A Theoretical Analysis of the Potential Role of [pi]-[pi] Stacking Interactions in the Photoprotolytic Cycle of Firefly Luciferin

• Published in: Chemphyschem Vol. 15; no. 17; p. 3761
• Main Authors: Pinto da Silva, Luís, Esteves da Silva, Joaquim C G
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.12.2014
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201402558

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.