The ability of night-migratory songbirds to sense the direction of the Earth's magnetic field is increasingly attributed to a photochemical mechanism, in which the magnetic field acts on transient radical pairs in cryptochrome flavoproteins located in the birds' eyes. The magnetically sensitive species is commonly assumed to be [FAD  TrpH + ], formed by sequential light-induced intra-protein electron transfers from a chain of tryptophan residues to the flavin adenine dinucleotide chromophore. However, some evidence points to superoxide, 2

It has been suggested that 31 P nuclear spins in Ca 9 (PO 4 ) 6 molecules could form the basis of a quantum mechanism for neural processing in the brain. A fundamental requirement of this proposal is that spins in different Ca 9 (PO 4 ) 6 molecules can become entangled and remain so for periods (estimated at many hours) that hugely exceed typical 31 P spin relaxation times. Here, we consider the coherent and incoherent spin dynamics of Ca 9 (PO 4 ) 6 arising from dipolar and scalar spin-spin interactions and derive an upper bound of 37 min on the entanglement lifetime under idealized physiological conditions. We argue that the spin relaxation in Ca 9 (PO 4 ) 6 is likely to be much faster than this estimate.

We present an analysis of reported magnetic field effects on the yield of formic acid produced by electrocatalytic reduction of carbon dioxide at a nanoparticle tin electrode (Pan et al., J. Phys. Chem. Lett. 11 (2020) 48-53). Radical pair spin dynamics simulations are used to show that: (1) the g mechanism favoured by Pan et al. is not sufficient to explain the observed magneto-current.(2) Field-dependent spin relaxation, resulting from the anisotropy of the g-tensor of 2 CO  , combined with the coherent singlet-triplet interconversion arising from isotropic hyperfine and Zeeman interactions, can quantitatively account for the observed magnetic field effect. (3) Modification of hyperfine interactions by isotopic substitution ( 1 H  2 H and/or 12 C  13 C) could be used to test both the proposed reaction mechanism and the interpretation presented here.

We explore the possibility that chemical feedback and autocatalysis in oscillating chemical reactions could amplify weak magnetic field effects on the rate constant of one of the constituent reactions, assumed to proceed via a radical pair mechanism. Using the Brusselator model oscillator, we find that the amplitude of limit cycle oscillations in the concentrations of reaction intermediates can be extraordinarily sensitive to minute changes in the rate constant of the initiation step. The relevance of such amplification to biological effects of 50/60 Hz electromagnetic fields is discussed.

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