Characterization of a Bio‐sourced, Fluorescent, Ratiometric pH Indicator with Alkaline pKa

• Published in: Photochemistry and photobiology Vol. 96; no. 6; pp. 1176 - 1181
• Main Authors: Alheety, Samar, Valenti, Domenic, Mujumdar, Nirvani, Ellis, Nakita, Campiglia, Andres D., Harper, James K., Heider, Emily C.
• Format: Journal Article
• Language: English
• Published: Lawrence Blackwell Publishing Ltd 01.11.2020
• Subjects: Chemical compounds; Computer applications; Density functional theory; Endophytes; Fluorescence; Fluorescence microscopy; Fluorophores; Fungi; Natural products; pH effects; Raw materials
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1111/php.13299

Utilizing organisms as sources of fluorophores relieves the demand for petroleum feedstock in organic synthesis of fluorescent products, and endophytic fungi provide a promising vein for natural fluorescent products. We report the characterization of a pH‐responsive fluorophore from an endophytic fungus isolated from sand pine. The endogenous fluorescence of the live organism was measured using fluorescence microscopy. Computational interpretation of the spectra was accomplished with time‐dependent density functional theory methods. The combined use of experimental and theoretically predicted spectra revealed the pH equilibria and photoexcited tautomerization of the natural product, 5‐methylmellein. This product shows promise both as a stand‐alone pH‐indicating fluorophore, with alkaline pKa, and as "green" feedstock for synthesis of custom fluorophores. The biomolecule, 5‐methylmellein, is a fluorescent, ratiometric pH indicator extracted from an endophytic organism. The highly alkaline pKa of the fluorophore fills a niche void in the variety of fluorescent pH indicators available for imaging. Computational methods further characterize photo‐induced tautomerization of 5‐methylmellein.