Two-photon "caging" groups: Effect of position isomery on the photorelease properties of aminoquinoline-derived photolabile protecting groups

Article abstract of DOI:10.1021/ol5035035

High two-photon photolysis cross sections and water solubility of probes are important to avoid toxicity in biomedical applications of photolysis. Systematic variation of the position of a carboxyl electron-withdrawing group (EWG) on photolysis of 8-dimet

Full text of DOI:10.1021/ol5035035

Letter
pubs.acs.org/OrgLett
Two-Photon “Caging” Groups: Effect of Position Isomery on the
Photorelease Properties of Aminoquinoline-Derived Photolabile
Protecting Groups
Christine Tran,† Thibault Gallavardin,† Morgane Petit,† Riadh Slimi,† Hamid Dhimane,†
Mireille Blanchard-Desce,‡ Francine C. Acher,† David Ogden,§ and Peter I. Dalko*,†
†Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques, UMR8601-CNRS, Universite
Saints-Peres, 75270, Paris Cedex 06, France
‡Institut des Sciences Molec
ulaires, UMR5255-CNRS, Bat
§Laboratoire de Physiologie Cer
ebrale, UMR8118-CNRS, Universite
̀
́
Paris Descartes, 45, rue des
̀
́
̂
iment A12 351, Cours de la Liber
́
ation, 33405, Talence Cedex, France
es, 75270, Paris Cedex 06,
́
́
́
Paris Descartes, 45, rue des Saints-Per
France
S
* Supporting Information
ABSTRACT: High two-photon photolysis cross sections and water solubility of probes are important
to avoid toxicity in biomedical applications of photolysis. Systematic variation of the position of a
carboxyl electron-withdrawing group (EWG) on photolysis of 8-dimethylaminoquinoline protecting
groups identified the C5-substituted isomer as a privileged dipole. The 5-benzoyl-8-DMAQ substitution
yields a caging group with an enhanced two-photon uncaging cross section (δu = 2.0 GM) and good
water solubility (c ≤ 50 mM, pH 7.4).
he light-controlled release of ligands is increasingly applied
Tin studies of organized biological systems. These probes
have the potential to trigger perturbations in cells and tissues at
the molecular level by site-selective interactions with high
temporal resolution and at controlled ligand concentrations.1
They can be activated by near-UV light, or by pulsed NIR light
under two-photon (TP) irradiation conditions. TP photolysis
has the advantage of maintaining the localization of excitation
deep within turbid biological tissue because scattered NIR
Figure 1. Quinolin-derived photosensitive probes.
photons do not produce out of focus excitation permitting better
spatiotemporal resolution than near-UV irradiation deep within
tissues. However, TP absorption is inherently much less efficient
at nontoxic laser intensities, requiring much larger TP cross
sections than in current probes such as nitrobenzyl or
nitroindolinyl cages, or in ruthenium coordination based
probes.1 The aim here was to develop small hydrophilic caging
groups with large TP cross sections. The quinoline platform has
the potential for applications under one- and two-photon
excitation conditions; these probes have high photolysis
efficiencies, long wavelength absorption, and low competing
fluorescence.2 Moreover, the photolysis of bromohydroxy-
quinoline (BHQ) and dimethylaminoquinoline (DMAQ)
derivatives (Figure 1) is fast enough not to extend the volume
of TP excitation beyond the illuminated region by diffusion of
intermediates3 and fast enough to activate synaptic glutamate
receptors.4
developed an empirical approach based on matching the
polarizations of the chromophore and photofragmenting
elements. This has allowed in fine high TP uncaging cross
sections to be achieved. We,5 and others6 have demonstrated that
the photophysical properties and the fragmentation of the
DMAQ scaffold are strongly influenced by structural variations.
In a recent systematic study we demonstrated a marked
dependency of the photolysis efficiency on the position of the
dimethylamino donor group under single-photon absorption
and TP absorption (TPA) conditions.5a We showed that the 8-
DMAQ caged acetate 2 (R = Ac, Figure 1) was considerably
more efficient in single and TP photolysis than analogues having
the dimethylamino substituent at the C5, C6, or C7 positions,
respectively, suggesting an optimal position for photolysis
In contrast to two-photon excited fluorophores, the rational
design of TP uncaging chromophores is less well understood and
appears more complex. Here and in forthcoming papers we have
Received: September 17, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ol5035035
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Organic Letters
Letter
efficiency. Here, the effect of an electron-withdrawing group
(EWG) on the photolysis of 8-DMAQ is examined.
