code: public static final boolean isNucleotide(String groupCode3) {
  		String code = groupCode3.trim();
  		return nucleotides30.containsKey(code)
  				|| nucleotides23.containsKey(code);
  	}
docs: Test if the three-letter code of an ATOM entry corresponds to a
  nucleotide or to an aminoacid.
  
  @param groupCode3
  3-character code for a group.