Elucidating the physical origins of the animal magnetic sense by cavity-enhanced spectroscopy
Keywords: cavity ring-down spectroscopy
spin chemistry
magnetic field effect
spectroscopy
cryptochromes

Many animal species have the ability to navigate using the Earth’s magnetic field. This is particularly true of migratory birds, who are able to fly thousands of kilometres only to nest in exactly the same place year on year. The best current hypothesis as to the method by which this magnetoreception – the ability to sense magnetic fields – takes place is that of magnetically sensitive radical pairs in cryptochromes – blue light photoreceptors found in the retinae of birds and other animals. Chapter 1 explores the theory needed for understanding the phenomenon of magnetoreception, especially spin chemistry.
The study of magnetically sensitive processes requires the ability to detect tiny changes in the concentrations of the chemical species involved. This has driven the development of state-of-the-art and high-sensitivity techniques, with a particular focus on optical spectroscopy. One such method is the use of cavity-based absorption measurements which increase the optical path length by passing the probe light through the sample multiple times. In this work, a spectrometer that employs the use of an optical cavity – cavity ring-down spectroscopy (CRDS) – was entirely redesigned, rebuilt and recoded. The work to improve the spectrometer, along with an introduction to the optical technique, is described in Chapter 2.
In Chapter 3, the performance of the spectrometer is scrutinised using a model system – flavin mononucleotide (FMN) and hen egg-white lysozyme (HEWL) – allowing for exploration of the detection sensitivity and stability of the apparatus. Time-resolved and magnetically altered reaction yield (MARY) experiments yield results similar to those reported previously, particularly by Dr D. Sheppard. The sensitivity of the rebuilt spectrometer is highlighted, revealing the importance of storage conditions of this widely used “calibration sample” with a slight deviation in the time- and wavelength-resolved spectra obtained when compared to previously reported results. Once shown that the spectrometer is fully functional, the effect of pump laser power on ring-down time is also explored with the hope of improving the signal-to-noise ratio by compensating for fluctuations from the laser.
The core of the thesis describes the measurements performed on cryptochrome 4 from three avian species: the migratory European robin (Erithacus rubecula); the pigeon (Columba livia), which is non-migratory, but known to orient using magnetic cues; and the chicken (Gallus gallus), which is not known to use the geomagnetic field for any purpose. The cryptochromes from the three species share strongly conserved amino acid sequences, but should, to support the cryptochrome hypothesis of magnetoreception, show differences that are based on the function. A marked difference in the spectroscopic results is shown between the migratory robin and the other two species. Additional measurements are performed on a point-mutant of the three cryptochromes in which the final electron transfer partner – a tryptophan residue at position 369 – is replaced so that it cannot be involved in forming the radical pair. It is shown that, while all cryptochrome proteins show some sensitivity of their radical concentrations to applied magnetic fields, that of the migratory robin is most remarkable – both in its magnitude and lifetime. The results of this thesis, presented in Chapter 4, hence provide further support of the radical pair hypothesis of magnetoreception and the data and conclusion formed a substantial part of a Nature publication in 2021.