Ultrafast intermolecular proton transfer from 10-hydroxy-camptothecin

Fluorescence probes are widely used for the investigation of biologically important systems. In most cases, the steady-state spectral data is used for the characterization of properties of their microenvironment. Surprisingly, few synthetic probes are known to utilize their excited-state reactivity, such as excited-state proton transfer, as an additional tool for the investigation of complex systems. Proton transfer, in both the ground and excited states, is a fundamental process in chemistry and biology. Camptothecin (CPT) is a pentacyclic alkaloid, first isolated from extracts of the Chinese tree Camptotheca acuminata. This brightly fluorescent compound was found to be a potent inhibitor of the growth of leukemia cells by exhibiting a unique mechanism of action: inhibition of DNA topoisomerase. A more potent water-soluble analog of CPT, 10- hydroxycamptothecin (10-CPT), has a subunit identical to 6- hydroxyquinoline (6HQ). Hydroxyquinoline derivatives are known to be both strong photo-acids and strong photo-bases l0-CPT and, therefore, undergo efficient tautomerization in the very wide range of pH resulting in weak tautomer (zwitterion) emission. This chapter presents the study of the ultrafast excited state intermolecular proton transfer processes of 10-CPT to a water-methanol mixture.