Scheme 2. Synthesis of the 6-Carboxy-8-DMAQ Acetate, 17
The presence of strong donor−acceptor couples that promote
internal charge transfer (ICT), as well as extended conjugation, is
a key parameter of increased TPA.7 The synthesis of the donor−
acceptor heteroaryl platform was motivated by the earlier
observation that these structures promote strong ICT and the
compounds show distinct TPA and photochemical properties, as
either fluorescent dyes8a or photoremovable protecting groups.8b
The next step of our study was thus the examination of the
influence of an EWG group such as carboxylate on the photolysis
of the most efficient phototrigger 8-DMAQ 2. The choice of
carboxylate was dictated by the observation that a stronger EWG
such as nitrile or nitro groups invariably inhibited the
photofragmentation of the probe.9
Scheme 3. Synthesis of the 7-Carboxy-8-DMAQ Acetate, 23
Preparations of carboxy-substituted 8-DMAQ-analogues 3−5
are depicted in Schemes 1−3. The synthesis of the C5-carboxy
Scheme 1. Synthesis of the 5-Carboxy-8-DMAQ Acetate, 12
the methyl ester (45%), formation of the lateral C2 chain by
oxidation/reduction, saponification, and acetylation (92%).
Unlike the previously prepared DMAQ derivatives5,6 the
carboxy substituted reagents 12, 17, and 23 were found to be
highly water-soluble to concentrations ≤50 mM at pH 7.4 in
TRIS buffer.10 However, for comparison with previous data all
analyses were performed in a 1/1 mixture of acetonitrile/TRIS
buffer (20 mM, pH 7.4, 293 K) for consistency. (For the
calculated molar extinction spectra, see Supporting Informa-
tion.) While all compounds had qualitatively similar absorption
spectra, C6 and C7 isomers showed lower εmax compared to the
C5 isomer. Carboxylates 12, 17, and 23 were slightly red-shifted
compared to the reference 8-DMAQ acetate, 2 (λmax = 347 nm)
showing λmax 349−368 nm (Table 1), a fact that is consistent
with the earlier observations of 6- and 7-DMAQ bearing
carboxylic ester/acid substituents in position C2 and C4.5a
Interestingly 7-carboxy-8-DMAQ acetate showed more complex
spectra compared to the other two indicating either different type
of associations or a cooperative effect between the ortho
substituents. Molar extinctions at the absorption maxima εmax
varied between 1.9 and 3.6 mM−1 cm−1 (Table 1), similar to
other DMAQ derivatives.3,5,6
DMAQ acetate samples 12, 17, and 23 were photolyzed by
366 nm light (for further details see SI). Photolysis products were
compared to the free carbinols 11, 16, and 22, respectively. No
quantitative analysis of the photoreleased acetic acid was made.
Before irradiation it was ascertained that all acetate samples 12,
17, and 23 are hydrolytically stable and the background
hydrolysis in the dark is less than 1% per hour at rt and at pH
7.4 (see SI). All carboxy-DMAQ-OAc (12, 17, and 23) undergo
photolysis at 366 nm, and the fragmentation followed the C5 >
C6 > C7 order (Table 1). While the C6 isomer 17 was slightly
less efficient than the reference compound 2 under UV
irradiation, compound 12 was photolyzed 1.5-fold more
efficiently than the reference 8-DMAQ acetate, 2 (Table 1).
