Purines theoretical considerations

The most important substructures that have been subject to theoretical consideration in solution and belong to the present category, are the five-member rings with two to four hetero atoms (Figures 6.4 and 6.5), six-member rings as pyridine and pyrimidine derivatives, condensed systems as the purine bases (Figure 6.6), and lactam-lactim systems. 

The formation of purines in interstellar space has been considered feasible for some considerable time. A theoretical study (using ab initio methods) on the mechanism of adenine formation from monocyclic HCN pentamers has been reported and has afforded a deeper insight into the gas-phase chemistry of possible purine syntheses. The authors drew the following conclusions from their results ... 

Theoretically, the purine- and pyrimidine-based nucleic acid constituents and the barbiturates have the potential to occur in several tautomeric forms of the keto/ enol and amino/imino type where the aromatic character of the six-membered pyrimidine ring is fully or, as in the barbiturates, partially retained, as illustrated in Fig. 15.4. In these molecular species, which are all feasible on the basis of organic chemical considerations, the hydrogen-bonding donor/acceptor properties of the functional amino, imino, enol and keto groups vary considerably, being donor in one form and acceptor in the other. 

Because of their intimate involvement in the genetic code purines anc pyrimidines have received considerable attention from both experimental and theoretical viewpoints. The numbering systems for these compounds are noted below ... 

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