pteridines pteridine 1,3,5,8-tetraazanaphthalene 132.044 132.12292 Beilstein:115341 C6H4N4 CAS:91-18-9 CHEBI:26372 CHEBI:27601 CHEBI:8626 CPNGPNLZQNNVQM-UHFFFAOYSA-N Gmelin:364077 InChI=1S/C6H4N4/c1-2-9-6-5(8-1)3-7-4-10-6/h1-4H KEGG:C07581 Pteridine azinepurine c1cnc2ncncc2n1 chebi_ontology pteridine mancude organic heterobicyclic parent azaarene ortho-fused heteroarene