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One of the great puzzles of biology is how the molecular machinery of life is so finely coordinated. Even the simplest cells are complex three dimensional biochemical factories in which a dazzling array of machines fill the shop floor.

These machines pump, push, copy, and compute in a dance of extraordinarily detailed complexity. Indeed, it is hard to imagine how the ordinary processes of conduction and electron transport allow this complexity to emerge given the losses that inevitably arise, even in much simpler circuits.

Today, Stuart Kauffmann at the University of Calgary in Canada and a few pals provide some extraordinary new insight into how all this might happen. These show that most biomolecules are quantum critical conductors; their electronic properties are precisely tuned to the transition point between a metal and an insulator.

In other words, biomolecules belong to an entirely new class of conductor that is not bound by the ordinary rules of electron transport, a discovery that has profound implications for our understanding of the nature of life and its origin.

Quantum criticality describes the behaviour of electrons in large molecules when they occupy the exotic state that sits at the knife edge between conduction and insulation. When these molecules are conductors, some of their bound electrons are able to move freely under the influence of an electric field. By contrast, when these molecules are insulators, the electrons are not free to move.

The quantum critical state occurs when the electronic states are balanced between conduction and insulation. In that case, the current is unpredictable and flows in avalanches that can vary in size by many orders of magnitude. It is a condition utterly unlike anything that exists in conventional electronic circuits.

The state of the electrons in a large molecule is determined by the pattern of energy levels the electrons can occupy. Molecules that are conductors have different patterns from those that are insulators. Indeed, it is possible to work out whether a molecule is a conductor or an insulator by calculating the pattern of electron levels.

That’s a difficult task given the number of electrons at work in large molecules. For instance, an ordinary biomolecule such as the protein myoglobin consists of 153 amino acids and over 1000 atoms. However, it has recently become possible to model the pattern of energy levels on powerful computers using NMR data about the physical structure of the molecule and this reveals the tell tale signature of a conductor or an insulator.

The central idea in Kauffmann and co’s paper is that molecules in the quantum critical also have a unique pattern of energy levels that can be determined by this modelling process. The question they set out to answer is: how common is quantum criticality in biomolecules?

The answer is something of a surprise. Kauffmann and co have calculated the pattern of energy levels in biomolecules from a wide range of essential classes and say that the vast majority of them are quantum critical. These molecules include myoglobin, which plays a central role in oxygen storage in the muscles, linoleic acid, sucrose, vitamin D3, vitamin B12, and the largest amino acid leucine and so on.

There are a few molecules that are insulators both by this measure and in experiments. They tend to be structural materials such as silk, a gum-like substance called dextrin and octadecane, an alkane hydrocarbon found in mineral oil. This list also includes DNA, which is otherwise thought of as a wide bandgap semiconductor, or as Kauffmann and co put it, practically an insulator.

The team found only a few biomolecules that are good conductors. These tend to have aromatic rings, such as the hormones testosterone and progesterone.

The team summarise their findings in the following way. “Most of the molecules taking part actively in biochemical processes are tuned exactly to the transition point and are critical conductors,” they say.

That’s a discovery that is as important as it is unexpected. “These findings suggest an entirely new and universal mechanism of conductance in biology very different from the one used in electrical circuits.”

The permutations of possible energy levels of biomolecules is huge so the possibility of finding even one that is in the quantum critical state by accident is mind-bogglingly small and, to all intents and purposes, impossible.

So that fact that most of the molecules they modelled fall into this category is hugely significant. “This shows that chemical and biological evolution selected only a tiny fraction— of the order of 10^?50—of possible small biomolecules and even less for proteins,” say Kauffmann and co. In other words, there is an important evolutionary advantage for molecules in this quantum critical state.

That’s fascinating work that raises many important questions about the nature and origin of life and the role of quantum criticality. For example, what exactly is the advantage that criticality confers? How does this new mode of conduction operate in living things? And why is it such a crucial ingredient in the complex factories of live that evolution has produced?

No doubt Kauffmann and co are pondering these and other questions. They are likely to be joined by cohorts of other chemists, biologists and physicists in the not too distant future.