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#4. 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.
They all consist of a central carbon atom linked to an R group (also called a side chain), a hydrogen atom, an amino (NH2) group and a carboxylic acid (COOH) group. The different amino acids (like the 4 examples above) differ in the structure of their R groups. Glycine's R group is a single H atom. Phenylalanine's R group differs from alanine's R group by a phenyl group. Alanine's R group is simply CH3 (a methyl group).