Carbanion-mediated photocages: Rapid and efficient photorelease with aqueous compatibility

Article abstract of DOI:10.1021/ja0517062

A new photocage is proposed, based on ketoprofen-derived compounds and mediated by carbanions. The new photocage has significant advantages over the widely used o-nitrobenzyl derivatives, including aqueous compatibility, faster photorelease, higher quantu

Full text of DOI:10.1021/ja0517062

Published on Web 05/06/2005
Carbanion-Mediated Photocages: Rapid and Efficient Photorelease with
Aqueous Compatibility
Matthew Lukeman and Juan C. Scaiano*
Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada
Received March 17, 2005; E-mail: tito@photo.chem.uottawa.ca
Photoremovable protecting groups or “photocages” provide
biochemists with powerful tools for the rapid introduction of a
variety of compounds to biological systems with spatial and
temporal control, allowing the time-resolved study of the ensuing
events.1 They have found use in other fields ranging from molecular
array (biochip) synthesis to time-resolved X-ray crystallography.
The most widely used photocage, the o-nitrobenzyl (oNB) group,
suffers from major disadvantages, specifically, slow release rate
following excitation (typically several microseconds or longer), poor
aqueous solubility, and formation of reactive nitrosoaldehyde
byproducts.2 Much effort has been devoted to the development of
new photocages that overcome one or more of the drawbacks
associated with the oNB group, although no candidate has emerged
with a sufficiently desirable combination of properties to challenge
its dominance. We report the development of a new photocage that
is superior to the oNB group in several respects, including a release
rate improvement of over 1000-fold, a higher photorelease quantum
yield, excellent aqueous solubility, and formation of benign
photoproducts. This photocage is capable of releasing a wide variety
of biologically relevant substrates, including alcohols and carboxylic
acids.
10 with the anhydride or acid chloride of acetic acid and ibuprofen,
respectively. Alcohol 10 was obtained via addition of gaseous
formaldehyde to the anion of the tert-butyl ester of 1, followed by
deprotection with TiCl4. Methyl ether 5 was obtained by treatment
of the anion of the methyl ester of 1 with iodomethyl methyl ether,
followed by hydrolysis in dilute NaOH solution. Derivatives 7-9
were prepared by addition of diiodomethane, dibromomethane, or
iodochloromethane, respectively, to the anion of 2-benzoylphenyl-
propionitrile, followed by hydrolysis to the corresponding acid in
50% H2SO4.
The design of the new photocage has its roots in the photo-
chemistry of ketoprofen (1), a nonsteroidal anti-inflammatory drug
(NSAID). On electronic excitation in neutral aqueous media, 1
undergoes rapid and efficient decarboxylation (Φ ) 0.75) to give
a benzylic carbanion, which is rapidly protonated by the solvent (k
∼ 4.9 × 106 s-1) to give 3-ethylbenzophenone (2) (eq 1).3 We
sought to harness this efficient photochemistry for effecting
photorelease by creating derivatives that include an appropriate
leaving group substituted â to the photogenerated carbanion.
Elimination would release the leaving group in its active form, along
with 3-(2′-allyl)benzophenone (3) (eq 2). This reaction represents
a novel carbanion-mediated mechanism for photodecaging and is
mechanistically unrelated to other photodecaging reactions that
involve a benzophenone moiety.4
Dilute deoxygenated solutions (4 mM) in phosphate buffer (pH
7.4) were prepared and irradiated briefly (1-10 min) with UVA
light (λ 350 nm) and, following workup, were analyzed by 1H and
13C NMR and MS. The only photoproduct observed on irradiation
under these conditions for derivatives 4 and 6-9 was 3, indicating
that the photogenerated carbanion gives quantitative photorelease
of all carboxylic acids and halides examined, with protonation of
the intermediate carbanion by water not being competitive. The
efficiency of the photorelease was very high for these derivatives
(Φ ∼ 0.7) and is more than sufficient for biochemical applications.
Derivative 5 bearing a strongly basic leaving group (methoxide)
gave 3 in 20% yield, along with an 80% yield of 11, the product
corresponding to protonation of the intermediate carbanion (eq 3).
While elimination of simple alcohols from the decarboxylated
ketoprofenate cage is not quantitative, the quantum yield is still in
the range of 0.10-0.15, which is adequate for most applications.
Derivatives 4 and 5 were synthesized to test the viability of this
design toward photorelease of carboxylic acids and alcohols,
respectively, while derivative 6 was designed to photorelease
ibuprofen to demonstrate possible drug release applications. In
addition, derivatives 7-9 bearing iodide, bromide, and chloride
leaving groups, respectively, were also prepared and studied. Esters
4 and 6 were synthesized by treatment of the corresponding alcohol
An important concern in application of “ketoprofenate” photo-
cages is the possible formation of radicals or other reactive species
resulting from secondary photolysis of the photoproduct 3 that might
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J. AM. CHEM. SOC. 2005, 127, 7698-7699
10.1021/ja0517062 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
