A quantum leap in biology

971 The most esoteric research field in the natural sciences is probably quantum physics. Despite the fact that Werner Heisenberg first proposed its central concepts nearly 80 years ago, it continues to baffle physicists and to cause headaches among non-physicists. Even Albert Einstein was unwilling to accept the central tenet that everything is just a matter of possibilities; he famously dismissed Heisenberg’s ideas by asserting that “God does not throw dice.” As quantum physics seems too mystical to be relevant to anything as real as a living organism, it might come as a surprise that its first applications have arrived in biology, rather than physics. The seeds of contemporary quantum biology were sown as early as 1930, a mere three years after Heisenberg postulated his uncertainty principle describing the inability to measure related quantities exactly (see sidebar). At that time, Erich Hückel, a German chemist and physicist, developed simplified methods based on quantum mechanics (QM) for analysing the structure of unsaturated organic molecules, in particular to explain the state of electrons in aromatic compounds. But Hückel was too far ahead of his time, and his concepts went almost completely unrecognized until the 1950s, when the arrival of computers made it possible to perform more detailed calculations. It was not until the 1990s, however, that the field of quantum biology became established with the development of density functional theory (DFT), which allows accurate calculations of electronic structure (see sidebar). By that time, high-resolution structures of protein complexes obtained using X-ray crystallography and nuclear magnetic resonance produced sufficiently accurate descriptions of crucial molecules for QM methods to unravel the details of key reactions, such as ATP hydrolysis. QM has also made a significant impact on the study of photoreception and the detection of colour, on research into the sensing of magnetic location and directional information by migratory birds, and, most controversially, in understanding the processes underlying consciousness. The last example relies on certain unprovable assumptions about the scientific basis of perception, whereas research on catalytic reaction centres (such as analysing substrate binding) hinges on solving the Schrödinger wave equation. Described in 1926, and central to the theory of QM, this equation describes the probability that a given electron is in a particular location at a certain time (see sidebar). Such QM-based applications calculate the sequence of events at the atomic level by analysing the electronic properties during the formation and breakage of chemical bonds or the orientation of electron orbitals, as determined by their quantum wave function.