Improved Synthesis of Caged Glutamate and Caging Each Functional Group

Article abstract of DOI:10.1021/acschemneuro.8b00152

Glutamate is an excitatory neurotransmitter that controls numerous pathways in the brain. Neuroscientists make use of photoremovable protecting groups, also known as cages, to release glutamate with precise spatial and temporal control. Various cage designs have been developed and amongst the most effective has been the nitroindolinyl caging of glutamate. We, hereby, report an improved synthesis of one of the current leading molecules of caged glutamate, 4-carboxymethoxy-5,7-dinitroindolinyl glutamate (CDNI-Glu), which possesses efficiencies with the highest reported quantum yield of at least 0.5. We present the shortest route, to date, for the synthesis of CDNI-Glu in 4 steps, with a total reaction time of 40 h and an overall yield of 20%. We also caged glutamate at the other two functional groups, thereby, introducing two new cage designs: α-CDNI-Glu and N-CDNI-Glu. We included a study of their photocleavage properties using UV-vis, NMR, as well as a physiology experiment of a two-photon uncaging of CDNI-Glu in acute hippocampal brain slices. The newly introduced cage designs may have the potential to minimize the interference that CDNI-Glu has with the GABA_A receptor. We are broadly disseminating this to enable neuroscientists to use these photoactivatable tools.

Full text of DOI:10.1021/acschemneuro.8b00152

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Article
Improved Synthesis of Caged Glutamate and Caging Each Functional Group
Charitha Guruge, Yannick P. Ouedraogo, Richard Louis
Comitz, Jingxuan Ma, Attila Losonczy, and Nasri Nesnas
ACS Chem. Neurosci., Just Accepted Manuscript • DOI: 10.1021/acschemneuro.8b00152 • Publication Date (Web): 11 May 2018
Downloaded from http://pubs.acs.org on May 16, 2018
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Improved Synthesis of Caged Glutamate and Caging Each Functional Group
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Charitha Guruge1, Yannick P. Ouedraogo1, Richard L. Comitz1, Jingxuan Ma1, Attila Losonczy2, Nasri
Nesnas1,*
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1 Department of Chemistry, Florida Institute of Technology, Melbourne FL 32901, United States
2 Department of Neuroscience, Columbia University, New York, NY 10032, United States
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* Corresponding Author
Professor Nasri Nesnas, Ph.D.
Department of Chemistry
Florida Institute of Technology
150 West University Blvd., Melbourne, FL 32901, United States
E-mail: nesnas@fit.edu
Key words: CDNI-Glu, α-CDNI-Glu, N-CDNI-Glu, photochemistry, cages, photoprotecting groups, caged
Glu
Abstract
Glutamate is an excitatory neurotransmitter that controls numerous pathways in the brain.
Neuroscientists make use of photoremovable protecting groups, also known as cages, to
release glutamate with precise spatial and temporal control. Various cage designs have been
developed and amongst the most effective has been the nitroindolinyl caging of glutamate. We,
hereby, report an improved synthesis of one of the current leading molecules of caged
glutamate, 4-carboxymethoxy-5,7-dinitroindolinyl glutamate (CDNI-Glu), which possesses
efficiencies with the highest reported quantum yield of at least 0.5. We present the shortest
route, to date, for the synthesis of CDNI-Glu in 4 steps, with a total reaction time of 40 h and an
overall yield of 20%. We also caged glutamate at the other two functional groups, thereby,
introducing two new cage designs: α-CDNI-Glu and N-CDNI-Glu. We included a study of their
photocleavage properties using UV-vis, NMR, as well as a physiology experiment of a two-
photon uncaging of CDNI-Glu in acute hippocampal brain slices. The newly introduced cage
designs may have the potential to minimize the interference that CDNI-Glu has with the GABAA
receptor. We are broadly disseminating this to enable neuroscientists to use these
photoactivatable tools.
