Radical Scavenging Could Answer the Challenge Posed by Electron- Electron Dipolar Interactions in the Cryptochrome Compass Model

Many birds are endowed with a visual magnetic sense that may exploit magnetosensitive radical recombination processes in the protein cryptochrome. In this widely accepted but unproven model, geomagnetic sensitivity is suggested to arise from variations in the recombination rate of a pair of radicals, whose unpaired electron spins undergo coherent singlet-triplet inter-conversion in the geomagnetic field by coupling to nuclear spins via hyperfine interactions. However, simulations of this conventional radical pair mechanism (RPM) predicted only tiny magnetosensitivities for realistic conditions because the RPM's directional sensitivity is strongly suppressed by the intrinsic electron-electron dipolar (EED) interactions, casting doubt on its viability as a magnetic sensor. We show how this RPM-suppression problem is overcome in a three-radical system in which a third scavenger radical reacts with one member of the primary pair. We use this finding to predict substantial magnetic field effects that exceed those of the RPM in the presence of EED interactions in animal cryptochromes.

Automated summary

This study investigates the visual magnetic sense in birds, suggesting that geomagnetic sensitivity may be achieved through magnetosensitive radical recombination processes in the protein cryptochrome. By introducing a third radical to overcome the suppression problem of the traditional radical pair mechanism, the research predicts substantial magnetic field effects in animal cryptochromes that exceed those of the traditional mechanism in the presence of electron-electron dipolar interactions.