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How Quantum Mechanics help us breath.

Posted on 30/01/2015

Why don't we suffocate whenever we try to take a breath? Researchers from the Theory & Simulation of Condensed Matter research group in the Department of Physics used quantum mechanics, a formulation of physics to describe phenomena at small length scale, to solve this human-sized mystery. Quantum mechanics has long proved its value in understanding physical phenomena, and in particular how electrons are involved in physical and chemical reactions, with implications for macroscopic scales and every day applications.

The new research, led by Dr Cedric Weber in the Department of Physics at King's College London, and reported in the journal Proceedings of the National Academy of Sciences, confirms that point. "This work," said Cedric Weber, "helps to illustrate the reality of quantum-mechanical effects, which may sometimes be viewed as exotic. It also illustrates the promises of cross-disciplinary approaches, where biology, chemistry, and materials science operate."

The conundrum elucidated by Weber's team stems from the way in which carbon monoxide reacts with proteins that carry oxygen around our bodies. Those proteins, which contain iron atoms, transport oxygen molecules through the bloodstream to wherever the body needs them. According to density functional theory, the current state of the art approach, the proteins should typically link up more often with molecules of carbon monoxide – from inside and outside the body – than with oxygen molecules. If that happened regularly, it would result in asphyxiation, killing off humans and animals. The small amounts of carbon monoxide naturally produced in our bodies would not be enough to displace oxygen fully, even if it had a greater binding ability. But we would be more vulnerable to poisoning by carbon monoxide in the atmosphere than experience shows to be the case. The fact that this doesn't happen means that oxygen molecules bind more effectively to proteins than theory forecasts.

"The problem that scientists have had is explaining how the proteins achieve this discrimination in favour of oxygen," said Cedric Weber.

To do so, the team applied a computer simulation technique based on quantum mechanics to reactions of oxygen and carbon monoxide with myoglobin, the main oxygen-carrying protein in muscle tissue. A typical method used in the community is the density functional theory, or DFT. DFT has been the standard tool for simulating electronic properties of materials and molecules for a number of years. This technique was applied in the past to study reactions between the iron atom inside myoglobin and a molecule of oxygen or carbon monoxide. These reactions involve electrostatics, the arrangement of electric charges in atoms and molecules. When the iron atom transfers negative electric charges to an oxygen or carbon monoxide molecule, it enables the molecule to attach itself to the entire myoglobin protein.

Unfortunately, all precedent theories consistently predicted that carbon monoxide should bind to myoglobin much more readily than oxygen. In the Department of Physics, we merged two cutting edges techniques, the so called linear scaling density-functional theory (a formulation of DFT which has a computational cost scaling linearly with the number of atom, instead of a cubic scaling as with other DFT implementations), and the dynamical mean field theory, which deals with quantum many body effects, was obtained for the first time and applied to this long standing problem. 

"Using DMFT, we showed that, in fact, close to one electron is transferred to the oxygen molecule,” Weber explained. "This provides much greater electrostatic stabilization than previously thought. It means that our estimate of the relative binding of oxygen and carbon dioxide is now in excellent agreement with experiment."

The analysis revealed that an effect called entanglement plays a critical role in binding oxygen molecules to the protein. Entanglement is a quintessential characteristic of quantum mechanics that links pairs of electrons so strongly that they no longer act independently. The process also involves Hund's exchange, another quantum-mechanical property that previous simulations had ignored.

"These effects strengthen the direct bonding between iron and oxygen, and also enhance electrostatic interactions with the protein," Weber explained. The overall result: "We significantly improve the agreement of theory with experiment in terms of the relative tendencies to bind oxygen and carbon monoxide”. The research has potential uses beyond understanding the molecular basis of breathing. The better understanding of how molecules bind to iron-containing proteins could help the drug-development process and possibly facilitate the design of artificial photosynthesis devices that would capture and store energy from the sun.

For further information, please contact Dr Cedric Weber.


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