Does quantum tunneling play a role in the activity of biological molecules?

Prof. David Kahn, from the Department of Materials and Surfaces at the Weizmann Institute examined the issue and hypothesized that tunneling occurs in some proteins as a way of dealing with excess electrons that may damage the protein

When talking about quantum tunneling, one usually thinks of physical experiments under unique control conditions, of nuclear fusion in the cores of stars, or of futuristic systems of quantum computing. Recently, Weizmann Institute of Science scientists were able to observe this quantum phenomenon also in proteins found in various biological systems, including the human body. These new observations raise the possibility that the quantum tunneling process plays a more extensive role than previously thought in the activity of proteins. These surprising findings, recently published in the scientific journal "Records of the American Academy of Sciences" (PNAS), may have significant implications for biochemical research of essential biological processes based on electron conduction, as well as for the possibility of bioelectronic developments.

Tunneling is a quantum process, usually observed in solid matter under controlled laboratory conditions, usually at very low temperatures and in dimensions much smaller than a protein. In classical physics, matter particles cannot pass through physical or energetic barriers. In quantum physics, on the other hand, there is a chance that particles will pass from one side of the barrier 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, as well as because they are flexible and interact with their environment.

Prof. David Kahan, from the Department of Materials and Surfaces, says that the research was born out of strange results obtained a few years ago in an experiment conducted in collaboration with Prof. Mordechai Shevs from the Department of Organic Chemistry, and Prof. Israel Pecht from the Department of Immunology. The experiment tested the electrical conductivity of proteins, and it was discovered that proteins conduct electricity much better than could be expected from these molecules, since electrical conduction, as we know it and use it in electronics, is not similar to what occurs in biological processes. The researchers observed the conduction of electrons in proteins under 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 you learn in high school teaches that the strength of the current decreases with distance and changes due to heating (increases in semiconductors and decreases in metals). Therefore, there is only one known mechanism that can explain the lack of temperature effect we saw, and that is quantum tunneling."

Proteins are supposed to be 'bad' at controlled quantum phenomena. We repeated the experiment again and again to make sure that the findings clearly indicated tunneling. In the future, the findings may grow new insights regarding activity in our bodies, and even point to new directions for creating interfaces between electronic systems and biological systems"

Dr. Jerry Ferreiro, recipient of an Azrieli scholarship for post-doctoral researchers, and Dr. Yu Shi, also a post-doctoral researcher, both members of the research team led by Professors Kahn, Shabs and Pecht, participated in the current study. The two designed and implemented an experimental setup to search for quantum phenomena in biological molecules - a complex task that combines biology, electronics, chemistry and physics. The research group also included a theoretician: Prof. Juan Carlos Cuevas from the Autonomous University of Madrid.

The first hurdle they had to overcome was dealing with "vibrating" and fragile protein molecules. This is where the ability of proteins to conduct electrons at any temperature comes to their aid. The researchers realized that they could perform the experiment at a very low temperature - rapid freezing of the proteins to about 15 degrees above absolute zero - which would eliminate most of the molecular vibration. The cooled proteins were gently analyzed between two thin metal plates, where one end of each protein was firmly anchored by a chemical bond to one metal plate, and the other end remained free to move, but slightly. A low electrical voltage was then applied between the plates. The experimental setup allowed the researchers to conduct electrons through the protein to the other metal plate, thus examining their behavior. In accordance with the hypothesis, tracking the patterns of fluctuations in the protein molecules revealed a unique signature for tunneling.

New findings - new questions

The new findings do not agree with the accepted models, both in the field of 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 dealing with excess electrons that could damage the protein. Many proteins have active chemical groups that may function as intermediate stations, which can be reached by tunneling to "park" an excess electron until this dangerous but essential "guest" can be passed on. Since tunneling appears to be an efficient way to transport electrons within or out of proteins, it may be involved in essential functions that rely on electron transport, such as cellular respiration and photosynthesis.

"The experiment lasted several years, and we repeated the experiments again and again to make sure that the findings clearly indicate tunneling," says Prof. Shaves. "At this point we have no choice but to accept the fact that this evidence does indicate that the passage of electrons through proteins occurs through tunneling. The findings indicate that they do this at room temperature as well, and they raise more questions than answers. In the future, the findings of these experiments may grow new insights regarding activity in our bodies, and even point to new directions for creating interfaces between electronic systems and biological systems."

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