8-Bromo-7-hydroxyquinoline as a Photoremovable Protecting Group for Physiological Use: Mechanism and Scope
Two-photon excitation (2PE) of \"caged\" biomols. represents a powerful method to investigate the temporal and spatial relevance of physiol. function in real time and on living tissue, because the excitation vol. can be restricted to 1 fL. Addnl., low-energy IR light is used, which minimizes tissue destruction and enables deeper penetration into tissue prepns. Exploitation of this technol. for studying cell physiol. requires the further development of photoremovable protecting groups with sufficient sensitivity to 2PE for use in \"caged\" compds. 8-Bromo-7-hydroxyquinoline (BHQ) is efficiently photolyzed by classic 1PE (365 nm) and 2PE (740 nm) under simulated physiol. conditions (aq. buffer of high ionic strength, pH 7.2) to release carboxylates, phosphates, and diols-functional groups commonly found on bioactive mols. such as neurotransmitters, nucleic acids, and drugs. It is stable in the dark, sol. in water, and exhibits low levels of fluorescence, which will enable use in conjunction with fluorescent indicators of biol. function. BHQ-protected effectors are synthetically accessible. Stern-Volmer quenching, time-resolved IR (TRIR), and 18O-labeling expts. suggest that the photolysis occurs through a solvent-assisted photoheterolysis (SN1) reaction mechanism on the sub-microsecond time scale. BHQ has the requisite photochem. and photophys. properties as a photoremovable protecting group to regulate the action of biol. effectors in cell and tissue culture with light, esp. 2PE.