Lyso Flipper-TR 29, a Flipper probe, specifically labels lysosomes and late endosomes. It achieves entry into these organelles by transient deprotonation, allowing it to cross their membranes in a neutral form [1].

Lyso Flipper-TR 29 enters lysosomes through transient deprotonation and diffuses across hydrophobic membranes in a neutral form; however, it cannot return to the cytosol due to the lysosomal low pH of approximately 5, which inhibits transient deprotonation. Additionally, Lyso Flipper-TR 29 tracks late endosomes but cannot label early endosomes (EE) because their internal pH is too high, greater than 6, which prevents transient deprotonation [1].