This order is analogous with the earlier observed trend showing
isomer 12 started from o-fluoro nitrobenzene 6. The
dimethylamino group was introduced by SNAr reaction (98%),
(Scheme 1), and the nitroaryl was then converted to the
corresponding aniline by Pd-catalyzed reduction. The formed
aniline was transformed to quinaldine, 7, in the presence of
crotonaldehyde following modified Dobner−Miller conditions
̈
(41% over two steps).5a The C5-nitrile derivative, 8, was
prepared in a short sequence that included selective mono-
bromination of the quinaldine (NBS, CHCl3) and trans-
formation of the bromo-derivative to the nitrile 8 using CuCN
(1.7 equiv) in DMF at 170 °C (92% two steps overall) (Scheme
1). The desired 12 was obtained after methanolysis of the nitrile
(68%) (Scheme 1), oxidation/reduction of the carbaldehyde,
saponification (LiOH, MeOH/H2O), and acetylation (63% over
two steps).
A similar strategy was used for the preparation of the C6-
carboxy isomer, 17. Compound 17 was prepared from 4-bromo-
2-fluoro nitrobenzene, 13, that was transformed to the 6-nitrile
derivative, 14, in 87% yield (Scheme 2). The dimethylamino
group was introduced by SNAr substitution of the fluorine, and
then the carbinol 15 was obtained by NaBH4 reduction of the
aldehyde. Finally, the C6-carboxy isomer 17 was obtained from
15 by methanolysis followed by saponification of the formed
methyl ester and acetylation (69%) (Scheme 2).
The C7-carboxy isomer, 23, was prepared from the advanced
7-bromoquinoline intermediate 185a by regioselective mono-
nitration followed by SnCl2 reduction and dimethylation of the
amino-bromoquinaldine intermediate (Scheme 3). The desired
product, 23, was obtained after a couple of functional group
adjustments that involved the transformation of the nitrile 20 to
B
DOI: 10.1021/ol5035035
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Photophysical Properties of Chromophores 12, 17, 23, 27, and 36
a
b
e
λabsmax (nm)
ε
max (mM−1 cm−1
)
ε
366 (mM−1 cm−1
)
λemmax (nm)
Stokes shift (cm−1
)
Φf
Qu (%)
ε
366Qu (M−1cm−1 ×100)
cd
,
12
17
368
349
3.6
2.4
3.5
2.0
533
533
8500
0.01
32
27
1120
540
c
10 100
0.02
0.03
d
cd
,
23
27
36
368
340
343
1.9
1.2
1.8
1.0
1.6
477
517
6200
9100
9600
<0.01
<0.01
<0.01
3
21
14
54
210
224
cd
,
cd
,
2.0
520
a
b
c
d
Stokes shift = 1/λabs − 1/λem. Φ = fluorescence quantum yield. Standard: quinine in H2SO4 (0.5 M) (Φ = 0.546). Standard: fluorescein in
e
NaOH (0.1 M) (Φ = 0.90). Standard: 7-DMAQ-OAc in TRIS (20 mM)/acetonitrile 1/1.
increased quantum yields for para- vs meta- or ortho-substituted
donor−acceptor (push−pull) chromophores.11 In summary, the
photolysis experiments have identified for 8-DMAQ the
privileged position of the EWG substituent for optimal uncaging
efficiency.
Scheme 4. Synthesis of the 5-Acryl- 27 and 5-Benzoyl-8-
DMAQ Acetates 36
The two-photon uncaging cross section at 730 nm (δu = σ2Qu)
of the best performing 12 was measured directly from the
fractional conversion of the acetate to the free carbinol. A 45 μL
volume was irradiated in a 3 mm path length quartz cuvette by
the beam of a Ti-Sapphire mode-locked laser (MaiTai BB;
SpectraPhysics) at 730 nm wavelength with 100 fs pulses at 80
MHz. The expanded beam was focused with a 50 mm lens so that
the whole of the excitation volume was contained in the cuvette.
Samples were irradiated for 2−4 h at 100 mW average power.
The loss of the cage was measured by HPLC, and the photolysis
cross sections calculated from the rate of reduction of the
fractional cage concentration at the laser beam parameters are
given above. The 5-carboxy-8-DMAQ acetate 12 had a δu = 0.11
GM (10−50 cm4 s/photon) photolysis cross section, roughly six
times less than that of the reference DMAQ acetate, 2 (δu = 0.67
GM).
