code: public static String convertThreeLetterCodeToOneLetterCode(String threeLetterCode) {
          AminoAcid[] monomers = createAAs();
          for (int i = 0; i < monomers.length; i++) {
              if (monomers[i].getProperty(RESIDUE_NAME).equals(threeLetterCode)) {
                  return (String) monomers[i].getProperty(RESIDUE_NAME_SHORT);
              }
          }
          return null;
      }
docs: Returns the one letter code of an amino acid given a three letter code.
  For example, it will return "V" when "Val" was passed.