Arginine / ARG / R (EnzDes coloring)
Arginine (ARG or R) is a hydrophilic amino acid. Its shape on Foldit is unique.
Because arginine's sidechain can be manipulated in so many different ways, it is often a good choice if you want to manually adjust a sidechain and then wiggle to gain points. Adjusting an arginine can change things quite radically.
In the Foldit Lua interface, the functions structure.GetAminoAcid and structure.SetAminoAcid use lowercase "r" for arginine.
Nitrogen atoms in arginine's sidechain can act as hydrogen bond donors.
Arginine's sidechain contains three nitrogen atoms, which can act as hydrogen bond donors. See the Sidechain Bonding Gallery and the Sidechain Bonding Table for other sidechain donors and acceptors.
For more, see Arginine on wikipedia.