Life at the End of Quantum Tunnels

Recently a biochemistry student told me that her classmates looked like they had seen a ghost when their professor seemingly took a left turn from a lecture on cellular respiration and started to discuss quantum tunnelling. But this 90-year discovery keeps surfacing in different contexts, reminding us that without the tunnelling effect, there would be no life in the universe.

Part of the lecture focused on iron–sulfur clusters, which play a role in the oxidation-reduction reactions of mitochondrial electron transport. The clusters are part of four protein complexes that sequentially shuttle electrons. The latter are ultimately gained from the breakdown of food molecules and are destined for oxygen. In so doing, protons are consumed inside the mitochondrial membrane while others are pumped out, creating a potential difference that helps motor the synthesis of adenosine triphosphate (ATP). Then ATP goes on to facilitate a host of energy-requiring reactions that keep an organism alive.

ElectronFlow
Each green arrow represents an electron jump due to quantum tunnelling.   http://www.pnas.org/content/107/45/19157/F2.large.jpg

But each time an iron cluster transfers an electron, it does so against a potential energy barrier. How does it do it? Because of the wave-like properties of a tiny particle like the electron, when it’s up against a thin-enough barrier, such as the 2.2 to 3.0 angstrom gaps (0.22 to 0.30 nanometers) shown in the diagram, there is a small but non-zero probability that the electron will be in the gap, and more importantly, also beyond it.  The best way to convince yourself that quantum tunnelling is physically possible is to go through the math and physics, and if you’re interested, it’s found here.  The author does not show every tedious algebraic step, but if you get stuck, I will gladly help in the comments section. It’s great fun while the laundry is being done.

Life involves a struggle against entropy made possible by a continuous energy source. For the planets and presumably moons that harbour life, the most important energy source is fusion from the sun. If you are like me in that you once assumed that the prodigious gravitational force at the core of a sun could provide hydrogen atoms with sufficient energy to overcome Coulombic repulsion and bring about fusion,

5-13-Nuclear-Fusion.jpg
Image credit: E. Siegel

then you were also incorrect. It turns out that the kinetic energies are too small by a factor of 1000. So how does fusion take place? Like electrons in iron clusters, hydrogen atoms, although more massive, are small enough, and thanks to gravity, close enough to overcome the thousandfold barrier working against them. So quantum tunnelling is ultimately working with gravity to make stars shine.

The fact that tunnelling probability decreases steeply with lower thermal velocities extends the duration of smaller stars, those weighing less than 1.5 solar masses. This is important in that it gives life enough time to evolve in solar systems with appropriate conditions. One of the prerequisites of life, we imagine, is the presence of water on the surface of a moon or planet. Whether water is out-gassed or brought in via a comet or asteroid, it has to be first synthesized in molecular clouds according to this reaction between molecular hydrogen and hydroxyl radicals:

OH + H2  →  H + H2O

The extremely cold temperatures combined with adsorption on dust particles create boundaries small enough for quantum tunnelling to allow the production of molecular hydrogen from its atomic counterparts. There is even evidence that the hydroxyl reaction itself benefits from the same phenomenon.

From deep space back to our bodies, can tunnelling cause unwelcome changes in the DNA molecule? In the double helix or “twisted ladder” of DNA, each nucleotide of one strand of the ladder is attracted to its complement on the other strand by means of a hydrogen bond. A hydrogen bond consists of a lone pair of electrons from one nucleotide attracted to the hydrogen bonded to an oxygen or nitrogen atom of the nucleotide on the other side of the strand.

c5cp00472a-f1_hi-res.gif
from Modelling Proton Tunnelling in the Adenine–Thymine Base Pair
A. D. Godbeer , J. S. Al-Khalili * and P. D. Stevenson

But there is a small possibility that the proton (hydrogen without electrons) can overcome the potential energy barrier and end up bonded to the hydrogen-less atom on the other strand. If the effect would be common enough, it could lead to a mutation. It should be noted that this a very active area of research and these authors have concluded that, at least in the adenine-thymine base pair, tunnelling does not occur. Less controversial is the ideas that quantum tunnelling plays a key role in the repair of DNA from ultraviolet damage, specifically in the electron-transfer needed to undo the dimerization of pyrimidines.

