Tautomeric properties, nucleic acid bases

A few chapters of the current volume describe different state-of-the-art experimental techniques used to unravel photophysical and photochemical properties of complex molecular systems. These chapters are especially tailored for the scholarly description of electronic excited state properties of nucleic acid bases and related species predicting different tautomeric distributions and possible nonra-diative deactivation processes. It is interesting to note that guanine provides particularly challenging case to discuss. Recent theoretical and experimental investigations show the existence of relatively significantly less stable imino tautomers in the... 

UV and NMR studies showed that l-methyl-A -hydroxycytosine and its methyl derivatives exist predominantly in the oxo-imino form (up to 90%) (98BPC87). A review of the structure and properties of isolated nucleic acid bases with the special attention given to metal-cation-assisted tautomerization and the solvent effects has been published (99CR3247). Tautomerism of protonated cytosine has been studied experimentally by IR and Raman spectra and theoretically by ab initio and DFT calculations (96JPC5578). Neutral and protonated forms of cytidine monophosphate have been studied by ultraviolet resonance Raman spectroscopy. The amino oxo tautomer was found to be the most stable, followed by the imino oxo form. The imino-hydroxy tautomer was determined to be the least stable one. Upon protonation, the amino-oxo structure is retained (93JA760). 

Acid Force Fields OPLS Force Fields Tautomeric Properties of Nucleic Acid Bases Ab Initio Study. 

TAUTOMERIC PROPERTIES OF NUCLEIC ACID BASES AB INITIO STUDY... 

The understanding of the tautomeric properties of the purine and pyrimidine bases of the nucleic acids and the determination of the electronic properties of the principal tautomers are of fundamental importance in molecular biology, in particular in connection with the theory of mutations (for general references see, e.g. refs. 1-6.) B. Pullman and A. Pullman have presented recently in these Advances3 a detailed review of the problem as it concerns the purine bases. The present paper... 

The chemical properties of the purine and pyrimidine bases include highly conjugated double bond systems within the ring structures. For this reason, nucleic acids have a very strong absorption maximum at about 260 nm, which is used for nucleic acid quantitation. Moreover, the bases can exist in two tautomeric forms, the keto and enol forms (Figure 10.2). In DNA and RNA, the keto forms are by far the more predominant, and this property makes it possible for the bases to form intermolecular hydrogen bonds (see Figure 10.18). 

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. 

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