a word for amino acids (protein letters) after they’ve been incorporated into proteins or peptides. Amino acids get their name because they have an amino group on one end and a carboxylic acid group on the other end. When amino acids join together through peptide bonds, they do so by joining the amino group of one to the carboxylic acid group of the other, losing a water equivalent in the process. So you no longer have amino acids, what you do have is the left-over “residues.” The residues include the unique side chain (aka R group) of the amino acids they came from, they’ve just lost a bit of the generic backbone in the bond forming process. Because the side chain is what’s most often “cared about,” the term “amino acid” is often used to refer to residues. I do this sometimes because the term “residue” can be confusing to people who aren’t deep in the throngs of biochemistry, but there is a distinction.