Using Catalysis to Drive Chemistry Away from Equilibrium: Relating Kinetic Asymmetry, Power Strokes, and the Curtin-Hammett Principle in Brownian Ratchets.

Chemically fueled autonomous molecular machines are catalysis-driven systems governed by Brownian information ratchet mechanisms. One fundamental principle behind their operation is kinetic asymmetry, which quantifies the directionality of molecular motors. However, it is difficult for synthetic chemists to apply this concept to molecular design because kinetic asymmetry is usually introduced in abstract mathematical terms involving experimentally inaccessible parameters. Furthermore, two seemingly contradictory mechanisms have been proposed for chemically driven autonomous molecular machines: Brownian ratchet and power stroke mechanisms. This Perspective addresses both these issues, providing accessible and experimentally useful design principles for catalysis-driven molecular machinery. We relate kinetic asymmetry to the Curtin-Hammett principle using a synthetic rotary motor and a kinesin walker as illustrative examples. Our approach describes these molecular motors in terms of the Brownian ratchet mechanism but pinpoints both chemical gating and power strokes as tunable design elements that can affect kinetic asymmetry. We explain why this approach to kinetic asymmetry is consistent with previous ones and outline conditions where power strokes can be useful design elements. Finally, we discuss the role of information, a concept used with different meanings in the literature. We hope that this Perspective will be accessible to a broad range of chemists, clarifying the parameters that can be usefully controlled in the design and synthesis of molecular machines and related systems. It may also aid a more comprehensive and interdisciplinary understanding of biomolecular machinery.

Insights from an information thermodynamics analysis of a synthetic molecular motor.

Information is physical, a realization that has transformed the physics of measurement and communication. However, the flow between information, energy and mechanics in chemical systems remains largely unexplored. Here we analyse a minimalist autonomous chemically driven molecular motor in terms of information thermodynamics, a framework that quantitatively relates information to other thermodynamic parameters. The treatment reveals how directional motion is generated by free energy transfer from chemical to mechanical (conformational and/or co-conformational) processes by 'energy flow' and 'information flow'. It provides a thermodynamic level of understanding of molecular motors that is general, complements previous analyses based on kinetics and has practical implications for machine design. In line with kinetic analysis, we find that power strokes do not affect the directionality of chemically driven machines. However, we find that power strokes can modulate motor velocity, the efficiency of free energy transfer and the number of fuel molecules consumed per cycle. This may help explain the role of such (co-)conformational changes in biomachines and illustrates the interplay between energy and information in chemical systems.

Chemical engines: driving systems away from equilibrium through catalyst reaction cycles.

Biological systems exhibit a range of complex functions at the micro- and nanoscales under non-equilibrium conditions (for example, transportation and motility, temporal control, information processing and so on). Chemists also employ out-of-equilibrium systems, for example in kinetic selection during catalysis, self-replication, dissipative self-assembly and synthetic molecular machinery, and in the form of chemical oscillators. Key to non-equilibrium behaviour are the mechanisms through which systems are able to extract energy from the chemical reactants ('fuel') that drive such processes. In this Perspective we relate different examples of such powering mechanisms using a common conceptual framework. We discuss how reaction cycles can be coupled to other dynamic processes through positive (acceleration) or negative (inhibition) catalysis to provide the thermodynamic impetus for diverse non-equilibrium behaviour, in effect acting as a 'chemical engine'. We explore the way in which the energy released from reaction cycles is harnessed through kinetic selection in a series of what have sometimes been considered somewhat disparate fields (systems chemistry, molecular machinery, dissipative assembly and chemical oscillators), highlight common mechanistic principles and the potential for the synchronization of chemical reaction cycles, and identify future challenges for the invention and application of non-equilibrium systems. Explicit recognition of the use of fuelling reactions to power structural change in catalysts may stimulate the investigation of known catalytic cycles as potential elements for chemical engines, a currently unexplored area of catalysis research.

A catalysis-driven artificial molecular pump.

All biological pumps are autonomous catalysts; they maintain the out-of-equilibrium conditions of the cell by harnessing the energy released from their catalytic decomposition of a chemical fuel1-3. A number of artificial molecular pumps have been reported to date4, but they are all either fuelled by light5-10 or require repetitive sequential additions of reagents or varying of an electric potential during each cycle to operate11-16. Here we describe an autonomous chemically fuelled information ratchet17-20 that in the presence of fuel continuously pumps crown ether macrocycles from bulk solution onto a molecular axle without the need for further intervention. The mechanism uses the position of a crown ether on an axle both to promote barrier attachment behind it upon threading and to suppress subsequent barrier removal until the ring has migrated to a catchment region. Tuning the dynamics of both processes20,21 enables the molecular machine22-25 to pump macrocycles continuously from their lowest energy state in bulk solution to a higher energy state on the axle. The ratchet action is experimentally demonstrated by the progressive pumping of up to three macrocycles onto the axle from bulk solution under conditions where barrier formation and removal occur continuously. The out-of-equilibrium [n]rotaxanes (characterized with n up to 4) are maintained for as long as unreacted fuel is present, after which the rings slowly de-thread. The use of catalysis to drive artificial molecular pumps opens up new opportunities, insights and research directions at the interface of catalysis and molecular machinery.

Coordination Nanosheets Based on Terpyridine-Zinc(II) Complexes: As Photoactive Host Materials.

Photoluminescent coordination nanosheets (CONASHs) comprising three-way terpyridine (tpy) ligands and zinc(II) ions are created by allowing the two constitutive components to react with each other at a liquid/liquid interface. Taking advantage of bottom-up CONASHs, or flexibility in organic ligand design and coordination modes, we demonstrate the diversity of the tpy-zinc(II) CONASH in structures and photofunctions. A combination of 1,3,5-tris[4-(4'-2,2':6',2″-terpyridyl)phenyl]benzene (1) and Zn(BF4)2 affords a cationic CONASH featuring the bis(tpy)Zn complex motif (1-Zn), while substitution of the zinc source with ZnSO4 realizes a charge-neutral CONASH with the [Zn2(µ-O2SO2)2(tpy)2] motif [1-Zn2(SO4)2]. The difference stems from the use of noncoordinating (BF4-) or coordinating and bridging (SO42-) anions. The change in the coordination mode alters the luminescence (480 nm blue in 1-Zn; 552 nm yellow in 1-Zn2(SO4)2). The photophysical property also differs in that 1-Zn2(SO4)2 shows solvatoluminochromism, whereas 1-Zn does not. Photoluminescence is also modulated by the tpy ligand structure. 2-Zn contains triarylamine-centered terpyridine ligand 2 and features the bis(tpy)Zn motif; its emission is substantially red-shifted (590 nm orange) compared with that of 1-Zn. CONASHs 1-Zn and 2-Zn possess cationic nanosheet frameworks with counteranions (BF4-), and thereby feature anion exchange capacities. Indeed, anionic xanthene dyes were taken up by these nanosheets, which undergo quasi-quantitative exciton migration from the host CONASH. This series of studies shows tpy-zinc(II) CONASHs as promising potential photofunctional nanomaterials.