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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.
Based on the physical and chemical properties of R groups, the 20 amino acids of proteins may be classified as follows. 1. Acidic: including aspartic acid (aspartate) and glutamatic acid (glutamate). In a neutral solution, the R group of an acidic amino acid may lose a proton and become negatively charged. 2. Basic: including lysine, arginine and histidine. In a neutral solution, the R group of a basic amino acid may gain a proton and become positively charged. Interaction between positive and negative R groups may form a salt bridge, which is an important stabilizing force in proteins. 3. Aromatic: including tyrosine, tryptophan and phenylalanine. Their R groups contain an aromatic ring. 4.