Substituent Effects on the Sensitivity of a Quinoline Photoremovable Protecting Group to One- and Two-Photon Excitation
Photoremovable protecting groups that can reveal biol. important functional groups through one- and two-photon excitation (1PE and 2PE, resp.) have promise in regulating physiol. function in a temporally and spatially restricted manner. Only a few chromophores have sufficient sensitivity to 2PE suitable for use as "caging groups" in physiol. expts. It would be useful to develop structure-property relationships of chromophores, so that chromophores with high two-photon uncaging action cross-sections (delta u) can be designed. The 8-bromo-7-hydroxyquinolinyl chromophore (BHQ) releases a variety of functional groups through 1PE and 2PE. Swapping the bromine substituent for a nitro (NHQ), cyano (CyHQ), or chloro (CHQ) or exchanging the hydroxy for dimethylamino (DMAQ and DMAQ-Cl) or sulfhydryl (TQ) significantly alters the photochem. and photophys. properties of the quinoline chromophore. CyHQ-OAc demonstrated a 3-fold increase in sensitivity for acetate release, whereas NHQ-OAc was photochem. insensitive. The quantum efficiencies (Qu) of the amino and sulfhydryl derivs. were about an order of magnitude lower than that of BHQ-OAc. All of the chromophores showed diminished sensitivity to 2PE compared to BHQ-OAc, but the CyHQ, DMAQ, and DMAQ-Cl chromophores are sufficiently sensitive for physiol. use. The high sensitivity of CyHQ to 1PE will be useful in biol. applications requiring short exposure with low light intensity.