Vibronic Structures in Absorption and Fluorescence Spectra of Firefly Oxyluciferin in Aqueous Solutions

Abstract

To elucidate the factors determining the spectral shapes and widths of the absorption and fluorescence spectra for keto and enol oxyluciferin and their conjugate bases in aqueous solutions, the intensities of vibronic transitions between their ground and first electronic excited states were calculated for the first time via estimation of the vibrational Franck-Condon factors. The major normal modes, overtones and combination tones in absorption and fluorescence spectra are similar for all species. The theoretical full widths at half maximum of absorption spectra are 0.4-0.7 eV and those for the fluorescence spectra are 0.4-0.5 eV, except for phenolate-keto that exhibits exceptionally sharp peak widths due to the dominance of the 0-0' or 0'-0 band. These spectral shapes and widths explain many relevant features of the experimentally observed spectra.

Theoretical insights into the effect of pH values on oxidation processes in the emission of firefly luciferin in aqueous solution

“…The equilibrium structures of the oxyluciferin anions in the S 0 and S 1 states previously determined by B3LYP/aug‐cc‐pVTZ and cam‐B3LYP/aug‐cc‐pVTZ calculations are similar to those determined by B3LYP/6–31G(d), B3LYP/6–31+G(d,p), and B3LYP‐D3/6–31+G(d,p) calculations in the current study. The difference in bond distance between the previous and current S 0 and S 1 equilibrium structures is less than two decimal places.…”

“…In previous studies, deprotonation in a general ‘AH’ molecule was described by the following calculation:…”

“…Because firefly bioluminescence is light emission caused by chemical reactions in proteins, the intensity of light signal increases when this quantum yield and/or reaction rate increases. This importance has motivated further studies on the mechanism of the chemical reaction in firefly bioluminescence . In 2006, the crystal structures of genjibotaru ( Luciola cruciata ) luciferase containing D‐luciferin bound to an adenosine monophosphate (AMP) analog or oxyluciferin were reported .…”

“…In 2006, the crystal structures of genjibotaru ( Luciola cruciata ) luciferase containing D‐luciferin bound to an adenosine monophosphate (AMP) analog or oxyluciferin were reported . Determination of these crystal structures stimulated theoretical studies on firefly bioluminescence …”

“…This means that the reaction process shown in Figure should change with the pH value . Based on studies of the absorption and photoluminescence of oxyluciferin, it has been noted that oxyluciferin and its conjugate acids and bases co‐exist in aqueous solution at pH 7. However, it is unclear whether protonated and/or deprotonated intermediate chemical species contribute to the luciferin emission process.…”