Cavity-Enhanced Detection of Biologically Relevant Magnetic Field Effects

Magnetoreception is the ability of some animals to use the weak magnetic field of the Earth for navigation over long-distance migrations. It is a well-known phenomenon, but its underlying biophysical mechanisms remain poorly understood. One proposal involves light-induced, magnetically sensitive chemical reactions occurring within cryptochrome proteins, rationalised via the radical pair mechanism (Chapter 1). The absence of evidence in support of this hypothesis is in part due to the lack of sufficiently sensitive techniques to measure magnetic field effects (MFEs) in biological samples. Cavity-enhanced detection, most commonly in the form of cavity ring-down spectroscopy (CRDS) or cavity-enhanced absorption spectroscopy (CEAS), is widely used in the gas phase to provide significant sensitivity gains over traditional single-pass measurements (Chapter 2). However, successful studies in the condensed phase are less prevalent due to the additional background losses inherent to the sample. This thesis reports on the application of broadband (i.e. monitoring > 100nm) variants of CRDS and CEAS to the study of MFEs on the radical recombination reactions of flavin-based systems in solution. The broadband CRDS (BBCRDS) instrument employed in Chapter 4 is able to monitor the spectral changes induced by magnetic fields with submicrosecond time resolution. However, the need to scan both the probe wavelength and time delay to construct time-resolved spectra leads to prohibitively long acquisition times, and hence exposure of sensitive samples to high numbers of photons. The broadband CEAS (BBCEAS) studies reported in Chapter 5 combine the high irradiance and spectral coverage of a supercontinuum radiation (SCR) source with a CCD detector to simultaneously acquire absorption spectra across the visible region (480–700nm). The CW nature of this technique precludes the possibility of following radical pair kinetics in real time. In an effort to combine the respective advantages of these two instruments, which individually have represented powerful advances in capability, a new cavity-enhanced technique is reported for the first time (Chapter 6). The result, optical cavity-enhanced transient absorption spectroscopy (OCTAS), is able to simultaneously monitor spectral evolution and associated MFEs on the microsecond timescale, with comparable sensitivity to the existing techniques. Magnetic responses in animal cryptochrome proteins have successfully been recorded using all three techniques, lending considerable weight to the hypothesis that these molecules are at the heart of the magnetic sense in animals.