Competing Pathways in the Photo-Favorskii Rearrangement and Release of Esters: Studies on Fluorinated p-Hydroxyphenacyl-Caged GABA and Glutamate Phototriggers

Abstract

Three new trifluoromethylated p-hydroxyphenacyl (pHP) caged γ-aminobutyric acid (GABA) and glutamate (Glu) derivatives have been examined for their efficacy as photoremovable protecting groups in aqueous solution. By replacing hydrogen with fluorine, e.g., a m-trifluoromethyl or a m-trifluoromethoxy vs. m-methoxy substituents on the pHP chromophore, modest increases in the quantum yields for release of the amino acids GABA and glutamate were realized as well as improved lipophilicity. The pHP triplet undergoes a photo-Favorskii rearrangement with concomitant release of the amino acid substrate. Deprotonation competes with the rearrangement from the triplet excited state and yields the pHP conjugate base that, upon reprotonation, regenerate the starting ketoester, a chemically unproductive or “energy wasting” process. Employing picosecond pump–probe spectroscopy, GABA derivatives 2 – 5 are characterized by short triplet lifetimes, a manifestation of their rapid release of GABA. The bioavailability of released GABA at the GABAA receptor improved when the release took place from m-OCF3 (2) but decreased for m-CF3 (3) when compared with the parent pHP derivative. These studies demonstrate that pKa and lipophilicity exert significant but sometimes opposing influences on the photochemistry and biological activity of pHP phototriggers.

2-Hydroxyphenacyl ester: a new photoremovable protecting group

“…This photoreaction was suppressed when dry solvents were used, which is also known from the photochemistry of 1 . 9,22 However, water obviously does not trap any reactive intermediates, which was demonstrated for 3a by isotopic labeling experiments (Scheme 6 and the related text in the Results).…”

“…Provided that energy transfer from 3 3 * to 2-naphthalene sulfonate in 95% water/acetonitrile (v/v) is a diffusion controlled process ( k diff ~ 4 × 10 9 M −1 s −1 ), 29 the lifetime of 3 3 * must be on the order of 1 ns or less, comparable to those of pHP carboxylates. 10,22 …”

Photochemical electrocyclic ring closure and leaving group expulsion from N-(9-oxothioxanthenyl)benzothiophene carboxamides

“…In both cases the direct photolyses result in expulsion of the various LG -in nearly quantitative yields, regardless of basicity of the LG -. As with 5, 15 the photochemical mechanism for 7-9 is thought to involve expulsion of a leaving group LG -from a zwitterionic intermediate phydroxyphenacyl λ max 282 nm (ε 14,000), 17 and desyl λ max 323 nm (ε 400). 18 A long wavelength absorber is 25 aminocoumarinylmethyl λ max 402 nm (ε 18,600).…”

An Optical Fiber-Based Uncaging System

“…The caged neurotransmitters are supplied by various vendors (Calbiochem, Dojindo Molecular Technologies, Inc., and Invitrogen) or can be synthesized in-house (e.g., Givens et al 1997; Stensrud et al 2009). …”

“…This system has been used to rapidly release glutamate or GABA from several different types of caged compounds (Givens et al 1997; Kandler et al 1998; Conrad et al 2000; Kim and Kandler 2003; Stensrud et al 2009). The two major neurobiological applications in which this system has been applied are focal dendritic stimulation with glutamate (Fig.…”