code: public static void addAcidicOxygen(IAminoAcid acid) throws CDKException {
          if (acid.getCTerminus() == null) throw new CDKException("Cannot add oxygen: C-terminus is not defined!");
  
          IAtom acidicOxygen = acid.getBuilder().newInstance(IAtom.class, "O");
          acid.addAtom(acidicOxygen);
          acid.addBond(acid.getBuilder().newInstance(IBond.class, acid.getCTerminus(), acidicOxygen,
                  Order.SINGLE));
      }
docs: Adds the singly bonded oxygen from the acid group of the AminoAcid.
  
  @param  acid         AminoAcid to which to add the oxygen
  @throws CDKException when the C-terminus is not defined for the given AminoAcid