Graphical abstract
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Introduction
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The brain is the most complex animal organ. It is the core of the nervous system and as a result
controls the entire organism. The average human brain contains at least 85 billion neurons
interconnected with a range of 1000 to 10,000 synapses.1–3 Neurons communicate with one
another using chemical messengers, known as neurotransmitters, which are released by the
presynaptic neuron across synaptic junctions. Postsynaptic neurons have receptors that bind
the neurotransmitters thereby activating pathways that lead to signal transmission. While
there are a handful of neurotransmitters, glutamate (Glu) and gamma aminobutyric acid
(GABA) are amongst the most common as they are the excitatory and inhibitory
neurotransmitters, respectively.4–8
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The immense complexity of the brain’s network renders brain mapping a colossal challenge.
The development of molecular tools that can be activated upon demand at specific synaptic
sites has become of apparent value. Neuroscientists have made use of photo responsive
protecting groups that can be released upon demand at very specific synapses.9 These are
known as cages, since their principal role is to completely inactivate the agonists, analogous to
incapacitating a caged lion. However, they have to respond to light in an efficient manner to
release the active agonist using specific wavelengths of laser.10,11 This process is known as
uncaging.
Several cage designs have been developed over the years.12–15 They vary in their photo
properties, which include: 1) wavelength 2) extinction coefficient and 3) quantum yield.
Amongst the most efficient cage designs are those developed by Corrie16-18 and Ellis-Davies19–22
:
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4-methoxy-7-nitroindolinyl glutamate (MNI-Glu) and 4-carboxymethoxy-5,7-dinitroindolinyl
glutamate (CDNI-Glu), respectively.
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We describe an improved synthesis of CDNI-Glu and introduce two new cage designs.
Glutamate has three functional groups: an α-amino, an α-carboxy, and an ω-carboxy. Ellis-
Davies and coworkers reported caging the latter,19 naming it CDNI-Glu in 2007, and
subsequently published a more detailed protocol in 2011.20 We are presenting the first
synthesis of caged Glu at the α-carboxy and at the amine groups, naming them: α-CDNI-Glu and
N-CDNI-Glu, respectively. One of the advantages of exploring these new caged agonists is their
potential at minimizing the observed inhibition of GABAA receptors when CDNI-Glu was
used.10,21 The structures of the three variants of caged Glu (A, B, and C), attached at each of the
three functional groups, are shown in Figure 1.
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O
OH
O
O
OH
O
O
OH
O
O2N
O2N
O2N
OH
O
N
O
N
N
O
OH
O2N
O2N
O2N
O
OH
O
N
O
OH
H
NH2
O
NH2
(A)
1
(B)
2
(C)
3
CDNI-Glu
α
-CDNI-Glu
N-CDNI-Glu
Figure 1. The three types of caged-Glu (A, B, and C); B and C have not been previously
reported; caging units are shown in red for clarity.
CDNI has become an essential photoprotecting tool amongst neuroscientists due to its superior
photochemical properties compared to other caging systems. Our earlier studies included a
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comparison with MNI, which has one nitro group instead of two. Using NMR techniques with
photolysis, we showed that the additional nitro group in the indole ring of CDNI increases the
relative rate of photocleavage by a factor of 5.8.23 The more recent protocol for the preparation
of CDNI-Glu and CDNI-GABA presents details of a 10-step synthesis carried out over 200 h of
reaction time, with overall yields of 7-11%.20 The immense significance that CDNI-Glu has had in
the field of optical neuroscience, especially due to its responsiveness to two-photon
irradiation,24 prompted us to develop a faster and more efficient route that we are currently
employing in other systems. Our improved synthesis employs 4 steps over a reaction time of 40
h, with an overall yield of 20%.
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Furthermore, CDNI-Glu causes off-target effects with the GABAA receptor at high
concentrations (3-12 mM).10,21 The proximity of the zwitterionic functions of COO– and NH3 in
+
CDNI-Glu mimics the curved conformation of GABA when binding the GABAA receptor. This
leads to interference between CDNI-Glu and the inhibitory GABAA receptor, effectively
mitigating the excitatory signals of photolyzed CDNI-Glu. There has been several attempts to
minimize or eliminate this interference via elaborate designs of different photocaged
glutamates.10 These designs are mainly based on bulking up the caged molecules25 thereby
minimizing their competitive interaction with the receptors.
