8-Bromo-7-hydroxyquinoline as a Photoremovable Protecting Group for Physiological Use:  Mechanism and Scope

Abstract

Two-photon excitation (2PE) of "caged" biomolecules represents a powerful method to investigate the temporal and spatial relevance of physiological function in real time and on living tissue, because the excitation volume can be restricted to 1 fL. Additionally, low-energy IR light is used, which minimizes tissue destruction and enables deeper penetration into tissue preparations. Exploitation of this technology for studying cell physiology requires the further development of photoremovable protecting groups with sufficient sensitivity to 2PE for use in "caged" compounds. 8-Bromo-7-hydroxyquinoline (BHQ) is efficiently photolyzed by classic 1PE (365 nm) and 2PE (740 nm) under simulated physiological conditions (aqueous buffer of high ionic strength, pH 7.2) to release carboxylates, phosphates, and diols-functional groups commonly found on bioactive molecules such as neurotransmitters, nucleic acids, and drugs. It is stable in the dark, soluble in water, and exhibits low levels of fluorescence, which will enable use in conjunction with fluorescent indicators of biological function. BHQ-protected effectors are synthetically accessible. Stern-Volmer quenching, time-resolved infrared (TRIR), and (18)O-labeling experiments suggest that the photolysis occurs through a solvent-assisted photoheterolysis (S(N)1) reaction mechanism on the sub-microsecond time scale. BHQ has the requisite photochemical and photophysical properties as a photoremovable protecting group to regulate the action of biological effectors in cell and tissue culture with light, especially 2PE.

Inverted Substrate Preferences for Photochemical Heterolysis Arise from Conical Intersection Control

“…For example, the meta-donor-substituted systems have an excited singlet ion diradical form that is electronically analogous to the classic meta xylylene diradical, 49 with a radical at the "carbenium center" and a cation radical donor substituent. There are numerous examples of cations that fall within this type (9,12,13,17,18,19,22,23,24,25,26,27,31). Thus, while the donor group does not act to stabilize the ground-state cation via resonance, it leads to stabilized singlet diradical excited states.…”

“…To date, no model connects structure to reactivity for these photoreactions, and many of the known photoremovable protecting groups, 9 or photocages, have been discovered serendipitously or through empirical investigations. The lack of a structure−reactivity relationship for photoheterolysis reactions has hindered the rational design of new structures that undergo photoheterolysis, which are reactions of applied importance in materials, 10 synthetic, 8,11 medicinal, 12 and biological chemistry. 13 In this Article, we attempt to understand why successful photochemical heterolysis reactions of C−LG bonds frequently generate unstabilized carbocations, the opposite of structural preferences for thermal heterolysis reactions.…”

Resonance Raman Characterization of Different Forms of Ground-State 8-Bromo-7-hydroxyquinoline Caged Acetate in Aqueous Solutions

“…This appears consistent with the hypothesis that the anionic form is the precursor of the photocleavage process as suggested in a previous experimental study. 6,7 …”

“…The 8-bromo-7-hydroxyquinolinyl group (BHQ) was found to be efficiently photolyzed by 1PE and 2PE in aqueous buffers at physiological pH with a high quantum efficiency and low levels of fluorescence that make this kind of phototrigger more attractive for utilization in biological experiments. 6,7 Recent work showed a very fast process of the deprotection and solvolysis of 8-bromo-7-hydroxyquinoline caged acetate (BHQ-OAc) to form the byproduct (BHQ-OH). 7 These results suggest that it would be useful to employ ultrafast (femtoseconds to picoseconds) and fast (nanoseconds to microseconds) spectroscopic methods to directly follow the photophysics and photochemistry associated with the deprotection and formation of the byproduct to better understand its reaction mechanism (particularly in aqueous environments).…”

A Model for Light‐Triggered Porphyrin Anticancer Prodrugs Based on an o‐Nitrobenzyl Photolabile Group

“…[19] In addition, the light-triggered porphyrin anticancer prodrug technique created herein may potentially be employed as binary cancer treatment in both chemotherapy and photodynamic therapy to potentiate the efficacy of anticancer drugs. Because two-photon excitation has better tissue penetration than UV excitation, and because it could also minimize the phototoxicity to cells and tissues, [35][36][37][38] for practical use of this light-triggered porphyrin anticancer prodrug technique, the UV photolabile o-nitrobenzyl moiety will be replaced with a two-photon photolabile species such as a 7-hydroxycoumarin moiety, [35] and the results will be reported in due course.…”