code: public static Character getOneLetterCode(ChemComp cc, ChemicalComponentDictionary dictionary){
  		if ( cc.getResidueType() == ResidueType.nonPolymer )
  			return null;
  
  		if ( cc.isStandard())
  			return cc.getOne_letter_code().charAt(0);
  
  		ChemComp parent = dictionary.getParent(cc);
  		if ( parent == null){
  			//System.err.println("parent is null " + cc);
  			return cc.getOne_letter_code().charAt(0);
  		}
  		PolymerType poly = cc.getPolymerType();
  		if (( poly == PolymerType.peptide) || ( poly == PolymerType.dpeptide)){
  			Character c = getAminoOneLetter(parent.getId());
  			if ( c == null)
  				c = UNKNOWN_ONE_LETTER_CODE;
  			return c;
  		}
  		if ( poly == PolymerType.dna){
  			Character c = getDNAOneLetter(parent.getId());
  			if (c == null)
  				c = UNKNOWN_NUCLEOTIDE;
  			return c;
  
  		}
  		return cc.getMon_nstd_parent_comp_id().charAt(0);
  	}
docs: so does: 4F3,5ZA and others