Fluorometric probe for the lipase level: Design, mechanism and biological imaging application

To realize accurate regulation for fluorescent substrate of lipase, two series compounds (I, II) with similar structure were designed and synthesized. The flexible diphenylmethane was permitted I to go deep into the catalytic site of lipase, while rigid structure of 9H-fluoren makes itself difficult to approach the center. Series I could be effectively hydrolyzed by porcine pancreatic lipase (PPL), and the product emitted fluorescence based on aggregation-induced emission (AIE) mechanism, meanwhile different substituent groups (-Br, -Cl, -Ph, -H) on I induced the tunable hydrolysis rate. The limit of detection (LOD) was 0.05 U/mL, and linear calibration range was 0.1-4.0 U/mL, this would be effectively applied to biological detection. This result was well supported by theories calculation and molecular docking. Moreover, substrate 1 was selected to describe the distribution of lipase in many living biology such as zebrafish, Locusta migratoria and Helianthus annuus seeds coat. It was found that lipase and lipid droplets (LDs) coexist in biological systems, but there is no lipase in LDs. The designed probe gives us an opportunity to describe lipase distribution in many important tissues, including biology and medical science.