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

被引:78
作者
Naumov, Pance [1 ]
Kochunnoonny, Manoj [1 ]
机构
[1] Osaka Univ, Grad Sch Engn, Dept Mat & Life Sci, Suita, Osaka 5650871, Japan
关键词
FLUORESCENT PROTEIN CHROMOPHORE; EXCITED-STATE DYNAMICS; COLOR-TUNING MECHANISM; FIREFLY BIOLUMINESCENCE; MOLECULAR-STRUCTURE; EMISSION-SPECTRA; PROTON-TRANSFER; LIGHT EMITTER; YELLOW-GREEN; WILD-TYPE;
D O I
10.1021/ja102885g
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
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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收藏
页码:11566 / 11579
页数:14
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