Tautomerism of guanine

Langer H, Doltsinis NL (2003) Excited state tautomerism of the DNA base guanine a restricted open-shell kohn-sham study. J Chem Phys 118 5400-5407... 

Mennucci B, Toniolo A, Tomasi J (2001) Theoretical study of guanine from gas phase to aqueous solution role of tautomerism and its implications in absorption and emission spectra. J Phys Chem A 105 7126... 

Sobolewski AL, Domcke W, Hattig C (2005) Tautomeric selectivity of the excited-state lifetime of guanine/cytosine base pairs The role of electron-driven proton-transfer processes. Proc Natl Acad Sci USA 102 17903-17906... 

C. Colominas, F. J. Luque, and M. Orozco, Tautomerism and protonation of guanine and cytosine. Implications in the formation of hydrogen-bonded complexes, J. Am. Chem. Soc. 118 6811 (1996). 

The amino groups are replaced with oxygen. Although here a biochemical reaction, the same can be achieved under acid-catalysed hydrolytic conditions, and resembles the nucleophilic substitution on pyrimidines (see Section 11.6.1). The first-formed hydroxy derivative would then tautomerize to the carbonyl structure. In the case of guanine, the product is xanthine, whereas adenine leads to hypoxanthine. The latter compound is also converted into xanthine by an oxidizing enzyme, xanthine oxidase. This enzyme also oxidizes xanthine at C-8, giving uric acid. 

Lee GYC, Chan SI. Tautomerism of nucleic acid bases. 11. Guanine. J Am Chem Soc 1972 94 3218-3229. 

Tautomeric equilibrium of guanine and adenine. The keto and amino forms are strongly favored for guanine and adenine, respectively. Comparable tautomeric equilibria exist for thymine, uracil, and cytosine. 

Books sometimes disagree on the preferred tautomer of guanine—6.f versus 6.g. A parallel can be found in 2-pyridone, which also has two likely tautomeric forms, 6.h and 6.i. To determine which form of 2-pyridone predominates, the ultraviolet spectra of two isomers, l-methyl-2-pyridone (6.j) and 2-methoxypyridine (6.k), were obtained with the observed Amax wavelengths shown. The Amax of 2-pyridone is at 229 nm. Does 2-pyridone likely favor tautomer 6.h or 6.i Which tautomer of guanine is likely favored, 6.f or 6.g (Elguero, J., Marzin, C., Katritzky, A. R., Linda, P. The Tautomer-ization of Heterocycles. Adv. Heterocycl. Chem., Suppl. 1 New York Academic Press, 1976.)... 

The tautomerism of functional derivatives of azines (e.g., pyridones) and other more complex systems (guanines, porphyrins, etc.) has been much studied both experimental and theoretically [40, 76-82], Our contribution to this field has been modest using a combination of 13C and 15N NMR in solution and in the solid state, together with GIAO//B3LYP/6-311++G calculations, the tautomerism of 2-pyri-done, 4(37/)-pyrimidone, uracil, and cytosine were determined [83],... 

Because of the presence of a carbonyl and an amino group and the possibilities for the imidazole proton to oscillate between nitrogens N-7, N-9, and N-3, the molecule of guanine offers numerous and complex possibilities of tautomerization. Among those two have received special attention. 

PPP calculations have been performed148 for a number of other tautomeric forms of guanine (imine and lactim). No discussion of the calculated results has, however, been given. 

Quantum chemistry provides an acute and consistent picture of the tautomeric landscape of guanine [8, 24-30], High level calculations at various levels of theory (Figure 13-2) distinguish basically four energetic ranges four tautomers lie in the... 

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