Herein, we present an improved synthesis of a critical photo responsive molecular tool, CDNI-
Glu, via a 4-step synthesis and overall yields of 20%. We also synthesized two new caged Glu (B
and C) by protecting the other two functional groups of the agonist. This may lead to
minimized interference with the GABAA receptor. We also deliver the photo properties of all
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three caged Glu molecules using UV-vis and NMR, per our previous work, as well as an in vitro
two-photon uncaging experiment in acute mouse brain slice. Future studies will involve the
evaluation of the new designs (cages B and C) in brain slices. We are receptive to collaboration
with neuroscience labs that are conducting brain slice experiments with two-photon laser, and
as such, we will gladly provide them with all three caged Glu A, B, and C.
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Results and Discussion
Synthetic Procedures
The improved procedures focus on two predominant factors: direct access to the indolinyl
system and direct deprotection and nitration pathways. Our nitration step takes advantage of
an old protocol, published in 1956 by G. A. Olah,26–29 that uses nitronium tetrafluoroborate,
-
+
NO2 BF4 , to directly introduce the nitronium ion for electrophilic aromatic substitution. We
found this to be a rapid and smooth technique to nitrate the indolinyl moiety directly to the
dinitro system. The use of freshly purchased reagent under anhydrous conditions led to a near
quantitative conversion to the dinitroindolinyl system, as observed by TLC. Our isolated yields
of 30% for that step suffered during the HPLC purification and lyophilization of the final product.
Scheme 1 outlines the 4 steps for the synthesis of CDNI-Glu along with their corresponding
reaction times and yields for each step.
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Total Reaction Time = 40 h
O
OtBu
O
O
OtBu
O
OH
O
O
OtBu
OH
7
NHBoc
7
8
a
b
c
9
18 h
87%
18 h
2 h
N
H
N
H
N
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H
80%
97%
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O
OtBu
O
O
OH
O
O2N
d
2 h
30%
N
O
N
O
O
OtBu
O2N
O
OH
NHBoc
NH2
(A)
8
CDNI-Glu
Scheme 1. Synthesis of CDNI-Glu (A); a) tert-butyl bromoacetate, K2CO3, acetone, RT; b)
NaBH3CN, AcOH, RT; c) 1-(3-dimethylaminopropyl)-3-ethylcarbodimide, DMAP, Boc-Glu-Ot-Bu 7,
CH2Cl2, RT; d) TFA, nitronium tetrafluoroborate (4 equiv), RT.26–29
Previous protocols20 make use of acetyl protecting groups to control the selectivity of the
functionalization of the N and O of the reduced hydroxyindole, i.e. indolinyl. Once the
hydroxyindole ring is reduced, the nucleophilic reactivity of the O and N of the indolinyl system
become comparable, thus necessitating the use of protecting groups. However, in the presence
of the 2,3-olefin in 4, the nucleophilic potency of the N is somewhat subdued by to the partial
aromaticity of the pyrrole ring. This enabled us to selectively functionalize the phenolic O with
the ester group under basic conditions of K2CO3 over 18 h, leading to 5 with 80% yield. Our
choice for a t-butyl ester instead of a methyl ester was mainly to enable a one-step
deprotection under TFA conditions needed for the subsequent nitration steps. Reduction of
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the indole double bond to form 6 proceeded with a 97% yield, in 2 h under acidic reductive
conditions of NaBH3CN in acetic acid. The attachment of the glutamate chain was
accomplished using standard EDC coupling with protected Glu 7. The reaction was carried out
for 18 h and afforded compound 8 in an 87% yield, which was purified through standard normal
phase chromatography. The final step was carried out in TFA, which served the dual purpose of
an instant deprotection of both tert-butyl groups of 8 and an acidic medium for the subsequent
nitration. The direct introduction of two nitro groups on the indolinyl ring was accomplished
using nitronium tetrafluoroborate in TFA, affording target compound, CDNI-Glu (A), in 30%
yield after HPLC purification and lyophilization.
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We employed this protocol to generate the other caged Glu designs (Figure 1). Caging the α-
carboxy group of Glu is unprecedented and may possess improved electrophysiological
properties; therefore, it is worthwhile to make it available to neuroscientists. Scheme 2
highlights the steps for an efficient synthesis of α-CDNI-Glu (B) in similar yields to our synthesis
of CDNI-Glu (A).
