Spectroscopic Studies of the Light-Color Modulation Mechanism of Firefly (Beetle) Bioluminescence

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• 作者：Takashi Hirano ; Yosuke Hasumi ; Kazuhiro Ohtsuka ; Shojiro Maki ; Haruki Niwa ; Minoru Yamaji ; Daisuke Hashizume
• 刊名：Journal of the American Chemical Society
• 出版年：2009
• 出版时间：February 18, 2009
• 年：2009
• 卷：131
• 期：6
• 页码：2385-2396
• 全文大小：433K
• 年卷期：v.131,no.6(February 18, 2009)
• ISSN：1520-5126

文摘

To reveal the light-color modulation mechanism of firefly (beetle) bioluminescence, we investigated the spectroscopic properties of the phenolate anion 1-O⁻ generated from 5,5-dimethyloxyluciferin (1-OH) using various base/solvent combinations. Phenolate anion 1-O⁻ is a model compound for the keto form of wild-type oxyluciferin phenolate anion (OL⁻), which is postulated to be the emitter of the bioluminescence. The fluorescence maxima of 1-O⁻ were found to depend on the base/solvent combination used, and they varied in the range 541−640 nm, which covers the almost whole range of the bioluminescence emission maximum. In a polar solvent, where ¹(1-O⁻)* and the countercation (the conjugate acid of a base) make a solvent-separated ion pair or a free ion couple, the emission maxima of 1-O⁻ were found to be modulated by the solvent polarity. In a less polar solvent, where ¹(1-O⁻)* and the countercation are formed as a contact ion pair, the strength of the covalent character of the O8′···H bond between ¹(1-O⁻)* and the countercation is operative. The effect of the base/solvent combination on the emission properties of ¹(1-O⁻)* was also verified using fluorescence lifetime measurements and density functional theory calculations on 1-O⁻ and its ion-pair models. On the basis of these results, we propose the following light-color modulation mechanism: (1) the light emitter is the excited singlet state of OL⁻ [¹(OL⁻)*], and (2) light emission from ¹(OL⁻)* is modulated by the polarity of the active-site environment of a luciferase and the degree of covalent character of the O8′···H bond between ¹(OL⁻)* and a protonated basic moiety in the active site. Mechanisms for variation of the bioluminescence colors and their applications are discussed.