Quantum physics explanation for smell gains traction

By Jason Palmer
Science and technology reporter, BBC News, Dallas

Girl smelling flower
Image caption,
The quantum details of a flower's chemicals may make the difference to our noses

The theory that our sense of smell has its basis in quantum physics events is gaining traction, say researchers.

The idea remains controversial, but scientists reporting at the American Physical Society meeting in Dallas, US, are slowly unpicking how it could work.

The key, they say, is tiny packets of energy, or quanta, lost by electrons.

Experiments using tiny wires show that as electrons move on proteins within the nose, odour molecules could absorb these quanta and thereby be detected.

If the theory is right, by extending these studies, an "electronic nose" superior to any chemical sensor could be devised.

Lock and key

The means by which a detected molecule is translated into a smell within the brain has already been the subject of Nobel prize-winning research.

But how precisely an odorant molecule is detected remains a mystery.

As with the picture of molecular interactions that drives our understanding of enzymes and drugs, the very shape of odorant molecules has been assumed to be the way it is detected in the nose.

In this scenario, molecules are seen to be the "key" that fits neatly into a detector molecule in the nose that acts as a lock.

But in 1996, Luca Turin, now of the Massachusetts Institute of Technology in the US, suggested that the "vibrational modes" of an odorant were its signature.

Molecules can be viewed as a collection of atoms on springs, and energy of just the right frequency - a quantum - can cause the spring to vibrate.

Since different assemblages of molecules have different characteristic frequencies, Turin proposed, these vibrations could act as a molecular signature.

The idea has been debated in the scientific literature, but presentations at the American Physical Society meeting put the theory on firmer footing.

Most recently, Dr Turin published a paper showing that flies can distinguish between molecules that are chemically similar but in which a heavier version of hydrogen had been substituted.

Like a spring with a heavier weight at one end, the vibration frequency is lowered, and flies appear to notice.

"There's still lots to understand, but the idea that it cannot possibly be right is no longer tenable really," said Andrew Horsfield of Imperial College London.

"The theory has to at least be considered respectable at this point," he told BBC News.

Dr Horsfield's research centres on demonstrating how the vibration might be detected.

The idea is that an electron on one part of a protein may move, and arrive at another part lacking a quantum of vibrational energy.

"The electron starts at one end of the room, if you like, and it can only make it to the other end if it gives up energy to the molecule in the middle of the room," he explained.

"Once it's arrived, you say 'Aha! The fact that it's here means that somewhere between where it started and where it is now there's a molecule with the right vibrational frequency'."

Room to move

The difficulty is demonstrating a physical system where this kind of detective work can be accomplished - to show a start and an endpoint to the process.

Dr Horsfield and his collaborators have demonstrated nanowires - wires just billionths of a metre across - that can act as the "room" of the analogy.

Image caption,
Nanowires exploiting the effect could be the basis for "electronic noses"

They showed how electrons could arrive at one end of these nanowires and give away what molecules they had encountered along the way.

Jennifer Brookes, a University College London researcher based at MIT, carries out computer simulations on the quantum physics at work in the process, in order to put it on a firmer mathematical footing.

"It's a very interesting idea; there's all sorts of interesting biological physics that implement quantum processes that's cropping up," she told BBC News.

"I believe it's time for the idea to develop and for us to get on with testing it."

Her presentation suggested that the vibrational theory of smell, at least as quantum physics is concerned, is a reasonable one.

"Mathematically, the theory is robust, and even if it's not happening in smell, it's interesting to think it might be a discriminatory process in nature in other ways," she said.

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