Kobe Team Models How Chiral Twists in a Protein May Sharpen the Magnetic Compass of Birds
Spectroscopy of cryptochrome suggests a spin-filtering effect could raise its directional sensitivity from about 0.3% to 1.6%. The authors call it a theoretical prediction.
Researchers in Japan have proposed a molecular mechanism that could make a light-sensitive protein a much better magnetic compass. The team, led by Yasuhiro Kobori of Kobe University, reports that a small chiral twist in the protein cryptochrome could strengthen its response to the direction of a magnetic field. The study was published Oct. 30 in The Journal of Physical Chemistry Letters.
Cryptochrome is a blue-light-sensitive protein linked to magnetoreception, the ability of some animals to sense Earth's magnetic field. A leading idea is that light creates a pair of molecules, each carrying an unpaired electron, and that the magnetic field changes how those electron spins evolve, which in turn changes the chemical yield. The effect in a weak field is tiny, and how it could be strong enough to guide a bird has been an open question.
Kobori's group used time-resolved electron paramagnetic resonance spectroscopy on cryptochrome-DASH from the African clawed frog, Xenopus laevis. They excited the protein with a laser pulse and measured it at temperatures from minus 153 degrees Celsius to minus 33 degrees Celsius. They then ran quantum-mechanical simulations that included chirality-induced spin selectivity, known as CISS, along with exchange coupling, electron-electron dipolar coupling and hyperfine interactions.
The spectra support a structural rearrangement in which an oxidized tryptophan residue, called WB(H)•+, undergoes a chiral rotation in the second charge-separated state. The researchers estimate the CISS contribution for that state at 8%. For the longer-range WC(H)•+ state they inferred a smaller contribution, no greater than 6%. Spin coherence from the earlier charge-separation step appears to transfer to the terminal radical pair.
Those numbers matter in the model. Without CISS, the modeled directional sensitivity, called reaction-yield anisotropy, is about 0.3%. With a 6% CISS contribution, it rises to about 1.6% at a field of 332.5 millitesla. The model also predicts differences in the response when the direction of the magnetic field is reversed, a signature that could be tested.
The authors describe the enhanced directional response as "a theoretical prediction based on spectroscopic analysis." It has not been shown to affect magnetic navigation in living animals, and the experiments used a frog protein rather than a bird's. Future work will test the predicted reversal of spin-polarization signals and measure how molecular dynamics affect signaling yields.
The team includes researchers from Toho University, Saitama University, Nagoya Institute of Technology and the University of Freiburg. If the chiral mechanism holds up, it would link two fields that rarely meet, spin physics in chiral molecules and the biology of animal navigation, and give experimenters a concrete effect to look for.