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Total Reaction Time = 40 h
4
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7
O
OtBu
O
OtBu
O
O
O
HO
OtBu
O
NHBoc
8
9
9
a
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18 h
87%
N
N
O
H
O
OtBu
6
O
OH
O
NHBoc
10
O2N
b
N
O
2 h
27%
O2N
O
OH
(B)
α-CDNI-Glu
NH2
Scheme 2. Synthesis of α-CDNI-Glu (B); a) 1-(3-dimethylaminopropyl)-3-ethycarbodimide,
DMAP, Boc-D-Glu(OtBu)-OH 9, CH2Cl2, RT; b) TFA, nitronium tetrafluoroborate, RT.
Starting from the common intermediate 6 (from Scheme 1), an EDC coupling reaction was
carried out with commercially available protected Glu 9 leading to the pre-nitration protected
intermediate 10 in 87% yield. The deprotection and nitration steps were carried out in TFA,
followed by the addition of nitronium tetrafluoroborate, similar to the procedure in Scheme 1.
The combined yield of the isolated and HPLC purified and lyophilized target α-CDNI-Glu (B), was
27%. The overall yield for the synthesis of caged Glu (B) from 4 was 18%.
The dinitroindolinyl caging system has only previously been used to cage carboxy groups apart
from one system on material assembly.30,31 We investigated the possibility of using the CDNI
system to protect the amine functionality of Glu, leading to a new form of a caged Glu, which
we hereby refer to as: N-CDNI-Glu (C). Clearly, the assembling route differed from the
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protocols used to cage the α- and ω-carboxy functionalities. We were able to access the newly
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6
caged design with only one additional step (see Scheme 3). The overall reaction time was 114 h.
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9
Total Reaction Time = 114 h
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OtBu
O
O
OtBu
O
OtBu
O
OtBu
O
H2N
O
12
a
b
18 h
90%
4 h
N
H
N
71%
11
6
O
Cl
O
OH
O
O
OtBu
O
c
O2N
72 h
77%
OH
O
OtBu
O
N
N
OH
OtBu
O2N
O
N
O
N
H
H
O
O
(C)
N-CDNI-Glu
13
Scheme 3. Synthesis of N-CDNI-Glu (C); a) triphosgene, pyridine, CH2Cl2, 0°C to RT; b) di-tert-
butyl glutamate 12, Et3N, CH2Cl2, 0°C to RT c) NaNO3, TFA, RT.
Also starting with common intermediate 6, triphosgene was added to form chlorocarbonyl
indolinyl 11. Then, it was coupled with di-tert-butyl ester protected Glu 12, under basic
conditions of triethylamine, to form pre-nitrated intermediate 13, which is easily purified.
Finally, the aromatic ring was nitrated with NaNO3 in TFA to afford N-CDNI-Glu (C) in 77% yield,
and overall yield of 35% from 4.
The latter nitration of 13 using nitronium tetrafluoroborate consistently led to the cleavage of
the Glu moiety, irrespective of our numerous attempts to use lower temperatures and milder
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conditions. It is likely that the more electron rich carbonyl of the urea unit of 13 interacts with
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+
the electrophilic NO2 leading to an activated labile carbonyl system, which may subsequently
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undergo an intramolecular decomposition via the α-carboxy group. This was not observed in
the less electron-enhanced carbonyl of the amide systems of 8 and 10, shown in Schemes 1 and
2, respectively. Therefore, we resorted to the alternative milder nitration pathway of NaNO3 in
TFA and achieved a yield of 77% after 3 days of reaction time.
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It is important to note that the easily accessible common intermediate 6 can be subjected to
only 2 additional steps to make α-CDNI-Glu (B), or just 3 additional steps to make N-CDNI-Glu
(C).
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Photocleavage Studies
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We also studied the photo properties of the three caged Glu designs and compared their
efficiency of uncaging under identical conditions. Although the CDNI cage has been extremely
valued for its two-photon uncaging properties, we focused our studies on a Rayonet
photochemical reactor (RPR 100) as the light source to compare the three designs. Each
molecule was irradiated with longer wavelength UV light at 350 nm. Our instrument has a calcd
light flux (light intensity) of 24 Wm-2.
