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      Spectral−Structural Effects of the Keto−Enol−Enolate and Phenol−Phenolate Equilibria of Oxyluciferin

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      Journal of the American Chemical Society
      American Chemical Society (ACS)

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          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.

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          Author and article information

          Journal
          Journal of the American Chemical Society
          J. Am. Chem. Soc.
          American Chemical Society (ACS)
          0002-7863
          1520-5126
          August 25 2010
          August 25 2010
          : 132
          : 33
          : 11566-11579
          Article
          10.1021/ja102885g
          20715859
          e987eb04-c0b3-42c0-8d37-74d34d933368
          © 2010
          History

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