title: A theoretical analysis of the potential role of π-π stacking interactions in the photoprotolytic cycle of firefly luciferin.
abstract: Firefly oxyluciferin is a photoacid that presents a pH-sensitive fluorescence, which results from pH-dependent changes on the conformation of self-aggregated π-π stacking complexes. Luciferin is a derivative of oxyluciferin with very similar fluorescence and photoacidic properties. This similarity indicates that luciferin is also expected to be able to form π-π stacking complexes, but no pH-sensitive fluorescence is found for this compound. Here, a theoretical approach is used to rationalize this finding. We have found that luciferin only forms π-π stacking complexes in the ground state at acidic pH. At basic pH and in the excited state, luciferin is present as a dianion. This species is not able to self-aggregate, owing to repulsive electrostatic interactions. Thus, this emissive species is not subject to π-π stacking interactions; this explains its pH-insensitive fluorescence.
authors.affiliation: Centro de Investigação em Química (CIQ-UP), Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, R. Campo Alegre 697, 4169-007 Porto (Portugal).
authors.forename: Luís
authors.initials: L
authors.lastname: Pinto da Silva
authors.forename: Joaquim C G
authors.initials: JC
authors.lastname: Esteves da Silva
mesh_terms: D000818:Animals; D048888:Fireflies / Q000737:chemistry; D049454:Firefly Luciferin / Q000737:chemistry* / Q000528:radiation effects*; D005453:Fluorescence; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D055668:Photochemical Processes / Q000528:radiation effects; D011789:Quantum Theory
chemical_list: D049454:Firefly Luciferin
keywords: fluorescence; oxyluciferin; photoacidity; photoprotolytic cycle; pi interactions