Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Abstract

In recent years ratchet mechanisms have transformed the understanding and design of stochastic molecular systems—biological, chemical and physical—in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale‐relevant concepts that underpin out‐of‐equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules ‘walk’ and track‐based synthesizers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts, and how dynamic (supra)molecular systems can be driven away from equilibrium. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, quantified by the outcome of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms, it is surely just as fundamentally important.

Photoswitchable Imines Drive Dynamic Covalent Systems to Nonequilibrium Steady States

“…Additionally, we are investigating applications of these dynamic covalent ratcheting systems in the context of systems chemistry. 39 ■ ASSOCIATED CONTENT…”

“…We are currently exploring how such systems can be rationally perturbed further from equilibrium to amplify the difference between the equilibrium and NESS states. Additionally, we are investigating applications of these dynamic covalent ratcheting systems in the context of systems chemistry …”

“…Systems that use light to modulate the reactivity of dynamic covalent motifs exhibit similarities to molecular ratchets, − particularly energy ratchets. ,,− Energy ratchets require spatial or temporal changes in conditions to perturb the equilibrium state of the system and are thus operated in a stepwise manner. ,, In contrast, information ratchets can operate continuously at a nonequilibrium steady state (NESS) . The first example of such an information ratchet was realized experimentally in 2007 by Leigh and coworkers, where a rotaxane with a photoisomerizable axle and a macrocycle functionalized with a photosensitizer could drive the position of the macrocycle on the axle away from thermodynamic equilibrium under photoirradiation .…”

Organic Synthesis Away from Equilibrium: Contrathermodynamic Transformations Enabled by Excited-State Electron Transfer

“…This kinetic asymmetry in the forward and reverse directions is enforced by the charge recombination steps, which gate the lifetimes of key intermediates and thus enable selective formation of the higher-energy products. These processes are reminiscent of related ratchet-type mechanisms commonly invoked in supramolecular chemistry to drive unidirectional behavior in the reactions of molecular machines and motors. , …”

“…These processes are reminiscent of related ratchet-type mechanisms commonly invoked in supramolecular chemistry to drive unidirectional behavior in the reactions of molecular machines and motors. 35,36 We later harnessed this photocatalytic alkoxy radicalmediated C−C bond β-scission chemistry for the depolymerization of hydroxylated polymers. The depolymerization of polymers lacking hydrolyzable functionality along their backbone (e.g., polyesters and polyamides) typically requires iterative cleavage of strong aliphatic C−C bonds and is therefore thermodynamically challenging.…”