Theoretical Study of Firefly Luciferin pK_a Values—Relative Absorption Intensity in Aqueous Solutions

Abstract

Ground-state vibrational analyses of firefly luciferin and its conjugate acids and bases are performed. The Gibbs free energies obtained from these analyses are used to estimate pKa values for phenolic hydroxy and carboxy groups and the N-H(+) bond in the N-protonated thiazoline or benzothiazole ring of firefly luciferin. The theoretical pKa values are corrected using the experimental values. The concentrations of these chemical species in solutions with different pH values are estimated from their corrected pKa values, and the pH dependence of their relative absorption intensities is elucidated. With the results obtained we assign the experimental spectra unequivocally. Especially, the small peak near 400 nm at pH 1-2 in experimental absorption spectra is clarified to be due to the excitation of carboxylate anion with N-protonated thiazoline ring of firefly luciferin. Our results show that the pKa values of chemical species, which are contained in the aqueous solutions, are effective to assign experimental absorption spectra.

Oxyluciferin Photoacidity: The Missing Element for Solving the Keto–Enol Mystery?

“…The experimental deduction that the protonation of a nitrogen heteroatom is responsible for the redshift in the absorption spectrum was corroborated by two theoretical computational studies 29. 30…”

Analysis of Photoexcitation Energy Dependence in the Photoluminescence of Firefly Luciferin

“…We have reported the molar fraction of chemical species in Fig. . Our previous study using those molar fractions showed slight discrepancies between theoretical and experimental fluorescence spectra because of overestimation of the molar fractions of N3 (N atom at 3‐position)‐protonated species.…”

“…Moreover, the widths of the absorption peaks for each chemical species were assumed to be 0.2 eV, following our previous paper , in which the theoretical absorption spectra using this width agreed well with the experimental curves. The relative absorption intensities for the photoexcitation between 350 and 600 nm in 10 nm steps were calculated by multiplying molar fractions with oscillator strengths evaluated from the Gaussian line shape with this width.…”

Theoretical Study of Fluorescence Spectra Utilizing the pKa Values of Acids in Their Excited States

“…Previously reported results for the geometry optimization and vibrational analysis, which were performed using the B3LYP/aug‐cc‐pVTZ method with solvent effects, of H 2 O and H 3 O + in the S 0 state were used, because the electronic excitation of these molecules does not occur in the energy region of concern in this study .…”