Anthraquinone-based fluorophore as electrostatic complexation-driven turn-on probe for Cardiolipin detection in model membranes

Cardiolipin (CL), being a mitochondria-specific phospholipid, is known to play a pivotal role in cellular respiration, and its levels are intricately connected to various neurodegenerative disorders. Owing to the simplicity compared to mass spectrometry-based lipidomics, optical detection of CL is an attractive option; however, only two probes, namely TTAPE-Me and 10- nonyl acridine orange (NAO), are currently available. Herein, we report the use of an anthraquinone-based fluorophore 1 as a turn-on sensor for CL in phospholipid membranes. 1 exhibited a robust fluorescence turn-on response upon selective binding to CL, with up to 5.8- fold enhancement in dioleoyl phosphatidylcholine (DOPC) vesicles and 5.1-fold in dipalmitoyl phosphatidylcholine (DPPC) vesicles. Based on the results from anisotropy, temperature studies, and salt dependence studies, it was concluded that the formation of an electrostatic complex was the driving factor behind the observed fluorescence enhancement. Mechanistic investigations, including anisotropy, salt-dependence, and temperature studies, revealed that electrostatic complex formation underlies the observed fluorescence enhancement. Interestingly, the probe also showed a high degree of selectivity toward CL compared to other biologically relevant analytes. These findings establish compound 1 as a promising optical probe for quantitative cardiolipin detection, providing an alternative to existing fluorophores.