Quantum Biochemical Compasses*

The fact that many animals are able to use the Earth's magnetic field to navigate and orient has been firmly established since the 1970s. The hypothesis that this capability might be driven by a quantum mechanical process involving a pair of photoinduced, highly spin polarized radicals, was originally proposed in the same decade. But only with the discovery of cryptochromes, a family of blue light photoreceptor proteins ubiquitous in Nature, did this radical pair hypothesis take centre stage in the discussion of animal magnetosensitivity and is now, arguably, the most likely mechanism to drive this fascinating process.
Here we report our comparative studies of magnetic field effects on the photo-induced electron transfer reactions in a series of proteins from the cryptochromes/photolyase family, including cryptochromes from Arabidopsis thaliana (a plant), Drosophila melanogaster (the fruit fly), Xenopous laevis (a cry-DASH protein from a frog) and E. coli photolyase. The magnetic sensitivity of these reactions is characterized by a combination of optical spectroscopy methods including sub-nanosecond transient and cavity based absorption spectroscopies (including Cavity Ringdown and Broad Band Cavity enhanced spectroscopies). Together, these techniques yield time-, field- and wavelength-resolved spectral data providing insights into the photo- and radical pair chemistry of blue-light photoreceptor proteins.