Molecular machines have been designed that can fulfill roles such as molecular switches, motors, and brakes. The mechanical behavior of these constructs differs from their macroscopic counterparts. For example, the concept of a conventional liquid lubricant makes no sense at the molecular scale. Despite this, water appears to have an accelerating effect on the movement of molecular machines based on hydrogen-bonded components, as described in the September 1 online edition of Nature Chemistry (DOI: 10.1038/NCHEM.1744).
M.R. Panman of the University of Amsterdam, E.R. Kay of the University of Edinburgh, D.A. Leigh of Edinburgh and the University of Manchester, and their colleagues studied a rotaxane “molecular shuttle” in which a peptide-based macrocycle can bind to either of two hydrogen accepting “stations.” In the neutral state, the succinamide site is the more stable position, but a short pulse of UV light excites the napthalimide group to a radical anion and induces the macrocycle to switch places.
The research team found that an addition of just 3% deuterated water to the system brought about a twofold increase in the rate of movement, while other hydrogen bonding solvents such as methanol, ethanol, and butanol had an insignificant or negative effect. A similar effect was also found for the thermal rotation of the macrocycle around the succinamide site, where the rotation rate could be doubled by adding 5% deuterated water.
Possible straightforward explanations for this phenomenon such as changes in viscosity and solvent polarity could be ruled out as the solvent properties do not correlate with the observed trends. Instead, the researchers propose that the ability of water to form a three-dimensional hydrogen-bonded structure is key, as this can stabilize the detached state of the macrocycle and increase the probability of it leaving a binding site. The small size of the molecule also allows the structure to quickly rearrange as the parts reposition. Evidence for this is provided by observation of hydrogen bonding between the water and the macrocycle peptide linkages in infrared spectroscopy.
This research provides useful insight into the translation of biological motors, where it has already been speculated that water plays an important role in “lubricating” protein folding and molecular rotors. While it is not possible to study biological systems in solvents other than water, synthetic machines such as these are able to put the principle to the test in a range of different solvents.
The structure and operation cycle of the molecular shuttle in acetonitrile. In the uncharged state, the macrocycle (blue) binds to the succinamide (green) site, but laser excitation of the napthalimide (red) site to a radical anion renders it a better hydrogen bond acceptor and induces the macrocycle to change places. Reproduced with permission from Nature Chem. 8 (2013), DOI: 10.1038/NCHEM.1744. © 2013 Macmillan Publishers Ltd.
