Erwin Schrödinger (1887-1961) is famous for the equation named after him, which describes the atom in quantum mechanical terms, and even more so for a thought experiment involving a cat that we don’t know if it is alive or dead. Unusually for a founding father of quantum mechanics, he also influenced the nascent field of molecular biology with his 1943 lecture: What is life?
At a time when there was virtually no research in the field we now know as biophysics, Schrödinger’s analysis of life processes from the perspective of theoretical physics was highly unusual. He considered how life can produce order from disorder, showing that it obeys the second law of thermodynamics in producing ordered structures at the expense of increased entropy in their environment. Separately, he discussed how living things might reproduce order from order, ie how they can faithfully replicate highly sophisticated functional architectures in every new generation of living beings.
While these theoretical considerations anticipated important findings that were made years later, even Schrödinger would have seen the sub-microscopic world of quantum mechanics as remote from the functions of living beings. The whole point of his famous thought experiment is that it projects the ‘weirdness’ of quantum mechanics, where two seemingly contradictory things can be valid at the same time, to the familiar scale of living organisms, where this appears inconceivable.
While we still don’t have cats that can be simultaneously dead and alive, Schrödinger might have been surprised to learn that many of the biochemical functions of all living organisms appear to make logical use even of those aspects of quantum mechanics that we tend to find highly illogical.
Quantum travelling
All biological structures depend on the electron distributions around atoms and the chemical bonds, which are determined by Schrödinger’s equation, and thus by quantum mechanics. Even though this part of physics also includes some seriously counter-intuitive aspects, such as the wave–particle dualism of the electron and Heisenberg’s uncertainty principle, it is widely taken for granted as part of the structural fabric of matter.
The kind of ‘quantum weirdness’ that we tend to regard as separate from our ‘real world’ can be exemplified by a quantum process known ‘tunnelling’. Whereas classical physics maintains that a solid impregnable barrier will not be breached under any circumstances, quantum particles coming up against an obstacle have a finite probability of showing up on the other side. ‘[Tunnelling] is like a kind of quantum teleportation,’ explained Johnjoe McFadden, professor of molecular genetics from the University of Surrey, UK, at a recent SCI lecture in London. McFadden is also co-author of a book about quantum biology with theoretical physicist, Jim Al-Khalili.1
Tunnelling is an aspect of quantum biology that can be traced back to the 1960s. US scientists, Don DeVault and Britton Chance had found anomalies in the temperature-dependence of photosynthesis reactions, which they interpreted as evidence for electron tunnelling.
Later in the 1970s, when the first high-resolution crystallographic structures of enzymes were solved, researchers hoped that they would yield definitive answers regarding the mechanisms of enzymatic reactions and the predicted lock-and-key, and induced-fit, models. It soon turned out that in some cases there was no space to accommodate the necessary movements of small particles such as protons or hydride ions. Studying isotope effects, ie replacing hydrogen with its heavier isotope deuterium, researchers confirmed that tunnelling of ions took place in certain kinds of enzymes and indeed was essential for their function.
Photosynthesis also appears to make use of another quantum phenomenon, ie coherent waves, to help its excitons (excited states that may produce charge separations consisting of an electron and the hole it leaves behind) arrive at their destination. Theoretical analyses suggested that excitons would not find their way efficiently enough to ensure the performance rate observed in photosynthesis.
In 2007, Graham Fleming at the University of California at Berkeley, US, and colleagues observed ‘quantum beats’ after using laser light pulses to excite the Fenna-Matthews-Olson (FMO) photosynthetic light-harvesting protein of green sulfur bacteria, suggesting that the excitons did not travel according to classical physics but as coherent waves that can follow different paths.2 Thus, the previously assumed random walk of the excitons through the photosynthetic reaction centre had to be replaced with a ‘quantum walk’, providing another example of how biology makes use of quantum phenomena.
A quantum compass
How migrating birds find their way across many thousands of kilometres is one of the most intriguing mysteries in biology. Behavioural experiments, conducted half a century ago, showed that birds can detect the Earth’s magnetic field. Later studies showed that light is also required for the avian compass to function, but the nature of this navigational instrument remained elusive.
Consider magnetotactic bacteria, which have magnetite crystals to help them find their way. In this case, the compass does not serve long-distant migration, but knowing which way is up. Gravity is not much help when you’re a bacterium suspended in water. However, if a high precision compass made of magnetite crystals exists in birds, it should surely have been found by now. Moreover, as Peter Hore, from the University of Oxford, UK, points out, ‘it is difficult to see how such a magnetite compass would depend on light’.
Hore has championed an alternative hypothesis. Theoretical analyses from his group in combination with spectroscopic measurements done by Christiane Timmel’s group, also in Oxford University’s chemistry department, have supported the idea that the avian compass is a chemical reaction involving a pair of radicals. The spins of the unpaired electrons, in the context of the surrounding nuclear spins, provide the compass needle that responds differently depending on its orientation to the external magnetic field. Specifically, the lifetime of the radical pair can change drastically depending on its orientation in the field.
