Chapter 9 – Emerging Biophysics Techniques 425
Two competing models for magnetoreception in birds use either classical physics in
alluding to a magnetic particle (the presence of small magnetic particles of magnetite is found
in the heads of many migratory birds) or a quantum mechanical model based on the gener
ation of radical pairs. The latter theory implicates a molecule called cryptochrome—a light-
sensitive protein that absorbs a blue light photon to generate two radical pairs (molecules
with a single unpaired electron).
This effect has been studied most extensively in the migratory patterns of a bird Erithacus
rubecula (known more commonly as the European robin, the one you have probably seen
yourself, if not in the flesh, then from its ubiquitous images on Christmas cards). Evidence
has grown since the original study published in 2004 (Ritz et al., 2004) that each robin poten
tially utilizes a chemical compass in the form of cryptochrome that is expressed in certain
cells in the retinas of the birds’ eyes (cryptochromes have also been found in photoreceptor
nerve cells in the eyes of other birds and insects and in some plant cells and bacteria).
Cryptochrome contains a flavin cofactor called “FADH” (the hydrogenated form of flavin
adenine dinucleotide, the same cofactor as occurs at various stages in Krebs tricarboxylic
acid cycle; see Chapter 2). One hypothesis is that molecular oxygen can enter a molecular
binding pocket of the cryptochrome molecule, and a radical pair can then be formed upon
the absorption of a single photon of blue light consisting of FADH· and the molecular oxygen
superoxide O2
•−, where “·” indicates an unpaired electron. In this case, an expectation is, from
the possible combinations of spins of the products, a proportion of 25% of pairs will be in the
singlet spin state and 75% in the triplet spin state, though magnetic coupling with the radical
states due to both internal and external magnetic fields, nuclear hyperfine, and Zeeman
coupling, respectively, to the Earth’s magnetic field (see Chapter 5), may also enable singlet/
triplet interconversion.
If the radical pair is in its singlet state, the unpaired electron from the superoxide radical
may transfer to the FADH radical, forming a singlet (FADH−A + O2) state, which has lower
free energy and is magnetically insensitive (note that this electron transfer is not possible
from the triplet state of the radical pair since this would not conserve total spin). That is, the
radicals are self-quenched. However, if the superoxide radical escapes from the molecular
pocket before this quenching electron transfer occurs, then this can result in an extended life
time of the magnetically sensitive radical states for as long as the spins between the electrons
of the FADH· and the O2
•− exhibit quantum entanglement. This presents an opportunity for
an external magnetic field to affect the reaction by modulating the relative orientation of the
electron spins.
If the singlet/triplet radical products have sufficiently long quantum coherence times, they
may therefore be biologically detectable by as a chemical compass—in essence, the entangle
ment would imply that the extent of any coupling observed depends on the local magnetic
field. One hypothesis is that the superoxide formed in the radical pair is particularly important
in magnetoreception. This is because the oxygen radical does not exhibit hyperfine coupling,
and so any observed coupling is a metric of the external magnetic field.
Quantum entanglement–based magnetoreception is speculative at several levels. First, it
implies that modulation of the reaction products by a magnetic field would lead to a behav
ioral modulation of the bird’s flight through, presumably, the bird’s sense of vision combined
with moving its head to scan the B-field landscape, for example, generating brighter or darker
regions on the bird’s retina. But there is no clear experimental evidence for this effect. Also, it
is unclear specifically where in the bird’s eye this putative radical pair reaction would actually
takes place. It may well be that high-resolution biophysical structural imaging tools, such as
AFM, may provide some future insight here, at least into mechanisms of free radical forma
tion upon light absorption.
Most challenging to this theory, however, are estimates that the entangled state would
need to last >100 μs to have sufficient sensitivity for use a chemical compass given the low
strength of the Earth’s B-field of ~5 mT. However, the longest entangled states observed to
date experimentally in the laboratory have lifetimes of ~80 μs, seen in a special inorganic
molecule from a class called endohedral fullerenes (or endofullerenes or simply fullerenes for
short) known as “N@C60” that consists of a shell of 60 carbon atoms with a single nitrogen
atom at the center and is highly optimized for extended quantum entanglement since the