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.

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Assignment of the fluorescence spectrum of firefly luciferin in aqueous solutions was achieved by utilizing not only emission energies but also theoretical absorption spectra and relative concentrations as estimated by pKa values. Calculated Gibbs free energies were utilized to estimate pKa values. These pKa values were then corrected by employing the experimental results. It was previously thought that the main peak near 550 nm observed in the experimental fluorescence spectra at all pH values corresponds to emission from the first excited state of the luciferin dianion [Ando et al. (2010) Jpn. J. Appl. Phys. 49, 117002-117008]. However, we found that the peak near 550 nm at low pH corresponds to emission from the first excited state of the phenolate monoanion of luciferin. Furthermore, we found that the causes of the red fluorescence at pH 1-2 are not only the emission from phenol monoanion but also the emission from the protonated species at nitrogen atom in the thiazoline ring of dianion.

Ab initio R-matrix techniques are combined with multi-channel quantum defect theory to determine the properties of arbitrarily highly excited electronic states of nitric oxide. Results are obtained for l = 0-4 channels associated with the four lowest NO+ target channels, namely 1Σ+, 3Σ+, 3Π and 3Δ. Energy and bond length variations of the quantum defect functions for all target states are reported. Certain small surface amplitude R-matrix poles also serve to determine the energies of valence states. Diabatic potential curves for Rydberg states converging to the 1Σ+ and 3Σ+ target states and for the valence states are presented. Systematic jumps in the pσ quantum defects for all target states as the bond length increases beyond R = 2.5 au are attributed to Rydbergization of the antibonding 2pσ* molecular orbital.

We measured the quantitative spectra of firefly (Photinus pyralis) bioluminescence at various temperatures to investigate the temperature dependence of the luciferin-luciferase reaction at 15–34 °C. The quantitative spectra were decomposed very well into red (1.9 eV), orange (2.0 eV), and green (2.2 eV) Gaussian components. The intensity of the green component was the only temperature sensitive quantity that linearly decreased as the temperature increased at pH 7 and 8. We found the quantitative bioluminescence spectra to be robust below 2.0 eV against temperature and other experimental conditions. The revealed robustness of the red emissions should be useful for quantitative applications such as adenosine-5′-triphosphate detection.