The excitation and emission energies of oxyluciferin are calculated by the TDDFT method and the absorption spectra in the aqueous solutions with various pH values are assigned using the relative absorption intensity estimated from the theoretical pK(a) values. The results suggest that the peak at 414 nm in the experimental absorption spectra at pH 8.3 corresponds to the excitation to the S-1 state of (6'O ,4O ) and that the peak at 371 nm in the strong acidic solution at pH 5.4 corresponds to the excitation to the S-1 state of (6'OH,4OH). (C) 2013 Elsevier B.V. All rights reserved.