Isoguanine, tautomerism

Figure 3.4 Representation of the main tautomeric forms of isoguanine in the gas phase and in aqueous solution. The relative stabilities (kcal moT j in the gas phase and in water are given in plain text and in italics, respectively.

The keto/enol or amino/imino equilibrium can be shifted, however, if the bases are chemically modified as in isoguanine (2-hydroxy,4-amino-purine). In this molecule, the tautomeric equilibrium depends largely on the dielectric constant of the solvent [525]. 

Purines show typical absorption spectra which are useful for the identification and the determination of their structure tautomeric equilibria can also be studied. By comparison of the spectra of natural and synthetic derivatives of purines, the N9 glycosylic linkage was established for nucleosides. The relatively simple spectra of purines show two main bands. Purine shows maxima around 220 nm and at 263 nm in neutral solution. Since the imidazole ring has no characteristic UV absorption, the spectra of purines and pyrimidines show similarities. Similar observations have been made for 7-deazapurines, 8-azapurines etc. In the series of adenine hypoxanthine, and xanthine (at pH 6) only one maximum is observed guanine, isoguanine, and purinc-2,6-diamine show two maxima. 

Jaworski, A., Kwiatkowski, J. S., and Lesyng, B. (1986). Inti. J. Quantum Chem. QBS12, 209—discussion of the relationship between tautomerism of isocytosine, isoguanine, and genetic code. 

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