The radical pair mechanism and the avian chemical compass: Quantum coherence and entanglement

Abstract

We review the spin radical pair mechanism which is a promising explanation of avian navigation. This mechanism is based on the dependence of product yields on (1) the hyperfine interaction involving electron spins and neighboring nuclear spins and (2) the intensity and orientation of the geomagnetic field. This review describes the general scheme of chemical reactions involving radical pairs generated from singlet and triplet precursors; the spin dynamics of the radical pairs; and the magnetic field dependence of product yields caused by the radical pair mechanism. The main part of the review includes a description of the chemical compass in birds. We review: the general properties of the avian compass; the basic scheme of the radical pair mechanism; the reaction kinetics in cryptochrome; quantum coherence and entanglement in the avian compass; and the effects of noise. We believe that the quantum avian compass can play an important role in avian navigation and can also provide the foundation for a new generation of sensitive and selective magnetic-sensing nanodevices. V C 2015 Wiley Periodicals, Inc. DOI: 10.1002/qua.24943 Radical Pair Mechanism IntroductionA radical is an atom, molecule or ion that has unpaired valence electrons. Radicals and radical pairs often play a very important role as intermediates in thermal, radiation, and photochemical reactions. [1] The presence of unpaired electron spins in these systems allows one to influence and control these reactions using interactions between external magnetic fields and electron spins. [2] However, until 1970, most scientists believed that ordinary magnetic fields had no significant effect on chemical or biochemical reactions, as the magnetic energy of typical molecules, under ordinary magnetic fields, is much smaller than the thermal energy at room temperature and is much smaller than the activation energies for those reactions. [1,2] This situation changed significantly in the 1970s after a series of experimental results were reported on magnetic field effects on chemical reactions. [3][4][5][6][7] Because of these experimental studies, a number of researchers have made an effort to theoretically explain the magnetic field effects on the chemical reactions. [8,9] Thanks to these and the subsequent efforts, we are now able to explain systematically magnetic field effects in terms of the radical pair mechanism. The radical pair mechanism was then successfully applied to explain the chemically induced nuclear polarization and electron polarization, which were shown to be based on the spin dynamics of radical pairs. [2] According to the radical pair mechanism, an external magnetic field affects chemical reactions by alternating the electron spin state of a weakly coupled radical pair, which is produced as an intermediate. The basic scheme of chemical reactions through the radical pairs is shown in Figure 1. Radical pairs are usually produced as short-lived intermediates through decomposition, electron transfer, or hydrogen transfer reactions from singlet or tr...

A ground-state-dominated magnetic field effect on the luminescence of stable organic radicals

“…20 1 (R/R)* can generate the singlet excimer state, 1 (R-R)*, with rate constant k Sex . We also assumed that a close RP, [26][27][28][29] (R*/R), is formed, where eldsensitive ISC does not happen due to the energy gap between the S and the T 0 states resulting from non-negligible exchange interactions (Fig. 3d).…”

The quantum mitochondrion and optimal health

“…Indeed, many biomolecules may have been selected for their ‘quantum criticality’, and thus behave somewhere between an insulator and a conductor, so also potentially acting as charge carriers [52]. Practical examples include quantum effects used in bird navigation [53], an explanation of how photosynthesis works [41], and possibly, even olfaction [54]. The recent discovery that lysozyme appears to demonstrate a ‘Fröhlich condensate’ [55], when combined with concept that strong electro-magnetic fields generated by mitochondria could generate ‘water order’, and thus protect against decoherence [56], is perhaps further evidence.…”

“…In effect, a quantum protective mechanism in photosynthesis [63]. If the triplet state can be used for bird navigation [53], could this hint that it is used in other biological processes as well? The link between triplet states and fields is particularly interesting–suggesting that ROS could be signalling in more ways than we realized.…”

“…Panel ( B ) Some predictions of our own: (1) coherence could be controlled by mitochondrial potential, which is turn could enhance quantum tunnelling of electrons (as well, as possibly, other coherence-dependent functions, such as enzyme function or ion channel status)–it could be biphasic as it could enhance efficiency (high ATP, low ROS), but if too high, it could start to hinder ATP by producing too much ROS. Thermodynamically it could initially aid in survival of the individual cell, then favour replication, and if necessary, induce cell death–thus would be hormetic; (2) it is now thought that a radical pair mechanism could be involved in avian navigation (effectively unpaired quantum entangled electron) [53], which might suggest that if such entities were produced in the ETC, it could act as a signal–implying there is more to ROS than thought; (3) if we combine (2) and (3), then any shift in electron flow and/or proton flow, either via electron input, or use, in particular, availability of oxygen, or even cell shape, or damage, would rapidly generate a signal that could instantaneously control mitochondrial function; (4) the effect of mitochondrial dynamics would thus have to viewed in a new light, as it is possible that fusion could enhance a quantum-based signalling system throughout the entire cell, which would be altered by fission.…”

The Hormesis of Thinking: A Deeper Quantum Thermodynamic Perspective?

“…If one of them is excited with extra energy and "rises" to a higher orbital but its spin direction switches, so as it now has the same spin as its partner, it cannot fall back due to Pauli's exclusion principle -so it can potentially be more reactive; the triplet designation comes from the quantum description of the two electrons now having spin=1, as they can exist in three different values. This triplet state may be important in how many organisms detect the earth's magnetic field, as the electrons remain entangled [66]. Putting this together with the observations that quantum tunnelling is important in ETC [46,47] there is a clear precedent not only for ROS in controlling lifespan and signalling, but that it is ultimately controlled by quantum effects.…”