title: Spectral-structural effects of the keto-enol-enolate and phenol-phenolate equilibria of oxyluciferin.
abstract: The effects of environmental polarity on the enolization of the keto form and the deprotonation of the enol, and the role of the neutral and ionized 6'-OH group in the fluorescence of the firefly emitter, oxyluciferin, were assessed through a detailed study of the structure and absorption and fluorescence spectra of its 6'-dehydroxylated analogue. It was found that the deprotonated 6'-O(-) group is a necessary, albeit insufficient, factor in accounting for the observed yellow-green and red emissions of oxyluciferin. Its negative charge is essential for effective excited-state charge transfer, which lowers the emission energy and broadens the emission spectrum. Deprotonation of the 6'-OH group changes its effect on the emission energy from blue- to red-shifting. Furthermore, the combination of these opposite effects and resonance stabilization of the phenolate-keto form causes switching of the order of maximum emission wavelengths of the three species involved in the keto-enol-enolate equilibrium from enol << keto < enolate in absence of 6'-OH to keto < enol << enolate with 6'-OH, to enol < enolate < keto with 6'-O(-). If only the keto-enol-enolate equilibrium is considered, solvents of medium polarity are the most effective in decreasing the excited-state energy. Polar or very polar environments also stimulate shift of the ground-state equilibrium toward the enol form. Under such circumstances, the enol group can be partly or completely deprotonated in the ground state or from the excited state: a polar environment facilitates the ionization, while a less polar environment requires the presence of a stronger base. In the absence of bases, the ground-state keto form exists only in solvents of very weak to medium polarity, but with stronger bases, it can also exist in a nonpolar or very weakly polar environment, usually together with the enolate anion. The phenol-enolate form of oxyluciferin, a species that could not be experimentally detected prior to this study, was identified as a yellow-emitting species.
authors: Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Osaka, Japan. npance@wakate.frc.eng.osaka-u.ac.jp
authors: Pance
authors: P
authors: Naumov
authors: Manoj
authors: M
authors: Kochunnoonny
mesh_terms: D007211:Indoles / Q000737:chemistry*; D007659:Ketones / Q000737:chemistry*; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry*; D011719:Pyrazines / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D007211:Indoles; D007659:Ketones; D010636:Phenols; D011719:Pyrazines; C014730:oxyluciferin