A Theoretical Analysis of the Potential Role of π-π Stacking Interactions in the Photoprotolytic Cycle of Firefly Luciferin

• Published in: Chemphyschem Vol. 15; no. 17; pp. 3761 - 3767
• Main Authors: Pinto da Silva, Luís, Esteves da Silva, Joaquim C. G.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.12.2014; WILEY‐VCH Verlag; Wiley
• Subjects: Animals; Biological and medical sciences; Chemistry; Exact sciences and technology; Fireflies - chemistry; Firefly Luciferin - chemistry; Firefly Luciferin - radiation effects; Fluorescence; Fundamental and applied biological sciences. Psychology; General and physical chemistry; Hydrogen-Ion Concentration; Molecular biophysics; Molecular Structure; oxyluciferin; photoacidity; Photochemical Processes - radiation effects; Photochemistry. Photosynthesis. Bioluminescence; photoprotolytic cycle; pi interactions; Quantum Theory; Radiation-biomolecule interaction; Fluorescence; photoprotolytic cycle; Luciferin; oxyluciferin; pi interactions; photoacidity; fluorescence
• ISSN: 1439-4235; 1439-7641; 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. Insensitive fluorescence: The potential role of π interactions in the fluorescence of luciferin is studied theoretically. This species is found to form π–π stacking complexes only in the ground state at acidic pH. Upon excitation, these complexes are disrupted, owing to repulsive electrostatic interactions. Thus, π interactions do not play any role in the fluorescence of luciferin, which is contrary to the case of oxyluciferin.