Counting integral numbers of amino acid residues per polypeptide chain

Proteins have integral numbers of each of the 20 amino acids. However, all the currently accepted methods of determining this number measure the ratio of moles of amino acid residue per mole of protein. This value is rarely close to an integer, due to experimental errors in determination of the molar amounts of both amino acid residues and polypeptide chain. A simple method which gives integral values of amino acid residues per polypeptide chain, independent of any other properties of the protein, would be useful in characterising proteins. We describe here one method; it is illustrated for the case of Cys residues, although the approach should be useful for many of the other 19 usual amino acids.