A typical photoacid is a weak acid in the ground state with a pKa in the range of 5-10, which leads to the manifestation of both acidic and basic forms of these molecules in their absorption spectra. In the excited state, the pKa constant decreases significantly due to the capability of these molecules to donate a proton via an Excited State Proton Transfer (ESPT) process.More recently, the photophysics and photochemistry of the oxyluciferin family of fluorophores have been studied due to the discovery that these molecule act as strong photoacids.1,2 It should be noted that oxyluciferin is the light emitter of firefly bioluminescence.3In solution, oxyluciferin exists in its enol form with two possible deprotonable groups: the phenolic and enolic ones. The observable ESPT was first attributed to the phenolic group, given previous results with oxyluciferin derivatives.1,2 However, another study revealed that the ESPT involved the enolic group.4 Oxyluciferin is then a very rare example of the photoacidity of a stable enol group in a five-membered ring.In this communication we intend to describe the characterization of this ESPT process, and explain why deprotonation occurs in a stable enol group (a rather exceptional situation) and not in the more traditional phenolic group. To this end, theoretical methods as density functional theory and topological approaches will be used, given their ability to present detailed and accurate electronic information. References1. L. Pinto da Silva, R. Simkovitch, D. Huppert, J.C.G. Esteves da Silva, ChemPhysChem 2013 , 14, 3441-3446.2. Y. Erez, I. Presiado, R. Gepshtein, L. Pinto da Silva, J.C.G. Esteves da Silva, D. Huppert, J. Phys. Chem. A 2012 , 116, 7452-7461.3. L. Pinto da Silva, J.C.G. Esteves da Silva, J. Chem. Theory Comput. 2011 , 7, 809-817.4. K.M. Solntsev, S.P. Laptenok, P. Naumov, J. Am. Chem. Soc. 2012 , 134, 16452-16455.

Characterization of the nontraditional enol-based photoacidity of firefly oxyluciferin

248th National Meeting (2014)

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