If those shocked biochemistry students read this blog, I am not sure that it would erase the “seen-a-ghost” expression from their faces. As educators we don’t often empathize enough with their survival-mode of trying to focus on the “essentials” that will get them through a given course. Quantum tunnelling and quantum phenomena are central ideas, but grasping them rests on an above average foundation of mathematics, physics and chemistry concepts. Is it realistic to assume that most biochemistry freshmen have already acquired that? We have to be patient, fuel them with enthusiasm and make sure that we don’t muddy the waters of key concepts with too much content in our courses.

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The Bad Dreams Of Big Science

Science does not always grow pretty when it gets too big. In 1939, at the eventual cost of over $20 billion (in 2014 equivalent dollars) and the involvement of over 100 000 workers, the Manhattan Project was initiated. Leo Szilard and Eugene Wigner had urged the U.S. president to set the wheels in motion in order to devise and build an atomic bomb before the Nazis could. But Szilard naively envisioned that the innovation would merely serve as a deterrent. Of course, the Allies did not rely on atomic warfare to defeat the Germans, but they used two such bombs to wipe out mostly innocent citizens in 2 cities in Japan, a country which had Kamikazes but no nuclear program. The horrific events haunted the consciences of some physicists, prompting some, including Szilard to switch fields to molecular biology.

Joseph Rotblatt was the only scientist¹ who had left the Project on moral grounds, after he realized the bomb was being developed for use against adversaries other than Nazi Germany.Joseph Rotblatt About a decade later, along with Bertrand Russell, Rotblatt organized the first of the Pugwash Conferences as a quest to seek peaceful solutions to the Cold War and its nuclear threat, and 40 years later he was awarded the Nobel Peace Prize.

Rotblatt himself was not interested in using nuclear energy as a power source. The peaceful use of the atom, in his mind, should focus only on smaller-scale medical and research applications which require much smaller reactors. I recall an anti-nuclear activist saying in the 1970s that using nuclear energy to boil water for powering turbines was akin to burning your house down to make toast.  To be fair, not all reactors are badly conceived. India is rekindling a better idea from the past, the use of  thorium reactors, which operate at lower temperatures, involve more energy-efficient recycling of material and create less waste. But such reactors were abandoned in the 1960s in favour of  our existing uranium-plutonium reactors to produce more stable byproducts used in nuclear weapons! ²

The nuclear accidents at Three Mile Island, Chernobyl and Fukishima received major media attention. The second one of these was the most serious. The forest is still thriving around the old site; unfortunately the human species is the most sensitive to radiation. At the very least, there were about 4000 cancers caused by the accident after exposing about 240 000 people to  worrisome levels of radiation.  Less known is the 2002 Oak Harbor incident where the control rods were corroded and the plant had to be shut down for two years at a cost of $124 million. Bruce Power may have to spend $15 billion just for a rehaul of 6 reactors in Ontario. The cost of new construction and indispensable implementation of safety measures is even higher. Despite this historical background and such economic realities,  those with conflicting interests focus on just one of its benefits and remind everyone that nuclear power is free of carbon dioxide. Meanwhile, there are still no long term storage sites for the wastes generated.

It’s perhaps because of all this that some believe that the ultimate big science solution to the world’s energy needs is fusion.  ITER is the first reactor designed to ‘ignite’ fusion plasma while generating more energy than it consumes after creating formidable “ignition” temperatures. But it has experienced a $40 billion dollar cost-overrun. Much of the excess expenses have been attributed to the bureaucratic burden of a large international effort. But the irony lies in the chosen isotope-combination of deuterium and tritium. Assuming they succeed, most of the energy produced will be carried off by neutrons, which will contaminate the containment walls and create more nuclear waste, which was supposedly one of the motivations for steering away from fission and moving towards fusion.

¹ We should also give credit to Franco Rasetti, one of the Via Panisperna Boys, who had worked with Enrico Fermi and who never joined the Manhattan Project because of conscientious objections. His friend Ettore Majorana, another brilliant physicist who proposed a connection between the neutrino and antineutrino, was also opposed to the military application of nuclear physics

²For a discussion of how lucky we have been, so far, to have survived nuclear attacks despite bad policies, see http://www.chomsky.info/articles/20140805.htm

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