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(B) α-CDNI-Glu
(A) CDNI-Glu
(C) N-CDNI-Glu
Figure 2. UV spectra for progressive photolysis of (A) CDNI-Glu (4.70×10-5 M), (B) α-CDNI-Glu
(7.94×10-5 M) and (C) N-CDNI-Glu (2.00×10-5 M) at a wavelength of 350 nm in DI water at pH 7.
Irradiation took place in the photo reactor at room temperature at specified intervals of 10s, for
CDNI-Glu (A) and α-CDNI-Glu (B), and for 30s for N-CDNI-Glu (C). It necessary to irradiate the
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latter for longer time, due to it lower quantum yield relative for the former two. The
absorbance changes, resulting from UV irradiation experiments on both CDNI-Glu and α-CDNI-
Glu, reflect a peak decrease at 272 nm and an increase at 350 nm (Figure 2 a and b).
Conversely, the N-CDNI-Glu (C) photolysis was relatively slow at UV light of 350 nm and the
absorption peaks increased around 400 nm (Figure 2c).
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Relative photolysis rates of each molecule were also measured using 1H NMR. A known amount
of sample molecule and maleic acid (internal standard reference) were dissolved in a specific
amount of deuterium oxide (0.5 mL), as NMR solvent, and transferred into a transparent NMR
tube. The NMR tube was then placed in the photochemical reactor (24 Wm-2) and irradiated at
1
350 nm for 30s time intervals. After each irradiation, the H NMR was collected using a Bruker
400 MHz NMR. The depletion of a selected peak (the aromatic Ha) was measured relative to
the internal standard peak, maleic acid. Based on peak integrals, the concentration of the
molecule was calcd (Figure 3).
The 7-nitro group, which is ortho to the labile amide, is key in the photolysis that leads to Glu
release. It effectively delivers an O to the amide leading to Glu release, and is subsequently
converted to a nitroso group,14 leading to an upfield shift in the aromatic proton Ha as it
becomes Hb. This process also leads to the aromatization of the indolinyl ring to indole, giving
rise to Hc and Hd with 1H NMR peaks at the 7.2 and 6.8 ppm, respectively (Figure 3).
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Figure 3. H NMR (400 MHz, D2O) spectra for progressive photolysis of CDNI-Glu (A) at a
concentration of 1.47 mM. The progressive photolysis spectra for α-CDNI-Glu (B) and N-CDNI-
Glu (C) are shown in SI.
The concentrations of these molecules were calcd based on the peak integrals of specific
protons (see SI for the calculations). The photolysis reactions for these molecules obey the first
order rate law (Figure 4). Of the three molecules, CDNI-Glu showed the highest rate of
photolysis, while N-CDNI-Glu showed the lowest. The time taken to completely photolyze
CDNI-Glu or α-CDNI-Glu was 300s, whereas it took N-CDNI-Glu more than 4000s to completely
release Glu. It is important to note that during neurological studies, it is not necessary for these
chemical tools to be completely photolyzed, as long as photolysis leads to the release of Glu
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and the caged molecule does not interfere in other competing signaling pathways. Therefore,
from the neuroscientists’ perspective, α-CDNI-Glu (B) and N-CDNI-Glu (C) may resolve the GABA
inhibition issue at relatively higher concentrations than CDNI-Glu (A).
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(B)
(A)
(C)
Figure 4. Rate of photolysis for CDNI-Glu (A), α-CDNI-Glu (B) and N-CDNI-Glu (C).
The rate constant for the reactions (k), maximum absorbance wavelength (λ max), quantum
yields (Φ) and photo cross sections (ε.Φ) were calcd using the data retrieved from Figures 2 and
4. Based on our results, the CDNI-Glu shows the most efficient photolytic properties, with a
higher quantum yield (0.5) and a photo cross section (3200 M-1cm-1); Table 1. The photolysis
reactions mentioned above were conducted in aqueous media.
Table 1. Photochemical properties of CDNI based glutamate cages.
