https://www.ks.uiuc.edu/Publications/Papers/PDF/RITZ2000/RITZ2000.pdf

A Model for Photoreceptor-Based Magnetoreception in Birds

Electron transfer in biological molecules provides both insight and inspiration for developing chemical systems having similar functionality. Photosynthesis is an example of an integrated system in which light harvesting, photoinduced charge separation, and catalysis combine to carry out two thermodynamically demanding processes, the oxidation of water and the reduction of carbon dioxide. The development of artificial photosynthetic systems for solar energy conversion requires a fundamental understanding of electron-transfer reactions between organic molecules. Since these reactions most often involve single-electron transfers, the spin dynamics of photogenerated radical ion pairs provide important information on how the rates and efficiencies of these reactions depend on molecular structure. Given this knowledge, the design and synthesis of large integrated structures to carry out artificial photosynthesis is moving forward. An important approach to achieving this goal is the development of small, functional building blocks, having a minimum number of covalent bonds, which also have the appropriate molecular recognition sites to facilitate self-assembly into a complete, functional artificial photosynthetic system.

Energy, charge, and spin transport in molecules and self-assembled nanostructures inspired by photosynthesis.

Migratory birds travel vast distances each year, finding their way by various means, including a remarkable ability to perceive the Earth's magnetic field. Although it has been known for 40 years that birds possess a magnetic compass, avian magnetoreception is poorly understood at all levels from the primary biophysical detection events, signal transduction pathways and neurophysiology, to the processing of information in the brain. It has been proposed that the primary detector is a specialized ocular photoreceptor that plays host to magnetically sensitive photochemical reactions having radical pairs as fleeting intermediates. Here, we present a physical chemist's perspective on the “radical pair mechanism” of compass magnetoreception in birds. We outline the essential chemical requirements for detecting the direction of an Earth-strength ≈50 μT magnetic field and comment on the likelihood that these might be satisfied in a biologically plausible receptor. Our survey concludes with a discussion of cryptochrome, the photoactive protein that has been put forward as the magnetoreceptor molecule.

https://www.pnas.org/content/pnas/106/2/353.full.pdf

Chemical magnetoreception in birds: The radical pair mechanism

Approximately 50 species, including birds, mammals, reptiles, amphibians, fish, crustaceans and insects, are known to use the Earth's magnetic field for orientation and navigation. Birds in particular have been intensively studied, but the biophysical mechanisms that underlie the avian magnetic compass are still poorly understood. One proposal, based on magnetically sensitive free radical reactions, is gaining support despite the fact that no chemical reaction in vitro has been shown to respond to magnetic fields as weak as the Earth's ( approximately 50 muT) or to be sensitive to the direction of such a field. Here we use spectroscopic observation of a carotenoid-porphyrin-fullerene model system to demonstrate that the lifetime of a photochemically formed radical pair is changed by application of < or =50 microT magnetic fields, and to measure the anisotropic chemical response that is essential for its operation as a chemical compass sensor. These experiments establish the feasibility of chemical magnetoreception and give insight into the structural and dynamic design features required for optimal detection of the direction of the Earth's magnetic field.

Chemical compass model of avian magnetoreception

The origin of the effect that a magnetic field has on various electronic properties of organic semiconductors is still controversial. It is now shown that substituting hydrogen for deuterium in conducting polymers changes the response to a magnetic field substantially, proving the essential part played by hyperfine interaction in this effect.

Isotope effect in spin response of pi-conjugated polymer films and devices.

The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of ...

Effects of weak magnetic fields on free radical recombination reactions

Exact analytical expressions for the evolution of a non-diffusing spin-correlated radical pair in a strong magnetic field are presented. The recombination of singlet and triplet pairs at different rates, and the coherent evolution caused by magnetic interactions, lead to radical pair concentrations which are the sum of four time-dependent terms : two decaying exponentials and two exponentially damped oscillations. The predicted time dependence is used to illuminate the recombination dynamics of the primary radical pair in photosynthetic bacterial reaction centres. Although mono-exponential decay is expected ˜ 20 ns after the formation of each individual radical pair, the inhomogeneous distribution of Larmor frequencies of the two electron spins leads to non-exponential kinetics and different time constants for the disappearance of singlet and triplet pairs, and the formation of singlet and triplet recombination products.

U. Till

Papers

Effects of weak magnetic fields on free radical recombination reactions

Radical pair kinetics in a magnetic field

The influence of very small magnetic fields on radical recombination reactions in the limit of slow recombination