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Cryptochrome may be key to organisms across the tree of life sensing the earths magnetic field during travel over large distances.

Cryptochrome responds to blue light exposure, by absorbing a blue photon, and transferring an electron from an orbitol near the surface of the protein, where it is paired with a second electron, to a location deeper in the protein, thus creating a radical pair. Experimentally, it is known that cryptochrome emits green photons as the electron transfers back to its original orbital and partner electron. Moreover, it was shown in 2016 by P.J. Hore and colleagues that turning on a magnetic field can decrease the rate of ba ck transfer of the electron. In their experiment, they continuously exposed a population of cryptochrome molecules to blue light, and measured the green fluorescence. For illustrative purposes, let us imagine that the population consisted of a million cryptochrome molecules. In their measurement, they counted 1000 green photon hitting their detector every millisecond, which means 1000 cryptochrome molecules were undergoing a back transfer of the electron. And they continuously measured 1000 green photons hitting the detector for every 1 ms interval. It appears that while 1000 were going through back transfer, the previous 1000 that had backtransferred 1 ms ago, had reabsorbed a blue photon and again had a radical pair. Therefore at any given moment 999000 cryptochromes had a radical pair, and 1000 cryptochromes had the electrons in one orbital together. Excitingly, when the researchers turned on a magnetic field, they measured a one percent decrease in green fluorescence - meaning, in our case, 990 green photon hit the detector over 1 ms, instead of 1000 - ten less cryptochromes were undergoing back transfer.

What happened?

The theory, is the electrons can spin up and down, and a given magnetic field of a given strength and direction, and frequency, can cause a new population distribution of ups and downs. With no magnetic field its 50 percent up and 50 percent down - whether or not there is blue light exposure. But with blue light exposure and a magnetic field, its a different story. No blue light and a magnetic field is still 50-50, because electron in the same orbital need to spin in opposite directions. But light decouples the electron and they can independently spin in either direction, both up, both down, or one up and one down. The magnetic field then enforces a non equal population distribution of up and down - it a distribution of up and downs that overall after adding all the direction vectors, will result in a vector that is more parallel to the direction of the magnetic field. 70 percent up and 30 percent down may be the result. Because of thermal energy and the entropic cost of being too organized, the ups and downs are always somewhere in between the ideal alignment to the magnetic field, and the original 50 -50 distribution.

This 70-30 distribution is important, because when two electron are spinning in the same way, but "trying" to repair, they cannot. Only once they eventually randomly spin in oppositve direction can they rejoin and back transfer occur. This explains the reduction from 1000 by 10 - since every millisecond 10 cryptochromes may no longer be excited by the blue photon, but still cannot backtransfer, because they are spinning in the same direction.

Amazingly through careful choice of three tryptophanes, electron transfer occurs with light, and is influenced by a magnetic field, such that the protein had a new liklihood of going back to its original state as a function of time.

Lastly, to speculate in this forum - perhaps the clusters and liqued phase transitions that cryptochromes undergoes and forms with blue light exposure, amplify this magnetic sensing effect. In the cluster, each cryptochrome is restricted from moving relative to the earth's magnetic field or any external magnetic field, because is held by its neighbors, and its "neighborhood", meaning all the cryptochromes in the cluster is large and also not moving much relative to the earth's coordinates. --- but does this matter?

If the movement of an electron back to its original orbital creates a local magnetic field, than the cryptochrome proteins with a path for back transfer that is an a particular orientation relative to the externally applied magnetic field, may had an additional energetic cost, as the external magnetic field "demands" that locals magnetic field line up with its direction.

See also Cryptochrome. </StructureSection>


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