The extension of π-conjugation has been shown to be, in some
cases, a viable strategy for increasing TP sensitivity;12 therefore, it
was interesting to examine the effect of the insertion of an extra
olefin, and also an aryl group in the dipole at position C5. The
synthesis of the acrylate derivative 27 started from quinaldine 7.
Regioselective halogenation followed by Heck coupling afforded
the α,β-unsaturated ester 24 in 62% yield (Scheme 4). The
oxidation/reduction of the quinaldine followed by acylation and
deprotection afforded the desired 27. Likewise, the trans-
formation of 7 to the carbinol 28 having the required halogen
linchpin at C5 was realized by selective monobromination of the
corresponding aldehyde followed by reduction (Scheme 4). The
TBS-protected carbinol, 29, was transformed to the boronate
ester 31 by Pd-mediated coupling. We found that the
electronically mismatched Suzuki−Miyaura reaction could be
realized at 80 °C resulting in the fully protected probe, 33, in a
slow reaction (48 h). The replacement of the TBS by acetate and
the deprotection of the carboxyl group afforded the desired 36.
The UV absorption of 27 and 36 showed qualitatively similar
be characterized by more efficient ICT. The diminished TP
sensitivity may eventually be attributed to the laser parameters
used, as the duration, shape, and intensity of the input pulse have
a marked influence on the control of population of the quantum
system: an eventual overexcitation may populate also higher
lying energy levels leading thus to an altered relaxation path.
In summary, the synthesis of 36, having a larger two-photon
uncaging cross section (δu = 2.0 GM) than 8-DMAQ, validates
the dipolar polarization based optimization strategy used here
and opens the way for the rational design of more efficient TP
optimized probes.
ASSOCIATED CONTENT
* Supporting Information
■
spectra to 12 with blue-shifted λmax at 340 and 343 nm with εmax
=
S
1.2 and 2.0 (mM−1 cm−1, Table 1), respectively. Also, the UV
photolysis of 27 and 36 by 366 nm irradiation showed similar
patterns to that of the parent 12 but was less efficient (Qu = 21
and 14%; εQu = 210 and 224 M−1 cm−1, respectively). The TP
photolysis at 730 nm fs pulsed laser light showed, however, a
marked difference with a modest TP uncaging cross section for
27 of δu = 0.25 GM, while a large increase of the two-photon
uncaging cross section was exhibited for 36 with δu = 2.0 GM.
The low TP uncaging cross section of 12 and 27 compared to
the parent 8-DMAQ acetate 2 is surprising, as these probes may
Full experimental procedures and analysis are described in the
Supporting Information. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: peter.dalko@parisdescartes.fr.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/ol5035035
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Organic Letters
Letter
ACKNOWLEDGMENTS
■
Authors acknowledge the generous financial support of the
ANR-2010-BLAN-1436-01 grant. Vincent Hugues is acknowl-
edged for photophysical measurements.
REFERENCES
■
(1) Selected reviews on “caged” compounds: (a) Bort, G.; Gallavardin,
T.; Ogden, D.; Dalko, P. I. Angew. Chem., Int. Ed. 2013, 51, 4526−4537.
̌
́
(b) Klan, P.; Solomek, T.; Bochet, C. G.; Blanc, A.; Givens, R.; Rubina,
M.; Popik, V.; Kostikov, A.; Wirz, J. Chem. Rev. 2013, 113, 119−191.
(c) Brieke, C.; Rohrbach, F.; Gottschalk, A.; Mayer, G.; Heckel, A.
Angew. Chem., Int. Ed. 2012, 51, 8446−8476. (d) Photochem. Photobiol.
Sci. 2012, 11 (Thematic issue). (e) Dore, T. M.; Wilson, H. C.
Photosensitive Molecules for Controlling Biological Function, Vol. 55;
Springer: Heidelberg, 2011, 57−92.
(2) (a) Ma, J.; Rea, A. C.; An, H.; Ma, C.; Guan, X.; Li, M. D.; Su, T.;
Yeung, C. S.; Harris, K. T.; Zhu, Y.; Nganga, J. L.; Fedoryak, O. D.; Dore,
T. M.; Phillips, D. L. Chem.Eur. J. 2012, 18, 6854−6864. (b) Abate-
Pella, D.; Zeliadt, N. A.; Ochocki, J. D.; Warmka, J. K.; Dore, T. M.;
Blank, D. A.; Wattenberg, E. V.; Distefano, M. D. ChemBioChem 2012,
13, 1009−1016 and cited references.
