What's special about the purine scaffold?
It is important to remember that the purine scaffold is one of the most fundamental organic structures in all lifeforms. From an evolutionary standpoint, this means that there must have been a simple, effective way for this scaffold to be prepared in the pre-biotic earth. After all, the first organisms to contain purine scaffolds did not have &quot;access&quot; to complex enzymes, proteins, etc.
Take a look at the molecular formula for adenine, a simple purine derivative. C5H5N5. We could rewrite this as (HCN)5. As the molecular formula alone might lead you to conclude, adenine can be made from only one chemical: hydrogen cyanide. Interestingly, hydrogen cyanide was a very abundant chemical in the earth's primordial atmosphere. Roy et al.1 replicated the atmospheric conditions of the primordial earth and found that substantial quantities of adenine could be produced! The mechanism for this reaction is quite beautiful, and I will include Peter Jacobi's rendition of it below. The mechanism for the formation of adenine also explains why other constitutional isomers are not produced. We must consider the alternating nature involved in these nucleophilic-electrophilic interactions (e.g. two nitrogen atoms will never &quot;decide&quot; to couple).

So, to conclude, the purine substructure is &quot;special&quot; because it not only has the unique ability to hydrogen bond and form a base pair for DNA replication but was also incredibly easy to synthesize in the primordial earth's atmosphere. Although life forms do not use HCN to synthesize the purine substructure anymore, its synthesis is still incredibly simple.
1 Roy, D.; Najafian, K.; Schleyer, P. V. R. Proc. Natl. Acad. Sci USA 2007, 104, 17272-17277.