title: Excited-state intermolecular proton transfer of firefly luciferin V. Direct proton transfer to fluoride and other mild bases.
abstract: We studied the direct proton transfer (PT) from electronically excited D-luciferin to several mild bases. The fluorescence up-conversion technique is used to measure the rise and decay of the fluorescence signals of the protonated and deprotonated species of D-luciferin. From a base concentration of 0.25 M or higher the proton transfer rates to the fluoride, dihdyrogen phosphate or acetate bases are fast and comparable. The fluorescence signals are nonexponential and complex. We suggest that the fastest decay component arises from a direct proton transfer process from the hydroxyl group of D-luciferin to the mild base. The proton donor and acceptor molecules form an ion pair prior to photoexcitation. Upon photoexcitation solvent rearrangement occurs on a 1 ps time-scale. The PT reaction time constant is ∼2 ps for all three bases. A second decay component of about 10 ps is attributed to the proton transfer in a contact pair bridged by one water molecule. The longest decay component is due to both the excited-state proton transfer (ESPT) to the solvent and the diffusion-assisted PT process between a photoacid and a base pair positioned remotely from each other prior to photoexcitation.
authors.affiliation: Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel.
authors.forename: Itay
authors.initials: I
authors.lastname: Presiado
authors.forename: Rinat
authors.initials: R
authors.lastname: Gepshtein
authors.forename: Yuval
authors.initials: Y
authors.lastname: Erez
authors.forename: Dan
authors.initials: D
authors.lastname: Huppert
mesh_terms: D000468:Alkalies / Q000737:chemistry*; D000818:Animals; D001517:Coleoptera; D049454:Firefly Luciferin / Q000737:chemistry*; D005453:Fluorescence; D005459:Fluorides / Q000737:chemistry*; D011522:Protons
chemical_list: D000468:Alkalies; D011522:Protons; D049454:Firefly Luciferin; D005459:Fluorides