Simulating the avian quantum compass with radical pair spin dynamics

Thesis

The mechanism underlying magnetic sensing in night migratory songbirds is believed to involve pairs of radicals, molecules containing a single unpaired electron, formed by photoexcitation of cryptochrome proteins in the avian retina. The overall quantum spin state of such radical pairs, and hence the concentration of subsequent reaction products, can be dependent on the direction of Earth strength external magnetic fields. While in vivo evidence for the so called Radical Pair Mechanism of magnetoreception is mostly circumstantial, the theory is well supported by photochemical experiments on purified cryptochrome proteins, which have been found in the avian retina and are known to form magnetically sensitive radical pairs in vitro. Reliable simulations of the underlying quantum spin dynamics face a number of challenges, but remain vital in guiding and interpreting experimental research.

Chapter 1 introduces the theoretical formalism of radical pair spin dynamics and provides an overview of the past experimental and computational findings that have given rise to a well-founded hypothesis for magnetoreception. The computational tools used to broach the viability of cryptochrome radical pair magnetoreceptors throughout the thesis are reviewed in Chapter 2.

The following chapters investigate aspects of a model radical pair based quantum compass. Several quantum mechanical and semiclassical techniques, most notably the Stochastic Schrödinger Equation approach, are utilised in Chapter 3 to obtain reliable estimates for the experimentally relevant half-field parameter, B_1/2 under realistic experimental conditions. Chapter 4 goes on to explore the effects of weak radiofrequency fields on the performance of the quantum compass, and pushes the limits of the conventional spin dynamics model to highlight the disparity between experimental observations of radiofrequency disruption in migratory birds, and the current best understanding of the underlying spin dynamics.

Chapters 5 and 6 consider, respectively, the impact of isotopic substitution and the inclusion of dynamic internal magnetic interactions on the cryptochrome quantum compass, using advanced computational techniques to consider the most realistic spin systems possible, and avoiding the common pitfalls of simplified toy-model approaches. Finally, a more flexible approach is taken in Chapter 7 to decipher unexpected results from the first experimental reports of anisotropic magnetic field effects on cryptochrome proteins.

Authors: Benjamin, PL

• DOI: 10.5287/ora-gakevjjok
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English