entry into the study of submillisecond biological processes. Meth-
oxide photorelease can be viewed as the limiting case; any better
leaving group should eliminate with a faster rate constant and higher
quantum yield, suggesting that the ketoprofenate cage could be
successfully applied to the photorelease of other biologically
significant functional groups, including phosphates.
In addition to the high quantum yields and fast photorelease rates,
the ketoprofenate cage offers other advantages over the oNB group.
The carboxylate function imparts excellent aqueous solubility to
the caged group. Also, photorelease can only proceed from the
carboxylate form (that is present at neutral pH), so no special
handling precautions are required to avoid exposure of the caged
group to light prior to loading in the aqueous medium.
In their recent review on photocages,1a Pelliciolli and Wirz
outlined what they believed to be the six most important criteria
that the ideal photocage would meet (in the context of biological
applications), and they include the following: (1) clean and efficient
photochemistry; (2) adequate absorption at wavelengths longer than
300 nm; (3) good aqueous solubility; (4) generation of inert and
nonabsorbing photoproducts; (5) high decaging rate constant; and
(6) absence of “dark” activity (from release via thermal hydrolysis,
for example). The ketoprofenate chromophore satisfies all six
criteria outlined by Pelliciolli and Wirz and, thus, is a significant
step forward in the evolution of photocaging chromophores.7 In
summary, we report the successful design of a new photocaging
group capable of effecting rapid and efficient photorelease of
carboxylic acids, halides, and alcohols, and potentially many other
functional groups, that possesses several advantages over the
conventional oNB group.
Figure 1. Transient absorption spectrum observed for 4 in phosphate buffer
solution (pH 7.4) at 72 (2), 180 (O), 460 (2), and 1650 (O) ns after the
laser pulse (τ ) 308 nm). Inset: decay profile for absorption at 600 nm
with first-order kinetic fit.
interfere with the system under study. To test for possible secondary
photochemistry, a pure sample of 3 was chromatographically
separated from the photolyzate of a solution of 4 and was irradiated
alongside an equimolar sample of 4 in aqueous acetonitrile solution
(1:1, aqueous portion buffered at pH 7.4). Under irradiation
conditions that gave 80% decaging of acetate from 4, absolutely
no decomposition of 3 was observed (by GC-MS), indicating that
3 can be considered photoinert under the conditions relevant to
photorelease applications; this probably reflects lower excited-state
energies (compared to that of benzophenone) and the π,π* character
of its low-lying excited states.
Acknowledgment. We acknowledge the generous financial
support of the Natural Sciences and Engineering Research Council
of Canada (NSERC). M.L. thanks NSERC for a postdoctoral
fellowship.
The protonation rate constants (in water) of a series of benzoyl
benzyl carbanions have been measured by laser flash photolysis
(LFP) and vary between 2 × 106 and 2 × 107 s-1, depending on
the nature of the benzylic substituents.5 Assuming that g99% of
the carbanions generated from 4 and 6-9 undergo elimination, a
minimum photorelease rate constant of g2 × 108 s-1 can be
estimated, which is more than 2 orders of magnitude faster than
that for release of carboxylic acids from the oNB chromophore.2a
The fast rate constant for carboxylate elimination precludes the
detection of the proposed intermediate carbanion by conventional
nanosecond LFP. Elimination of methoxide from 5, however, is
slower, as protonation of the intermediate carbanion is competitive,
thus nanosecond LFP was employed in an attempt to detect the
carbanion intermediate. LFP of a flowing solution of 5 (oxygen-
saturated phosphate buffer, pH 7.4, λex 308 nm) gave rise to two
kinetically distinct transient species, both present immediately after
the laser pulse (Figure 1). The longer-lived species at λmax 520 nm
is characteristic of the triplet excited state of the benzophenone
chromophore3b and is assigned as such. The shorter-lived species
with λmax 580 nm and τ ) 40 ns is assigned to the expected
carbanion intermediate based on the similarity of its maximum
absorption and lifetime with that of previously reported ketoprofen-
derived carbanions.3 From the partitioning between the elimination
and protonation routes, we calculate a rate constant for methoxide
elimination from 5 of 5.1 × 106 s-1. This rate constant is more
than 6 orders of magnitude higher than that for photorelease of
alcohols from oNB ethers and for alcohols linked to other
photocages through carbonate or carbamate linkers!2,6 Keto-
profenate-protected alcohols might, therefore, be the most attractive
Supporting Information Available: Experimental preparations for
4-9 and 1H NMR spectra for 4 and 5 before and after irradiation (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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(7) Photoproduct 3 has an absorption profile that is very similar to that of
the starting acids 4-9 at wavelengths longer than 300 nm, and while 3 is
not “nonabsorbing” (criterion 4), no highly absorbing photoproducts are
formed, unlike the photoproducts arising from the oNB group.
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