Reaction-yield detected magnetic resonance spectroscopy of radical pairs in cryptochrome-4a: a computational study

RYDMR (reaction-yield detected magnetic resonance) is a spectroscopic technique used to study transient radical pairs by measuring changes in the yields of their reaction products induced by a weak oscillating magnetic field in the presence of a static magnetic field. Here we report spin dynamics simulations that explore the experimental conditions required for an in vivo RYDMR study of radical pairs in the cryptochrome proteins thought to be the receptors in the magnetic compass sense of migratory songbirds. The main conclusions are as follows. (a) RYDMR signals at the Larmor resonance condition would provide strong support for a magnetoreception mechanism based on radical pair chemistry. (b) The signals should be stronger when the static and time-dependent fields are mutually perpendicular than when they are parallel. (c) Assuming one component of the pair is a flavin radical, the width of the RYDMR spectrum should depend on the direction of the static field with respect to the tricyclic flavin ring system. (d) The spectra could allow one to distinguish radical pairs in which a flavin radical is combined with either a tryptophanyl radical, a tyrosyl radical, or a radical with very weak hyperfine interactions. (e) For the clearest insights into the mechanism of magnetic sensing, the frequency of the oscillating field should exceed ∼100 MHz. (f) Harmonic signals and spin-locking effects may provide additional information on the nature of the magnetoreceptor.