This invention relates to the field of nucleic acid chemistry, more specifically to the field of compositions and processes that are nucleic acid analogs. More specifically, it relates to purine analogs that contain three rings, where the third ring bridges the exocyclic substituent at position 6 to position 7, using the purine numbering system. Still more specifically it relates to analogs having this structure that are able to form nucleobase pairs having the Watson-Crick geometry with a pyrimidine or pyrimidine analog, where the nucleobase pair is joined by hydrogen bonding patterns that either present a standard hydrogen bonding pattern, or a non-standard hydrogen bonding pattern. Most specifically, it to nucleoside analogs that are analogs of isoguanosine, but where the 5-6 ring system of the purine ring in isoguanosine is fused to another five- or six-membered ring, where the fused ring joins the exocyclic amino group with an atom that is, by analogy, at position 7 of the isoguanine ring system. Compositions having this structure can support a base pair with isocytosine joined by three hydrogen bonds, or a base pair with a diaminodiazanaphalene riboside, or 2'-deoxyriboside, or an analog, joined by four hydrogen bonds.