Lipid droplet specific BODIPY based rotors with viscosity sensitivity to distinguish normal and cancer cells: impact of molecular conformation.

Lipid droplets (LDs) are dynamic, multifunctional organelles critical for regulating energy balance, cell signaling, membrane formation, and trafficking. Recent studies have highlighted LDs as emerging cancer biomarkers, with cancer cells typically exhibiting a higher number and viscosity of LDs compared to normal cells. This discovery paves the way for developing molecular probes that can monitor intracellular viscosity changes within LDs, offering a powerful tool for early cancer diagnosis, recurrence monitoring, and therapeutic interventions. In this study, we designed and synthesized two series of donor-acceptor (D-A) conjugated BODIPY-cyanostilbene based fluorophores (5a-c and 6a-c) by fine-tuning the cyanostilbene unit with three distinct substituents (OMe, H, Cl) and modulating the molecular conformation rigidifying the indacene core. While the terminal substituents had a minimal effect on the optical properties, changes in molecular conformation significantly impacted the photophysical behavior of the fluorophores. Compounds 5a-c function as molecular rotors, with the free rotation of the -biphenyl rings leading to non-radiative deactivation of the excited state, resulting in weak emission. Additionally, this structural feature makes them highly responsive to changes in viscosity. As the glycerol concentration increased from 0% to 99%, the fluorescence intensity of compounds 5a, 5b, and 5c increased dramatically by 17-fold, 78-fold, and 43-fold, respectively. In contrast, compounds 6a-c, with restricted phenyl ring rotation due to tetra-methyls on the indacene unit, showed only a modest 2-3-fold increment in fluorescence intensity under similar conditions. These fluorophores possess several key advantages, including high selectivity for LDs, good photostability, sensitivity to viscosity, and responsiveness to polarity and pH. Moreover, they effectively differentiate between normal and cancer cells, making them valuable tools for cancer diagnosis and potential therapeutic applications.