(3) (a) Kiskin, N.; McCray, J.; Chillingworth, R.; Piston, D.; Ogden, D.
Eur. Biophys. J. 2002, 30, 588−604. (b) Kiskin, N.; Ogden, D. Eur.
Biophys. J. 2002, 30, 571−587.
(4) Dore, T. M.; Wilson, H. C. Neurometh. 2011, 55, 57−92. (b) For
the kinetics of receptor dynamics, see: Attwell, D.; Gibb, A. Nat. Rev.
Neurosci. 2005, 6, 841−849. (c) For recollected data of the dark
fragmentation rate of different caging groups, see: Dore, T. M.; Wilson,
H. C. Neurometh. 2011, 55, 57−92.
(5) (a) Petit, M.; Tran, C.; Roger, T.; Gallavardin, T.; Dhimane, H.;
Palma-Cerda, F.; Blanchard-Desce, M.; Acher, F. C.; Ogden, D.; Dalko,
P. I. Org. Lett. 2012, 14, 6366−6369. (b) Petit, M.; Bort, G.; Doan, B.-T.;
Sicard, C.; Ogden, D.; Scherman, D.; Ferroud, C.; Dalko, P. I. Angew.
Chem., Int. Ed. 2011, 50, 9708−9711. (c) Dunkel, P.; Tran, C.;
Gallavardin, T.; Dhimane, H.; Ogden, D.; Dalko, P. I. Org. Biomol. Chem.
2014, 12, 9899−9908.
(6) Li, Y.-M.; Shi, J.; Cai, R.; Chen, X.-Y.; Guo, Q.-X.; Liu, L.
Tetrahedron Lett. 2010, 51, 1609−1612.
(7) (a) He, G. S.; Yuan, L.; Cheng, N.; Bhawalkar, J. D.; Prasad, P. N.;
Brott, L. L.; Clarson, S. J.; Reinhardt, B. A. J. Opt. Soc. Am. B 1997, 14,
1079−1087. (b) Belfield, K. D.; Hagan, D. J.; Van Stryland, E. W.;
Schafer, K. J.; Negres, R. A. Org. Lett. 1999, 1, 1575−1578.
(8) (a) Genin, E.; Hugues, V.; Clermont, G.; Herbivo, C.; Castro, M. C.
R.; Comel, A.; Raposo, M. M. M.; Blanchard-Desce, M. Photochem.
Photobiol. Sci. 2012, 11, 1756−1766. (b) Aujard, I.; Benbrahim, C.;
Gouget, M.; Ruel, O.; Baudin, J. B.; Neveu, P.; Jullien, L. Chem.Eur. J.
2006, 12, 6865−6879.
(9) The synthesis and photophysical analysis of these derivatives will
be discussed elsewhere. For related observations, see: Jarrett Davis, M.;
Kragor, C. H.; Reddie, K. G.; Wilson, H. C.; Zhu, Y.; Dore, T. M. J. Org.
Chem. 2009, 74, 1721−1729.
(10) For the determination of water solubility from the absorbance as a
function of the concentration, see: Kim, H. M.; Kim, B. R.; Choo, H. J.;
Ko, Y. G.; Jeon, S. J.; Kim, C. H.; Joo, T.; Cho, B. R. ChemBioChem 2008,
9, 2830−2838 and references cited.
́
(11) Andraud, C.; Fortrie, R.; Barsu, C.; Stephan, O.; Chermette, H.;
Baldeck, P. L. In Photoresponsive Polymers II, Vol. 214; Marder, S. R., Lee,
K. S., Eds.; Springer: Berlin, Heidelberg, 2008; pp 149−203.
(12) He, S.; Tan, L.-S.; Zheng, Q.; Prasad, P. N. Chem. Rev. 2008, 108,
1245−1330.
D
DOI: 10.1021/ol5035035
Org. Lett. XXXX, XXX, XXX−XXX