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Tautomers are different forms of a molecule which have the same chemical formula and overall structure. Tautomers are a type of isomer.

In Foldit, the amino acid histidine has two tautomers. In one tautomer, the nitrogen at atom 10 acts as a hydrogen bond donor, and the nitrogen at atom 7 acts as a hydrogen bond acceptor. In the other tautomer, the atom 10 is an acceptor, and atom 7 is a donor.

In histidine, the difference between the tautomers is a where a hydrogen atom, often referred to as "proton", is located. The nitrogen bonded to the hydrogen becomes the hydrogen bond donor.

Histidine atoms 7 and 10 maybe switch between acceptor and donor when the sidechain moves to a different position, such as during a shake.

In contrast, leucine and isoleucine have the same chemical formula, so they are isomers. Their overall structure is different, so they are not tautomers.

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