catalysis ,,− , of fuel-to-waste reactions have recently been realized .…”

Switched “On” Transient Fluorescence Output from a Pulsed-Fuel Molecular Ratchet

“…An energy ratchet switches between different potential energy surfaces to reverse the relative depths of pairs of energy minima and the relative heights of pairs of energy maxima (Figure 2B). 1,7 This inexorably leads to statistically dictated directional transport without the position of the substrate needing to affect the kinetics of transport. 1,7 In the model energy ratchet (Figure 2A), the molecular thread contains dibenzylamine (green)/dibenzylammonium (blue) and Nmethyltriazolium (orange) binding sites for the crown ether.…”

“…Biology uses chemically fueled ratcheting − in numerous metabolic processes, including molecular-level information processing − and transportation . Chemically powered ratcheting also forms the basis of a number of artificial molecular motors, − pumps, ,− and dissipative materials. ,,− A molecular energy ratchet ,,− was recently reported in which a crown ether (a “reading head” that responds to particular “symbols” encoded on a molecular tape) is pumped from solution onto a molecular strand by a pulse ,,− of a chemical fuel .…”

“…An energy ratchet switches between different potential energy surfaces to reverse the relative depths of pairs of energy minima and the relative heights of pairs of energy maxima (Figure B). , This inexorably leads to statistically dictated directional transport without the position of the substrate needing to affect the kinetics of transport. , In the model energy ratchet (Figure A), the molecular thread contains dibenzylamine (green)/dibenzylammonium (blue) and N -methyltriazolium (orange) binding sites for the crown ether. Bulky groups at each end restrict the passage of the ring onto and off of the thread.…”