Unraveling the Mechanism of the Photodeprotection Reaction of 8‐Bromo‐ and 8‐Chloro‐7‐hydroxyquinoline Caged Acetates

Abstract

Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming “caged compounds” are a powerful means to regulate the action of physiologically active messengers in vivo through 1-photon excitation (1PE) and 2-photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8-chloro-7-hydroxyquinolin-2-yl)methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time-resolved spectroscopic studies using transient emission (ns-EM), transient absorption (ns-TA), transient resonance Raman (ns-TR2), and time-resolved resonance Raman (ns-TR3) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ-OAc, but not CHQ-OAc. A high fluorescence quantum yield and a more efficient excited-state proton transfer (ESPT) in CHQ-OAc compared to BHQ-OAc explain the lower quantum efficiency of CHQ-OAc relative to BHQ-OAc.

Photoactivatable, biologically-relevant phenols with sensitivity toward 2-photon excitation

“…34–37 The hydroxyquinoline PPG has also been shown to directly release phenols without the use of a carbonate linker, 3 and the timescale, measured for BHQ-OAc, is on the order of nanoseconds, 38 thus making this PPG suitable for use in studying temporally resolved signaling events. By directly releasing phenols, photoactivatable versions of relevant phenol-containing biological effectors with potentially rapid release kinetics are now possible.…”

Light-Activated Serotonin for Exploring Its Action in Biological Systems

“…We chose BHQ as the caging group for 5-HT because it has good sensitivity to 1PE-mediated photolysis at biologically compatible wavelengths (>350 nm) (Fedoryak and Dore, 2002; Zhu, et al, 2006) and rapid release kinetics (Ma, et al, 2012). High sensitivity to light is important for working in thick or pigmented biological tissues, such as whole larval zebrafish or Xenopus laevis embryos.…”

“…To test this, we synthesized 8-bromo-2-(phenoxymethyl)quinolin-7-ol (BHQ-OPh) from bromo-7-(methoxymethoxy)quinolin-2-yl)methanol (MOM-BHQ-OH, Figure 2a). MOM-BHQ-OH, prepared from 8-bromo-7-hydroxyquinaldine as previously described (Ma, et al, 2012), was converted to the corresponding mesylate, which was subsequently displaced by phenol in good yield to provide the desired phenyl ether. Removal of the methoxymethyl ether (MOM) protecting group with trifluoroacetic acid in methanol afforded BHQ-OPh.…”

“…Initially formed carbamic acid intermediates typically decompose to CO 2 and the amine on the 6-7 ms timescale (Papageorgiou and Corrie, 1997), which is slower than the diffusional escape time from the focal volume of 2PE excitation (estimated at 113-900 μs (Kiskin and Ogden, 2002)) and the opening of ionotrophic 5-HT-gated ion channels (5-HT 3 receptors open on the order of 1-2 ms timescales and remain open for up to 10 ms (Jackson and Yakel, 1995)). Time-resolved studies on BHQ-caged acetate (Ma, et al, 2012) suggest that BHQ- O -5HT releases 5-HT on nanosecond timescales—orders of magnitude shorter than diffusional timescales.…”

“…BHQ- O -5HT was found to be more sensitive to light at 368 and 740 nm than BHQ- N -5HT, and based on the behavior of similar compounds (Ma, et al, 2012), its release kinetics are expected to be faster than diffusion rates and the opening of 5-HT 3 receptors. BHQ- O -5HT mediated the light activation of 5-HT, subsequently depolarizing mammalian neurons in culture or in the nervous system of intact larval zebrafish (5-7 dpf).…”