Editing Magnetoreception (section)

==== Cryptochrome ====

[[File:Quantum Magnetoreception in Birds.svg|thumb|center|upright=2.5|The [[radical pair mechanism]] has been proposed for quantum magnetoreception in birds. It takes place in [[cryptochrome]] molecules in cells in the birds' [[retina]]s.<ref name="Hore Mouritsen 2022" />]]

According to the first model, magnetoreception is possible via the [[radical pair mechanism]],<ref name="Hore 2016">{{cite journal |last1=Hore |first1=Peter J. |author1-link=Peter Hore (chemist) |last2=Mouritsen |first2=Henrik |date=5 July 2016 |title=The Radical-Pair Mechanism of Magnetoreception |journal=[[Annual Review of Biophysics]] |volume=45 |issue=1 |pages=299–344 |doi=10.1146/annurev-biophys-032116-094545 |pmid=27216936 |s2cid=7099782 |url=https://ora.ox.ac.uk/objects/uuid:c1e3c8ca-98b3-4e9d-8efd-0b9ad9b965eb |doi-access=free }}</ref> which is well-established in [[spin chemistry]]. The mechanism requires two molecules, each with unpaired electrons, at a suitable distance from each other. When these can exist in states either with their [[Spin (physics)|spin]] axes in the same direction, or in opposite directions, the molecules oscillate rapidly between the two states. That oscillation is extremely sensitive to magnetic fields.<ref name="Rodgers 2009">{{cite journal |last=Rodgers |first=Christopher |date=1 January 2009 |title=Magnetic field effects in chemical systems |journal=[[Pure and Applied Chemistry]] |volume=81 |issue=1 |pages=19–43 |doi=10.1351/PAC-CON-08-10-18 |s2cid=96850994 |url=https://ora.ox.ac.uk/objects/uuid:db8c2eed-f8c8-4bf7-9e56-05375c725377 |doi-access=free }}</ref><ref name="Steiner Ulrich 1989">{{cite journal |last1=Steiner |first1=Ulrich E. |last2=Ulrich |first2=Thomas |date=1 January 1989 |title=Magnetic field effects in chemical kinetics and related phenomena |journal=Chemical Reviews |volume=89 |issue=1 |pages=51–147 |doi=10.1021/cr00091a003 |url=https://kops.uni-konstanz.de/bitstreams/8255f73d-49e1-4084-b2b7-c350f70ec767/download}}</ref><ref name="Woodward 2002">{{cite journal |last=Woodward |first=J. R. |date=1 September 2002 |title=Radical pairs in solution |journal=Progress in Reaction Kinetics and Mechanism |volume=27 |issue=3 |pages=165–207 |doi=10.3184/007967402103165388 |s2cid=197049448 |doi-access=free }}</ref><ref name="Rodgers Hore 2009" /> Because the Earth's magnetic field is extremely weak, at 0.5 [[gauss (unit)|gauss]], the radical pair mechanism is currently the only credible way that the Earth's magnetic field could cause chemical changes (as opposed to the mechanical forces which would be detected via magnetic crystals acting like a compass needle).<ref name="Rodgers Hore 2009">{{cite journal |last1=Rodgers |first1=C. T. |last2=Hore |first2=Peter J. |author1-link=Peter Hore (chemist) |year=2009 |title=Chemical magnetoreception in birds: The radical pair mechanism |journal=[[PNAS]] |volume=106 |issue=2 |pages=353–360 |doi=10.1073/pnas.0711968106 |pmid=19129499 |pmc=2626707 |bibcode=2009PNAS..106..353R |doi-access=free }}</ref>

In 1978, Schulten and colleagues proposed that this was the mechanism of magnetoreception.<ref>{{cite journal |last1=Schulten |first1=Klaus |last2=Swenberg |first2=Charles E. |last3=Weiler |first3=Albert |date=1 January 1978 |title=A Biomagnetic Sensory Mechanism Based on Magnetic Field Modulated Coherent Electron Spin Motion |url=https://experts.illinois.edu/en/publications/a-biomagnetic-sensory-mechanism-based-on-magnetic-field-modulated |journal=[[Zeitschrift für Physikalische Chemie]] |volume=111 |issue=1 |pages=1–5 |doi=10.1524/zpch.1978.111.1.001 |s2cid=124644286 |url-access=subscription }}</ref> In 2000, scientists proposed that [[cryptochrome]] – a [[flavoprotein]] in the [[rod cell]]s in the eyes of birds – was the "magnetic molecule" behind this effect.<ref>{{cite web |last1=Solov'yov |first1=Ilia |last2=Schulten |first2=Klaus |title=Cryptochrome and Magnetic Sensing |url=http://www.ks.uiuc.edu/Research/cryptochrome/ |access-date=10 January 2022 |website=Theoretical and Computational Biophysics Group, [[University of Illinois Urbana-Champaign]] }}</ref> It is the only protein known to form photoinduced radical-pairs in animals.<ref name="Hore 2016" /> The function of cryptochrome varies by species, but its mechanism is always the same: exposure to blue light excites an electron in a [[chromophore]], which causes the formation of a radical-pair whose electrons are [[Quantum entanglement|quantum entangled]], enabling the precision needed for magnetoreception.<ref name="Wiltschko Ahmad Nießner Gehring 2016" /><ref>{{cite journal |last1=Hiscock |first1=Hamish G. |last2=Worster |first2=Susannah |last3=Kattnig |first3=Daniel R. |last4=Steers |first4=Charlotte |last5=Jin |first5=Ye |last6=Manolopoulos |first6=David E. |last7=Mouritsen |first7=Henrik |last8=Hore |first8=P. J. |author8-link=Peter Hore (chemist) |date=26 April 2016 |title=The quantum needle of the avian magnetic compass |journal=[[PNAS]] |volume=113 |issue=17 |pages=4634–4639 |doi=10.1073/pnas.1600341113 |pmid=27044102 |pmc=4855607 |bibcode=2016PNAS..113.4634H |doi-access=free}}</ref>

