Yggdrasil

The R represents the side chain of the amino acids which make up the anti-parallel beta sheet. Since a wide variety of side chains can exist on a beta -sheet, they generalize the structure by writing Rs instead of specific amino acid side chains. In general, an R is an abbreviation in a chemical structure showing that something else goes where the R is, but the authors are too lazy to show the rest of the molecule, or the R-group is variable.