Soniat M. et al.: Relative Energies of Anionic Pterin Tautomers 124 Pteridines/Vol. 20/No. 4 Correspondence to: Christopher B. Martin, Department of Chemistry and Physics, Lamar University, Box 10022, Beaumont, Texas 77710, e-mail: Christopher.Martin@lamar.edu, astros435@yahoo.com Pteridines Vol. 20, 2009, pp. 124 - 129 Theoretical Study on the Relative Energies of Anionic Pterin Tautomers Michael Soniat, Christopher B. Martin Department of Chemistry and Physics, Lamar University, Box 10022, Beaumont, Texas 77710 Abstract Pterins exist as several tautomeric forms and behave as weak acids in aqueous solutions. Therefore, several acid- base equilibria may be present. For several pterin derivatives, the range of the experimental pKa lies in the range 6-8. The anionic form of the lactam structure is the only structure considered in the literature to represent the chemistry of pterins in alkaline solution. In the present study, density functional theory calculations were used to determine the relative energies of various anionic tautomers of pterin present in aqueous solution which may play a role in chemical and biological systems. Key words: Pterin, tautomer, energy Introduction Pterins and their derivatives are important members of heterocyclic compounds that are widespread in bio- logical systems. Pterins participate in relevant biologi- cal functions behaving as inhibitors (1), enzymes (2- 4), coenzymes (5, 6), sensitizers (7-9), and pigments (10). In the past decade, the photophysical and photo- chemical properties of pterins have received increased attention. (11) The photochemical properties of pterins have been found to be dependent on chemical struc- ture, pH, the availability of oxygen, and the immediate availability of appropriate electron donors (11-16). More recently, Lorente and Thomas reported their experimental study on pterins in aqueous solution assigning the absorption and emission spectra of pterins and proposed plausible decay mechanism of excited pterins. (17, 18) Pterins, chiefly 6-carboxy pterin, have been shown to photosensitize DNA dam- age by both type I (electron transfer) and type II (sin- glet oxygen) mechanisms. (19-22) Despite their impor- tant contribution, the electronic and geometrical fea- tures of related low-lying states as well as the explicit mechanism of photochemistry and photophysics of pterins are still unclear. In previously reported studies, only two tautomeric structures of pterins have been considered to be significant. These structures are the lactam and lactim structure. (23) (Figure 1) In recent theoretical studies which investigated pterins in aqueous media, we have shown that the lac- tam tautomer is the lowest energy pterin tautomer while the lactim structure was higher in energy than expected (∼6 kcal/mol) compared to the lactam. We also showed that three other neutral pterin structures were calculated to be less than 4 kcal/mol above the most stable lactam and therefore should also be con- sidered as structures which possibly play an important role in chemical and biological pterin systems. (24) N N N N H 3 C O NH 2 H N N N N H 3 C O NH 2 N N N N H 3 C O NH 2 - H + H + H - H Pterin Lactam Pterin Lactim Pterin Anion H Figure 1. Commonly accepted structures of the lactam, lactam, and anionic pterin forms.