Excited-State Intermolecular Proton Transfer of Firefly Luciferin IV. Temperature and pH Dependence

Time-resolved emission as well as steady-state UV-vis techniques were employed to study the photoprotolytic processes that D-luciferin, the natural substrate of the firefly luciferase, undergoes in both acidic aqueous solutions and ice. The emission spectrum of D-luciferin in a 20 mM HCl aqueous solution or higher has an additional emission band at 590 nm red-shifted with respect to the strongest emission band positioned at 530 nm of the deprotonated NRO-* form in a pH-neutral aqueous solution. We attribute this emission band to the zwitterion form designated as +HNRO-. The time-resolved emission signals show that the NRO-* emission band at 530 nm and the zwitterion emission band at 590 are strongly quenched by a recombination process with a proton in an acidic solution and in ice. In ice, the quenching rate is 10 times faster than in the liquid state:. We attribute the fast quenching rate to the high value of the proton diffusion constant in ice.