Quantum Entanglement Holds DNA Together, Say Physicists
A new theoretical model suggests that quantum entanglement helps prevent the molecules of life from breaking apart.
There was a time, not so long ago, when biologists swore black and blue that quantum mechanics could play no role in the hot, wet systems of life.
Since then, the discipline of quantum biology has emerged as one of the most exciting new fields in science. It's beginning to look as if quantum effects are crucial in a number of biological processes, such as photosynthesis and avian navigation which we've looked at here and here.
Now a group of physicists say that the weird laws of quantum mechanics may be more important for life than biologists could ever have imagined. Their new idea is that DNA is held together by quantum entanglement.
That's worth picking apart in more detail. Entanglement is the weird quantum process in which a single wavefunction describes two separate objects. When this happens, these objects effectively share the same existence, no matter how far apart they might be.
The question that Elisabeth Rieper at the National University of Singapore and a couple of buddies have asked is what role might entanglement play in DNA. To find out, they've constructed a simplified theoretical model of DNA in which each nucleotide consists of a cloud of electrons around a central positive nucleus. This negative cloud can move relative to the nucleus, creating a dipole. And the movement of the cloud back and forth is a harmonic oscillator.
When the nucleotides bond to form a base, these clouds must oscillate in opposite directions to ensure the stability of the structure.
Rieper and co ask what happens to these oscillations, or phonons as physicists call them, when the base pairs are stacked in a double helix.
Phonons are quantum objects, meaning they can exist in a superposition of states and become entangled, just like other quantum objects.
To start with, Rieper and co imagine the helix without any effect from outside heat. "Clearly the chain of coupled harmonic oscillators is entangled at zero temperature," they say. They then go on to show that the entanglement can also exist at room temperature.
That's possible because phonons have a wavelength which is similar in size to a DNA helix and this allows standing waves to form, a phenomenon known as phonon trapping. When this happens, the phonons cannot easily escape. A similar kind of phonon trapping is known to cause problems in silicon structures of the same size.
That would be of little significance if it had no overall effect on the helix. But the model developed by Rieper and co suggests that the effect is profound.
Although each nucleotide in a base pair is oscillating in opposite directions, this occurs as a superposition of states, so that the overall movement of the helix is zero. In a purely classical model, however, this cannot happen, in which case the helix would vibrate and shake itself apart.
So in this sense, these quantum effects are responsible for holding DNA together.
The question of course is how to prove this. They say that one line of evidence is that a purely classical analysis of the energy required to hold DNA together does not add up. However, their quantum model plugs the gap. That's interesting but they'll need to come up with something experimentally convincing to persuade biologists of these ideas.
One tantalising suggestion at the end of their paper is that the entanglement may have an influence on the way that information is read off a strand of DNA and that it may be possible to exploit this experimentally. Just how, they don't say.
Speculative but potentially explosive work.
Ref: arxiv.org/abs/1006.4053: The Relevance Of Continuous Variable Entanglement In DNA
http://www.technologyreview.com/blog/arxiv/25375/
Impact: Quantum Physics June 29, 2010
Once in a while science produces theoretical work that has tantalizing possibilities but also raises a strong skeptical response. This is another way of saying that a theory has a certain amount of plausibility but is without experimental evidence. Such is the case with a theory proposed by Elisabeth Rieper and colleagues at the National University of Singapore and submitted in a paper at arXiv.org on June 21, 2010: The relevance of continuous variable entanglement in DNA. They are saying that the stability of DNA is in part the result of quantum entanglement.
It’s a little early to be talking about the ‘field’ of quantum biology, although there is already strong evidence for quantum effects in photosynthesis [SciTechStory: Confirmation of quantum entanglement in photosynthesis] but if the existence of significant quantum effects in DNA can be substantiated (that means with experimental evidence) it would be a foundational discovery. However, at this point the idea is a working hypothesis based on mathematical modeling. It goes something like this…
Quantum effects, in this case quantum entanglement, are among the most counter-intuitive and challenging ideas in physics – ‘spookish’ is the word Einstein used. Scientists know quantum mechanics from many decades of mathematical theorizing and a couple of decades of experimentation with atomic behavior at temperatures close to absolute zero. It comes as an enormous intellectual and theoretical leap to grapple with the idea that natural (biological) phenomenon use quantum effects at or above room temperature. Yet, the evidence is accumulating that quantum entanglement is involved with biology in a very fundamental way – photosynthesis being the first to have experimental evidence.
This new theory, which is certain to provoke as many skeptical voices as words of interest, began with wondering what role, if any, might quantum entanglement play in DNA. Quantum entanglement is described, simply, as two separate particles that work together as if they were one particle no matter how far apart they might be. If one particle moves up, the other particle moves down, instantly, as if they were on the ends of a teeter-totter. They are a system that behaves as one particle. In the case of DNA, the ‘particles’ are the molecules of the DNA base pairs, formed by the nucleotides with adenine, guanine, thymine, and cytosine. Each nucleotide is surrounded by a cloud of electrons that behave as if the nucleotide were an atomic nucleus. The cloud shifts relative to the nucleus, perhaps influenced by what are called Van der Waals forces, from side to side so to speak forming a dipole (two poles), and this shifting is regular – a harmonic oscillation. In solid-state physics, the oscillation of molecules within a solid is known as a phonon, a kind of quasi-particle that vibrates at a specific frequency and gives the solid many of its electrical and physical properties. In DNA, when a base pair is formed the clouds of each nucleotide must oscillate in opposite directions if the bond is to hold together.
The key question for the researchers was what influence does the double helical structure of DNA have on this oscillation? To answer the question, they first modeled how the phonons would behave at absolute zero temperature. Here (mathematically) it was clear the phonons would be typical quantum objects, existing as both waves and particles exhibiting the property of quantum entanglement. As it turns out, the size of the DNA helix corresponds rather well to the wavelength (frequency) of the phonons. This correspondence causes the phonons to stay within this frequency, something called ‘phonon trapping.’ Though the nucleotide phonons in each base pair oscillate in opposite directions they do so in a quantum entangled system – they act together and at the same frequency, ensuring the stability of the pair bond and of the helix itself.
At least that’s what the model shows can happen. The model also shows that this configuration can maintain the bond at high temperatures – room temperatures or above (e.g. 20 degrees C or 68 degrees F). The quantum entanglement is vital to making this work, because under classical mechanics the vibration of the particles in the helix would shake it apart, especially at higher temperatures.
Of course, this is all modeling. What must come next is experimental evidence. It won’t come easily. The researchers point to the notion that using classical mechanics to add up the energy necessary to hold the helix together comes out short, and that adding the quantum effects makes up the difference. But this is indirect evidence.
Keep in mind that quantum mechanics existed as mathematical theory long before experimental evidence was provided. The situation may be similar with showing quantum effects in DNA. Or not. As is the case when there are potentially major shifts in scientific understanding – and finding quantum mechanics as a basis for some of the most fundamental aspects of biology certainly qualifies – the demands for evidence will be rigorous. In the meantime, scientists will engage in vigorous debate. It will be interesting to see how argument and evidence changes or nullifies the theoretical insight. The process represents the essence of the scientific method applied to a potentially revolutionary hypothesis.
http://scitechstory.com/2010/06/29/quantum-entanglement-helps-keep-dna-together/
