Insights from an information thermodynamics analysis of a synthetic molecular motor

Information is a physical quantity, the realisation of which transformed the physics of measurement and communication in the latter half of the 20th Century. However, the relationship and flow between information, energy and mechanics in chemical systems and mechanisms remains largely unexplored. Here we analyze a minimalist experimental example of an autonomous artificial chemically-driven molecular motor - a molecular information ratchet - in terms of information thermodynamics, a framework that quantitatively relates information to other thermodynamic parameters. This treatment reveals how directional motion is generated by free energy transfer from the chemical to the mechanical processes involving the motor. We find that the free energy transfer consists of two distinct contributions that can be considered as “energy flow” and “information flow”. We identify the efficiency with which the chemical fuel powers the free energy transfer and show that this is a useful quantity with which to compare and evaluate mechanisms of, and guide designs for, molecular machines. The study provides a thermodynamic level of understanding of molecular motors that is general, complements previous analyses based on kinetics, and has practical implications for designing and improving synthetic molecular machines, regardless of the particular type of machine or chemical structure. In particular, the study confirms that, in line with kinetic analysis, power strokes do not affect the directionality of chemically-driven molecular machines. However, we also find that under some conditions power strokes can modulate the molecular motor current (how fast the components rotate), efficiency with respect to how free energy is dissipated, and the number of fuel molecules consumed per cycle. This may help explain the role of such conformational changes in biomolecular machine mechanisms and illustrates the interplay between energy and information in chemical systems.