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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.
There are about 10 nonpolar amino acids with R-groups that are not stable when in contact with water. They are hydrophobic. About 5 amino acids have polar side chains, R-groups which do not ionize or become positively or negatively charged. These R-groups are neither strongly hydrophilic nor hydrophobic. Atoms in long molecules, such as polypeptides, are not rigidly fixed in space or position.