Molecular machines that seem to "walk" in living organisms transporting proteins between cells are the subject of a new study by University of California, Riverside researchers who hope to find out more about how these remarkable machines behave, in a development that could lead to important breakthroughs in medicine and the manufacturing of electronic devices.

The team investigated a class of molecular machines that, bizarre as this may sound, travel along flat surfaces with the aid of two (anthraquinone and pentaquinone) or four (pentacenetetrone and dimethyl pentacenetetrone) leg-like structures. They do this by carefully coordinating the movement of their appendages, effectively "walking" the distance as they carry their loads from cell to cell.

An artificial walking molecular machine would be of great value to the electronics industry and, more importantly, in the medical field, where the manipulation of molecular machines is already taking place to some degree. Patients affected by gastroesophageal reflux disease, for instance, are prescribed proton pump inhibitors that slow the pumping action of biological molecular machines, which reduces stomach acid levels.

But in order to build a controllable molecular walking machine, scientists first need to understand more about the mechanics that govern their movement. To try and achieve this, back in 2008, the UC Riverside scientists built a "two-legged" molecule capable of transporting small loads that was able to move relatively quickly along a metallic surface.

This time, looking to carry heavier loads, the team built a "four-legged" molecular structure in a shape resembling a horse, provided energy by raising the surrounding temperature, and then proceeded to study its movements. Apparently the structure, as with all other quadrupedal molecules, seems to prefer a pacing gait — with the legs on one side moving together, followed by the legs on the other side — to trotting (diagonally opposite hooves moving together), which seems to distort the molecule too much from its original shape.

Next, the researchers tried to established whether quantum tunneling, a phenomenon that is known to affect small particles like electrons and hydrogen atoms, could also affect the much bigger molecules. And indeed, they did find that the two-legged machines could tunnel right through barriers along the way thanks to the wonders of quantum mechanics, even though their four-legged counterparts seemed too bulky and, despite the extra legs, too slow to take advantage of the phenomenon.

Despite this minor setback the researchers did not despair, as the development of artificial molecular machines is just at its very beginning stages and future developments may well make tunneling possible ever for bigger molecules. Next, the researchers plan to focus their efforts on developing molecular machines whose motion can be controlled by light.

The research was supported by a grant from the Department of Energy and the National Science Foundation. The study appeared online last week in the Journal of the American Chemical Society.