The Protein at the End of the Tunnel

Nuclei in the core of stars, or on futuristic systems of quantum computing. Recently scientists at the Weizmann Institute of Science have been able to observe this quantum phenomenon in proteins found in various biological systems, including the human body.

These new observations suggest that the quantum tunneling process may play a larger role than previously thought in the activity of proteins. These surprising findings, published recently in the Proceedings of the American Academy of Sciences (PNAS), may have significant implications for biochemical research of essential biological processes based on electron transmission and on the possibility of bio-electronic developments.

Tunneling is a quantum process, usually observed in solid material under controlled laboratory conditions, often at very low temperatures and in much smaller dimensions than protein. In classical physics, matter particles can not pass through physical or energetic barriers. In quantum physics, however, there is a chance that particles will pass from one side of the checkpoint to the other, in a process known as tunneling. In proteins it is very difficult to observe this phenomenon because of the size of the protein molecules, and because they are flexible and interact with their environment.

Prof. David Kahan, of the Department of Materials and Interfaces, said that the study was created following strange results obtained a few years ago in an experiment conducted jointly by Prof. Mordechai Sheves of the Department of Organic Chemistry and Prof. Israel Pacht of the Immunology Department. The experiment examined the electrical conductivity of proteins and found that proteins conduct electricity far better than could be expected of these molecules, since electrical conductivity, as we know it and use it in electronics, is not the same as what happens in biological processes. The researchers saw electrons going to proteins in very diverse conditions, including at different temperatures and at different distances between the electrodes in the experiment. “It was very strange,” says Prof. Kahn. “Because the physics taught in high school shows that the intensity of the current decreases with distance and changes as a result of heating, there is only one known mechanism that can explain the lack of temperature effect we have seen, and this is quantum tunneling.”

A post-doctoral researcher, both members of the research team headed by Professors Cahn, Sheves and Fecht. The two designed and implemented an experimental system to search for quantum phenomena in biological molecules – a complex task that combines biology, electronics, chemistry and physics. The research group also included a theorist: Prof. Juan Carlos Quabas of the Autonomous University of Madrid.

The first hurdle they had to undergo was coping with “vibrating” protein molecules and fragility. Here comes the aid of proteins to conduct electrons at any temperature. The researchers understood that they could conduct the experiment at a very low temperature – freezing the proteins to about 15 degrees above absolute zero – which would eliminate most of the molecular vibration. The cooled proteins were delicately placed between two thin metal plates, with one end of each protein firmly anchored in a chemical bond to one metal plate, and the other end remained free to move, but slightly. A low voltage was then applied between the plates. The experimental system enabled the researchers to conduct electrons through the protein to the other metal plate, thus examining their behavior. According to the hypothesis, monitoring the patterns of fluctuations in the protein molecules revealed a unique signature for the tunnel.

New findings – new questions

The new findings are not consistent with conventional models, both in physics and in the field of protein research. “Proteins are supposed to be ‘bad’ in controlled quantum phenomena,” explains Prof. Kahn. However, in view of the findings, there was no escaping the hypothesis that tunneling occurs in some proteins as a way of coping with excess electrons that may damage the protein. Many proteins have active chemical groups that can function as intermediate stations, which can be reached by tunneling to “park” an excess electron until such a dangerous but essential “guest” can be passed on. Because tunneling appears to be an efficient way to transport electrons into or out of proteins, it may be involved in vital functions based on electron transfer, such as cellular respiration and photosynthesis.

“The experiment took several years, and we repeated the experiments again and again to make sure that the findings do indeed indicate a tunneling,” says Prof. Sheves. “At this point, we have no choice but to accept the fact that these evidence does indicate that electrons travel through proteins through tunneling, which suggests that they do so at room temperature and raise more questions than answers. With regard to activity in our bodies, and even to point out new directions for creating interfaces between electronic systems and biological systems. “