Spin-Correlated Radical Pairs in Cryptochromes: Fit for Purpose as Biological Compass?

One molecular candidate for the magnetic compass of migratory birds still is light-induced spin-correlated radical pairs in cryptochromes. Only few experimental methods allow for directly probing spin-correlated radical pairs and their magnetic interactions responsible for the proposed compass role. Here, we show how time-resolved electron paramagnetic resonance
(TREPR) spectroscopy can help elucidate crucial characteristics of spin-correlated radical pairs. This starts with their very existence and proceeds via their precursor states and their magnetic interactions to the kinetics of the spin polarisation. Taken together, while TREPR spectroscopy cannot by itself prove a molecule to be (part of) the magnetic compass of migratory birds, it allows to make informed judgements based on its characterisation of the underlying radical pairs