Molecular machines are very common in nature, for example - transport proteins that enter and leave cells and proteins that help with metabolism. In order to develop artificial molecular machines, scientists must properly understand the principles governing the laws of motion that exist in this molecular/nanometric world.
In order to face this challenge, a research team from the University of California (Riverside) tested a family of molecular machines that "walk" on a smooth metal surface. They tested both two-legged and four-legged machines.
"We created a four-legged horse-like structure in order to study how molecular machines are able to handle the movement of multiple parts," said Ludwig Bartels, professor of chemistry. "Several years ago, we discovered how it is possible to move carbon dioxide particles along a surface in a straight line using a molecular machine with two legs that move one after the other. In our new research, we wanted to make a structure capable of carrying more cargo - that is, one that would require more legs. However, if the structure will have more than two legs, how will it organize their movement?"
The researchers discovered that the four-legged compounds use a walking form of striding - the two legs from the same side of the mare move together, and then the two legs from the opposite side of the mare. The structures they developed move correctly in a straight line, and do not veer to the side or fall off course. The researchers also performed a simulation of structures that move in the form of tapping, in which the two legs that are diagonally opposite to each other move together, and found that this form of movement is too problematic to be stable.
After the researchers understood how particles move, they turned to another fundamental question regarding molecular machines: do particles - or part of them - simply pass through barriers that originate from the roughness of the surface on which they move?
"If this is indeed the case, it will be a fundamental deviation from the laws of motion in the macroscopic world and this will lead to a great acceleration of the motion," said the researcher. "It will be like driving on a bumpy road when the wheels of your car go through the bumps instead of going over them. It is known that four-legged machines enable such behavior for very light particles such as electrons and hydrogen atoms, however - the question arises, will this also be realized in large particles?"
The researchers varied the temperature in their experiments to provide the molecular machines with varying amounts of energy and tested how the speed of the machines changed depending on this. They discovered that a two-legged machine is able to take advantage of the "tunneling" phenomenon to quickly pass through surface bumps. However, a machine with four legs (or more) is unable to take advantage of the tunneling method; The researchers discovered that although such a machine is indeed able to organize the movement of its legs in the form of a stride, it is not able to move using the tunneling method. That is, if you are interested in transporting cargo, even on the smallest scale, you need a light-weight and agile bipedal structure," the researcher points out. "Larger moving structures can carry more cargo, but since they are unable to effectively utilize the tunneling phenomenon, the result will be their slow movement. Is this finding supposed to relax the hands of the researchers? Not really, since the field of molecular machines is still in its infancy. As a matter of fact, there is also an advantage in the slow movement of mares, since in this situation it is possible for us to notice more precisely the movement itself and learn from the individuals how to control it."
The research findings were published in the scientific journal Journal of the American Chemical Society.
In the next step, the researchers plan to develop molecular machines whose movement is controlled by light. Today, feverish research is being done in the field of molecular machines due to their biological function and due to their medical/therapeutic importance. For example, people with heartburn are treated with hydrogen-pump-inhibiting drugs, which slow the pumping action of biological molecular machinery and thus reduce the acidity levels in the stomach.
"In general, the picture that scientists have of such biological molecular machines completely ignores the possibility of tunneling," said the lead researcher. "Our research corrects this perception, which may lead to novel ways to control or correct the behavior of biological molecular machines."
Artificial molecular machines are important to the microelectronics industry due to its pursuit of ever smaller active components for computers and information storage. In principle, artificial molecular machines will also be able to operate inside cells as a substitute for their biological equivalents and will significantly improve medicine.
The scientists used a variety of compounds in their research: as bipedal components - anthraquinone and pentaquinone; as four-legged components - pentacenetetrone and pentacenetetrone and by advanced spectroscopic methods.
