Investigation of magnetic field effects on protein photochemistry using cavity enhanced spectroscopy

Thesis

Some animals, such as migratory birds, are known to have the ability to sense and use magnetic fields for orientation. At the heart of this remarkable ability is believed to be a photoinduced chemical reaction taking place within a blue-light receptor protein called cryptochrome (Cry). It is hypothesised that magnetic fields can play a decisive role in determining reaction rates (and, hence, yields) by acting on the spin states of spin correlated radical pairs (RPs) formed between a flavin chromophore and a tryptophan (Trp) electron-transfer (ET) chain within Crys.

In this work, cavity ring-down spectroscopy (CRDS) has been used for the detection of magnetic field effects (MFEs) via small changes in the differential absorbance (ΔΔA~10⁻⁶) of aqueous protein samples, with sub-microsecond time resolution. The photochemistry and effects of magnetic fields on European robin (Er) Cry, fruit fly Cry, and a plant Cry are characterised. In a key result for this field of research, an MFE in the photochemistry of ErCry is revealed for the first time, lending considerable support to its proposed involvement in bird magnetoreception. Using Cry mutants with modified Trp ET chains, the role of individual Trps in the manifestation of MFEs was explored. Replacing the terminal (i.e. fourth) Trp of the ET chain in ErCry with a redox inert amino acid is shown to significantly increase the optically-detected MFE, indicating that the RP formed between the flavin and the third Trp might be the primarily magnetically sensitive species.

To better understand the relevant biochemical processes in Cry, de novo designed artificial flavoproteins (maquettes) have been used to model the photochemistry of Cry (Chapter 4). The comparatively simple and adaptable design of these model proteins facilitates detailed MFE studies by reducing the complexity of natural Cry. In these maquettes, photoinduced ET leads to the formation of a RP which exhibits MFEs analogous to those in Crys. The profound effect of the donor-acceptor distance on the MFE is demonstrated using flavomaquettes with varying flavin-Trp distances.

The application of cavity enhanced spectroscopy to both natural cryptochromes and artificial flavoproteins, opens up new pathways for the detection and characterisation of MFEs in biologically relevant environments. The enhanced sensitivity of these approaches constitutes a significant step forward in understanding the underlying biochemical processes of animal magnetoreception.

Authors: Zollitsch, T

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Maquette; Cryptochrome; Radical Pair Mechanism; Cavity Ring Down Spectroscopy; Magnetoreception