Many lines of evidence point to cryptochrome and radical pairs as the mechanism of magnetoreception in birds:<ref name="Hore Mouritsen 2022">{{cite journal |last1=Hore |first1=Peter J. |author1-link=Peter Hore (chemist) |last2=Mouritsen |first2=Henrik |title=The Quantum Nature of Bird Migration |journal=[[Scientific American]] |url=https://www.scientificamerican.com/article/how-migrating-birds-use-quantum-effects-to-navigate/ |date=April 2022 |pages=24–29}}</ref>

* Despite 20 years of searching, no biomolecule other than cryptochrome has been identified capable of supporting radical pairs.<ref name="Hore Mouritsen 2022" /><!--page 28-->
* In cryptochrome, a yellow molecule [[flavin adenine dinucleotide]] (FAD) can absorb a photon of blue light, putting the cryptochrome into an activated state: an electron is transferred from a tryptophan amino acid to the FAD molecule, forming a radical pair.<ref name="Hore Mouritsen 2022" /><!--page 28-->
* Of the six types of cryptochrome in birds, cryptochrome-4a (Cry4a) binds FAD much more tightly than the rest.<ref name="Hore Mouritsen 2022" /><!--page 28-->
* Cry4a levels in [[migratory birds]], which rely on navigation for their survival, are highest during the spring and autumn migration periods, when navigation is most critical.<ref name="Hore Mouritsen 2022" /><!--page 28-->
* The Cry4a protein from the [[European robin]], a migratory bird, is much more sensitive to magnetic fields than similar but not identical Cry4a from pigeons and chickens, which are non-migratory.<ref name="Hore Mouritsen 2022" /><!--page 29-->

These findings together suggest that the Cry4a of migratory birds has been [[Natural selection|selected]] for its magnetic sensitivity.<ref name="Hore Mouritsen 2022" /><!--page 29-->

Behavioral experiments on migratory birds support this theory. Caged migratory birds such as robins display migratory restlessness, known by [[ethologist]]s as ''[[Zugunruhe]]'', in spring and autumn: they often orient themselves in the direction in which they would migrate. In 2004, Thorsten Ritz showed that a weak radio-frequency electromagnetic field, chosen to be at the same frequency as the singlet-triplet oscillation of cryptochrome radical pairs, effectively interfered with the birds' orientation. The field would not have interfered with an iron-based compass. Further, birds are unable to detect a 180 degree reversal of the magnetic field, something they would straightforwardly detect with an iron-based compass.<!--Except at the equator, north and south can be told apart by the fact that the field dips downwards towards the poles.--><ref name="Hore Mouritsen 2022" />

[[File:Effect of RF interference on Magnetoreception in Birds.svg|thumb|center|upright=2.5|Very weak [[Electromagnetic interference|radio-frequency interference]] prevents [[Bird migration|migratory]] robins from orienting correctly to the [[Earth's magnetic field]]. Since this would not interfere with an iron compass, the experiments imply that the birds use a radical-pair mechanism.<ref name="Hore Mouritsen 2022"/>]]

From 2007 onwards, Henrik Mouritsen attempted to replicate this experiment. Instead, he found that robins were unable to orient themselves in the wooden huts he used. Suspecting extremely weak radio-frequency interference from other electrical equipment on the campus, he tried shielding the huts with aluminium sheeting, which blocks electrical noise but not magnetic fields. When he earthed the sheeting, the robins oriented correctly; when the earthing was removed, the robins oriented at random. Finally, when the robins were tested in a hut far from electrical equipment, the birds oriented correctly. These effects imply a radical-pair compass, not an iron one.<ref name="Hore Mouritsen 2022"/>

In 2016, Wiltschko and colleagues showed that European robins were unaffected by [[Local anesthesia|local anaesthesia]] of the upper beak, showing that in these test conditions orientation was not from iron-based receptors in the beak. In their view, cryptochrome and its radical pairs provide the only model that can explain the avian magnetic compass.<ref name="Wiltschko Ahmad Nießner Gehring 2016">{{cite journal |last1=Wiltschko |first1=Roswitha |last2=Ahmad |first2=Margaret |last3=Nießner |first3=Christine |last4=Gehring |first4=Dennis |last5=Wiltschko |first5=Wolfgang |title=Light-dependent magnetoreception in birds: the crucial step occurs in the dark |journal=[[Journal of the Royal Society Interface]] |volume=13 |issue=118 |year=2016 |doi=10.1098/rsif.2015.1010 |page=20151010 |pmid=27146685 |pmc=4892254 }} A supplement to the paper summarizes alternative hypotheses on avian compass mechanisms.</ref> A scheme with three radicals rather than two has been proposed as more resistant to spin relaxation and explaining the observed behaviour better.<ref name="Kattnig 2017">{{cite journal |last=Kattnig |first=Daniel R. |title=Radical-Pair-Based Magnetoreception Amplified by Radical Scavenging: Resilience to Spin Relaxation |journal=The Journal of Physical Chemistry B |volume=121 |issue=44 |date=26 October 2017 |doi=10.1021/acs.jpcb.7b07672 |pages=10215–10227 |pmid=29028342 |hdl=10871/30371 | hdl-access=free }}</ref>