In 2008, the researchers presented a first model system that demonstrated in vitro the sensitivity of such a chemical compass to a magnetic field as weak as that of the Earth. Light-sensitive proteins, known as cryptochromes, which are present in the retina of birds, are presumed to be the carriers of the radical pair in vivo. So far, there is no crystal structure of bird cryptochromes, although the analogous protein structures from mice, flies and plants have been solved. Initial mechanistic work directly analysing the response of bird cryptochromes to light and magnetic fields has been consistent with their suspected role as magnetosensors.
Work on model compounds and on computational simulations has uncovered a number of ways in which evolution may have enhanced the compass effect to make it more robust and useful in practice. In a recent publication, Timmel’s group demonstrated chemical amplification mechanisms that could enhance the sensitivity of the avian compass by more than one order of magnitude.5 To translate the detection of magnetic fields into a sense of direction, the cryptochromes would also have to be anchored to cellular structures in a well-defined orientation. This is another open question waiting to be answered.
Conducting behavioural experiments with zebra finches, Rachel Muheim and colleagues at the University of Lund, Sweden, discovered that the polarisation of ambient sky light appears to assist with the detection of the magnetic field. These researchers found that the compass only works if the polarisation of light is parallel to the magnetic field, not when it is perpendicular.6
Recently, Hore’s group showed that the remarkably good precision of the avian compass can be achieved when the coherence between the two electron spins in the radical pair is maintained for longer than a few microseconds.7 Hore clarified that quantum entanglement between the two spins, even though it is observable, is not a fundamental requirement for the function of the compass. The coherence, analogous to the alignment of light waves in a laser, still makes this a typical quantum mechanical system. Decoherence is the process – much feared by those who want to build quantum computers – by which quantum systems become normal.
Thus, while the definitive mechanism of the avian compass remains to be uncovered, the answer is likely to include some quantum mechanics.
Quantum nose
There is yet another group of biological sensors that have defied many attempts of mechanistic explanation and may involve quantum mechanics, namely the odour receptors in our noses. One might expect them to recognise molecules by their shape, but this assumption performs very poorly at correlating odorants to their smells. Molecules that confer the same olfactory sensation can have very different shapes and sizes, while even enantiomers of the same substance can produce different smells.
Similar smelling molecules of different sizes may share a characteristic functional group, such as a thiol group, but there are also substances that smell like thiols but aren’t.
A visionary explanation first proposed by Malcolm Dyson in the 1920s and inspired by the then newly discovered Raman effect is that the nose somehow conducts vibrational spectroscopy measurements of characteristic chemical bonds in the odorant. Thus, the odour of a given chemical group like a thiol could be shared with others that happen to have similar vibrational frequency. But how would our olfactory epithelium measure vibrational frequencies?
In 1996, Luca Turin at University College London, UK, proposed a quantum mechanical mechanism that would enable cells to do so. It is based on inelastic tunnelling – a kind of tunnelling in which the particle in question has to shed some energy and this energy must be absorbed by some entity with a suitable resonance frequency, such as a chemical bond with the matching vibrational mode. Thus the tunnelling event could serve as a sensor for the presence of a chemical bond with a given vibration frequency.
Turin’s model predicts that isotope exchanges in odorant molecules should be able to change the characteristic smell, which he confirmed in his experiments, but other researchers came to results contradicting his theory, and the issue remains controversial to this day.
Even more controversial is idea from mathematician Roger Penrose and psychologist Stuart Hameroff that consciousness is somehow related to quantum coherence and the mind acts as a quantum computer. Although Al-Khalili and McFadden have shown a lot of patience with some of the more adventurous hypothesis in the field, in this case they side with the majority of experts and declare themselves ‘not convinced’.
Outlook
While there are many examples of biological processes where quantum effects may play a crucial role, Hore appears to be a bit sceptical of the fashionable label, ‘quantum biology’. Some of the things now called quantum biology are not fundamentally new, he points out, and in a sense all phenomena in the universe go back to quantum mechanics if they are dissected at the nanoscale.
However, Hore concludes, ‘this could become really exciting, as we may be able to learn from biology to make better devices, for instance, organic materials for electronics, or photovoltaic devices, or to make better magnetic sensors’. Thus, following up on biomimetics from the metre to the molecule scale, the quantum weirdness of life may offer another opportunity for us to learn from nature.
References
1 J. Al-Khalili and J. McFadden, Life on the edge, the coming of age of quantum biology. Town: Bantam Press, 2015.
2 G. S. Engel et al, Nature, 2007, 446, 782.
3 K. Maeda et al, Nature, 2008, 453, 387.
4 K. Maeda, et al, Proc. Natl. Acad. Sci. USA, 2012, 109, 4774.
5 D. R. Kattnig et al, Nature Chem., 2016, doi: 10.1038/NCHEM.2447.
6 R. Muheim et al, Proc. Natl. Acad. Sci. USA, 2016, 113, 1654.
7 H. G. Hiscock et al, Proc. Natl. Acad. Sci. USA, 2016, 113, in press.
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