Chemical amplification of magnetic field effects relevant to avian magnetoreception

Abstract

Magnetic fields as weak as the Earth's can change the yields of radical pair reactions even though the energies involved are orders of magnitude smaller than k B T at room temperature. Proposed as the source of the light-dependent magnetic compass in migratory birds, the radical pair mechanism is thought to operate in cryptochrome flavoproteins in the retina. Here we demonstrate that the primary magnetic field effect on flavin photoreactions can be chemically amplified by slow radical termination reactions under conditions of continuous photoexcitation. The nature and origin of the amplification are revealed by studies of the intermolecular flavin-tryptophan and flavin-ascorbic acid photocycles and the closely related intramolecular flavin-tryptophan radical pair in cryptochrome. Amplification factors of up to 5.6 have been observed for magnetic fields weaker than 1 mT. Substantial chemical amplification could have a significant impact on the viability of a cryptochrome-based magnetic compass sensor.2 Amongst other directional cues, migratory birds use a light-dependent geomagnetic compass for orientation and navigation [1][2][3] . Although the primary detection mechanism is unclear, the evidence currently points to magnetically sensitive photochemical reactions in cryptochrome proteins located in the retina [4][5][6] . Cryptochromes 7 contain the chromophore flavin adenine dinucleotide (FAD), photoexcitation of which can trigger three consecutive intra-protein electron transfers along a conserved triad of tryptophan (Trp) residues to produce a (FAD  Trp  ) radical pair [8][9][10] . This form of cryptochrome is magnetically sensitive in vitro and possibly also in vivo [11][12][13] . Similar magnetosensitivity is conceivable in other cryptochrome-derived radical pairs in which FAD  or its protonated form, FADH  , is paired with an ascorbic acid radical 14 or, less plausibly, superoxide, 2 O ·- 15,16 . There appear to be different electron transfer pathways in some cryptochromes 17,18 , but no evidence so far that they give rise to magnetic field effects.The radical pair mechanism is well established as the source of magnetic field effects on chemical reactions 19 . Remarkably, the magnetic interactions of electron spins in organic radicals can result in significant changes in reaction kinetics and product yields even though those interactions are many orders of magnitude weaker than the thermal energy, k B T. The sensitivity to applied magnetic fields derives from the coherent spin dynamics of pairs of radicals formed in spincorrelated states. Several conditions need to be satisfied for a radical pair to be suitable as a geomagnetic compass sensor 5 : (a) the electron spin in at least one of the radicals must interact anisotropically with nuclear spins; (b) the mutual interaction of the two electron spins should be small; (c) the radical pair must react spin-selectively; (d) its lifetime should not be too short; and (e) the electron spin relaxation must be relatively slow. Studies of isolated cry...

Ambient and supplemental magnetic fields promote myogenesis via a TRPC1‐mitochondrial axis: evidence of a magnetic mitohormetic mechanism

“…Our finding that ambient magnetism sustains basal myogenesis (Fig. 5) is consistent with the existence of a mitohormetic myogenic mechanism (51, 63), monitoring and adapting to the environment, as well as reflecting its quantum biological origin (50,60,62). The electromagnetic efficacy window described in this study (Supplemental Fig.…”

Biological effects of the hypomagnetic field: An analytical review of experiments and theories

“…[197]. In this latter study, a chemical amplification of the magnetic effect in cryptochrom biradicals have been found.…”

Upper bound on the biological effects of 50/60 Hz magnetic fields mediated by radical pairs

“…We make no attempt to link this change to any specific biological process; rather we seek to estimate the maximum primary magnetic field effect under chemically and physically plausible conditions. Calculations are based on the [FAD  TrpH + ] radical pair that accounts for the observed effects of static magnetic fields on the photochemistry of purified cryptochromes (24,(43)(44)(45). This species is formed by the transfer of an electron from a tryptophan residue (TrpH) in the protein to the photoexcited, non-covalently bound, flavin adenine dinucleotide (FAD) chromophore, Figure 1(a).…”