8-chloro-7-hydroxyquinoline and xanthone acetic acid derivatives as photoremovable protecting groups with susceptibility to two-photon excitation
Wilson, Hunter Callahan
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Photochemistry offers scientists a powerful method of exploring biological processes. Photoremovable protecting groups (PPGs) are quickly removed with a flash of light, allowing researchers to explore how the timing and location of events triggered by messengers, such as nucleotides, neurotransmitters, peptides, drugs, etc., impact cellular function. Although most caging groups rely on single-photon excitation (1PE) mechanisms to govern their release, caging groups susceptible to two-photon excitation (2PE) enable precise control of uncaging events and are better-adapted to the study of biological systems. Recently, the synthesis and photochemical analyses of 8-bromo-7-hydroxyquinoline (BHQ), 4-xanthone acetic acid (4-XAA) and 2-xanthone acetic (2-XAA) acid have indicated their photochemical sensitivity. To enhance understanding of quinoline photochemistry, 8-chloro-7-hydroxyquinoline (CHQ) was synthesized and its photochemistry measured based on cleavage of an acetyl group. Irradiation of CHQ with a UV lamp afforded the quantum efficiency (Qu), and the 2PE cross-section (δu) was determined with a Ti:Sapphire laser. δu values for 4-XAA and 2-XAA in aqueous phosphate buffer were measured. The stability of CHQ was measured in an aqueous buffer solution of high ionic strength, which mimicked biological conditions. Results show that CHQ demonstrates a lower rate of both 1PE and 2PE cleavage than BHQ. However, δu measurements for 4-XAA and 2-XAA indicate excellent susceptibility to 2PE. Future work will involve mechanistic studies of quinoline photorelease and will examine 2-XAA and 4-XAA as 2PE cages for biomolecules.