The bird magnetic compass may get a boost from a small chiral twist inside a light-sensing protein, according to a Kobe University-led team. Using time-resolved electron spin resonance, the researchers tracked structural changes in cryptochrome, a protein thought to help migratory birds sense Earth’s magnetic field, and linked the rotation of one amino acid to a spin effect that strengthens direction sensing in their model. The work appeared in The Journal of Physical Chemistry Letters on 30 October (Japan time).
The team was led by Professor Yasuhiro Kobori of Kobe University’s Research Center for Light-Life Innovation, with Masaaki Fuki and Misato Hamada of Kobe, Tatsuya Iwata of Toho University, Hiroki Nagashima of Saitama University, Hideki Kandori of Nagoya Institute of Technology and Stefan Weber of the University of Freiburg.
How the bird magnetic compass is thought to work
Cryptochromes are found in migratory birds, plants and other organisms. They contain a pigment called FAD that absorbs blue light and then pulls electrons step by step from nearby tryptophan amino acids. This creates magnetic “radical pairs.” A long-standing hypothesis holds that the spin states of these pairs respond to the direction of an external magnetic field, changing the yield of a chemical signal and letting the animal sense direction.
Kobe University says this compass effect has not been fully tested at the molecular or atomic level. A separate phenomenon, chirality-induced spin selectivity (CISS), describes how twisted, handed structures can favor electrons of one spin direction. Its link to biological magnetic sensing was largely unexplored.
What the Kobe team measured
The researchers expressed cryptochrome-DASH from the African clawed frog, Xenopus laevis, in E. coli and hit it with pulsed laser light. They recorded microwave absorption and emission signals from the resulting radical pairs with 0.1-microsecond precision, first at −153 °C and then at warmer temperatures of −60 °C and −33 °C.
- In the second charge-separated state, the analysis showed that a tryptophan residue undergoes a chiral rotation, producing a CISS effect of about 8%.
- In the third, longer-range state, a smaller CISS effect of 6% or less remained.
- The team concluded that the CISS effect seen later is carried over from the twist that happens at the second step.
A stronger signal in the bird magnetic compass model
The researchers then used the crystal structure of pigeon cryptochrome 4 to predict how the signaling yield varies with magnetic field direction. Without CISS, the conventional radical pair model gave an anisotropy of only 0.3%, which the team says would make a poor compass.
Including even a small 6% CISS effect amplified the anisotropy to 1.6% at the 332.5 millitesla field used in the analysis. The yield dropped sharply when the field pointed along FAD’s long axis and rose along its short axis, giving the reaction a clearer directional preference.
Kobe University describes this as the first demonstration that chiral rotation of a specific amino acid can enhance the magnetic compass effect.
What comes next
The team plans spectroscopic measurements to study the chiral rotation in more detail and to track how reversing the field flips the polarized signals and changes signal yields. The researchers say the findings could guide weak magnetic sensors built from biomolecules and improve understanding of light-driven magnetic sensing in living things.
In my view, the gap between frozen protein samples and a bird in flight is still wide, since the compass enhancement here is a model prediction rather than a measurement in live animals. For more on magnetism and spin in research, see our stories on magnetism in an ammonia photocatalyst and a diamond chip for quantum networks.
FAQ
What is the bird magnetic compass?
It is the ability of migratory birds to sense Earth’s magnetic field direction, thought to involve light-triggered radical pairs in cryptochrome proteins.
What did Kobe University find?
A tryptophan in cryptochrome rotates in a chiral way after light absorption, producing a CISS effect of about 8% that, in modeling, strengthens direction-dependent signaling.
Was this tested in live birds?
No. Measurements used frog cryptochrome at low temperatures, and the compass enhancement was predicted using pigeon cryptochrome’s structure.