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. For more, see Arginine on wikipedia.