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Physical constraints for the Stoneham model for light-dependent magnetoreception

A new biophysical model for magnetoreception in migratory birds has recently been proposed by Stoneham et al. In this photo-induced radical pair (RP) model the signal transduction mechanism was physical rather than chemical in nature, as otherwise generally assumed in the literature. The proposal contains a magnetosensor and a signal transduction mechanism. The sensor would be an electric dipole related to a long lived triplet state of an RP. This makes it sensitive to the geomagnetic field via the Zeeman interaction. The field of the electric dipole moment would then promote isomerization from cis-to-trans in the retinal of a nearby rhodopsin. This would trigger the neuronal signal. Here we gather several observations from different works that constrain the feasibility of this physical model. In particular we argue that the perturbation of rhodopsin by a local electric field from a nearby electric dipole (10^6 V/m) cannot modify the field in the binding pocket of rhodopsin (10^9 V/m) sufficiently to trigger the isomerization of cis-retinal. The dipole field is much weaker than those from other sources in the vicinity which are known not to promote isomerization.