Work theoretically extracted from molecular motor

Recently published computational work from the laboratories of Feynman Prize winners Mark A. Ratner (Theoretical, 2001) and George C. Schatz (Theoretical, 2008) has demonstrated a molecular motor in which work is done on a scanning probe microscope tip by a change in molecular shape caused by light. Although light activated molecular motors have been reported before (see for example “Molecular machine switches magnetic state at room temperature“), this result is noteworthy because the motor performs mechanical work on an external device. The accomplishment is described at, “Researchers turn photons into work using DNA“:

By using light to change the elasticity of a DNA molecule, scientists have designed a molecular motor that can turn light into mechanical work. Unlike most previously reported molecular motors, the proposed setup involves an atomic force microscope, which acts as an interface with the outside world and enables the work to be extracted. …

“The greatest significance of this work is showing how the structure of DNA can be exploited to amplify the transduction ability of azobenzene in a setup in which the work can be extracted,” Schatz told “To our knowledge, this is the first proposed DNA-based molecular motor with an interface to the outside world.” …

From the abstract of “DNA-Based Optomechanical Molecular Motor”:

An azobenzene-capped DNA hairpin coupled to an AFM is presented as an optically triggered single-molecule motor. The photoinduced trans to cis isomerization of azobenzene affects both the overall length of the molecule and the ability of the DNA bases to hybridize. Using a combination of molecular dynamics simulations and free energy calculations the unfolding of both isomers along the O5′−O3′ extension coordinate is monitored. The potentials of mean force (PMFs) along this coordinate indicate that there are two major differences induced by photoisomerization. The first is that the interbase hydrogen bond and stacking interactions are stable for a greater range of extensions in the trans system than in the cis system. The second difference is due to a decreased chain length of the cis isomer with respect to the trans isomer. These differences are exploited to extract work in optomechanical cycles. The disruption of the hairpin structure gives a maximum of 3.4 kcal mol−1 of extractable work per cycle with an estimated maximum efficiency of 2.4%. …

This is the same reversible azobenzene cis-trans isomerization used in previously published, experimentally demonstrated, molecular motors, but in this case, the effect is amplified because the shorter cis isomer destabilizes the DNA base pair, rendering the DNA hairpin less stiff. A clever idea that we look forward to seeing experimentally demonstrated.

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