Caged Glu
CDNI-Glu
k / s-1
λ max / nm ε (λ max) /M-1cm-1
Φ
ε.Φ / M-1cm-1
3200
8.9×10-3
2.2×10-3
330
330
6400
6400
0.5
α-CDNI-Glu
0.13
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N-CDNI-Glu
6.2×10-5
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0.003
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Two-photon uncaging CDNI-Glu across the synapse in hippocampal brain slices
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We have confirmed the responsiveness of CDNI-Glu for two-photon uncaging in acute mouse
hippocampal slices (Figure 5). Simultaneous two-photon uncaging of CDNI-Glu (2mM) on
multiple dendritic spines of CA1 pyramidal cells evoked large amplitude membrane potential
depolarizations recorded at the cell body in whole-cell current-clamp configuration. The CDNI-
Glu uncaging evoked responses were completely blocked by antagonists of ionotropic
glutamate receptors. These experiments indicate that CDNI-Glu can be used to effectively
activate glutamate receptors on postsynaptic spines in vitro.
Figure 5. Two-photon uncaging of CDNI-Glu (A) in acute hippocampal brain slices. a) Left:
schematics of the experimental configuration; CA1 pyramidal neurons were recorded in in
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whole-cell patch clamp mode in acute mouse hippocampal slices and individual spines on radial
obliques dendritic region (red box) were selected for two-photon uncaging. The resulting
responses are recorded at the soma. Right: CDNI-Glu (2mM applied locally with a pipette)
uncaging evoked excitatory postsynaptic potentials (gluEPSPs) recorded from a CA1 pyramidal
cells by uncaging on 7 (grey trace) and 30 (black trace) spines on a dendritic segment using with
0.2 ms uncaging laser duration (D) and 1 ms interstimulus interval (ISI) at 720 nm. b) Left: CDNI-
Glu uncaging-evoked excitatory currents (gluEPSCs) are mediated by ionotropic glutamate
receptors. Left: CDNI-Glu uncaging evoked excitatory postsynaptic current traces (gluEPSC) at a
single spine in control (black) and in the presence of ionotropic glutamate receptor antagonists
(grey, NBQX: 2 ꢀM, DL-AP5: 50 ꢀM). Right: summary of the CDNI-Glu evoked single-spine
gluEPSC amplitude in control and in the presence of ionotropic glutamate receptor antagonists
(mean ± s.e.m, n=5 recordings).
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CONCLUSION
We have improved the synthesis of CDNI-Glu, an important light triggered molecular tool that is
critical for neurological studies. Relative to published methods, our synthesis uses half the
number of steps, one fifth of the reaction time, and affords nearly double the yields. We have
also introduced two novel caged Glu, from the common intermediate 6 (used in CDNI-Glu
synthesis) with 2 or 3 additional steps to generate α-CDNI-Glu and N-CDNI-Glu, respectively.
We reported the photophysical properties of all three caged-Glu variants and compared them
using both UV-vis and NMR. We have also shown the electrophysiological response of CDNI-
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Glu in a brain slice. The quantum efficiency of CDNI-Glu is best amongst the three reported.
The two newly described caged Glu may have the potential to minimize interference with
GABAA receptor, and we wish to present these types of caged Glu here to enable further
investigation by neuroscientists. We have successfully employed our synthetic protocol to
generate other photo cleavable agonists that will be published in due course.
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METHODS
The starting material for all three caged Glu molecules (Scheme 1, 2 and 3) presented was the
commercially available 4-hydroxyindole. Unless otherwise mentioned, chemicals were
purchased from Fisher or Aldrich in highly purified form and used without further purification.
The solvents used: dichloromethane (CH2Cl2), hexanes, and ethyl acetate (EtOAc) were
purchased from VWR. The anhydrous solvent CH2Cl2 was obtained from a solvent purification
system (MBRAUN, MB-SPS), which uses an activated alumina column. The reactions were
conducted under N2 or Ar using standard Schlenk line techniques. The reaction vessels were
flame dried before conducting reactions and charged with N2/Ar (The flame dry vacuum/N2
charge was repeated three times). Reaction progress was monitored using TLC (EMD Millipore
TLC Silica Gel 60 F254) and observed under short and long wavelength ultraviolet light or stains
(p-anisaldehyde). The products of each step in the schemes were purified using column
chromatography with 60 Å silica gel (Fisher scientific). All final products of caged Glu A, B, and
C, which are quite polar, were purified using reverse phase HPLC: Waters 2695 separation
module with Waters 996 photodiode array detector. The separations were conducted using an
XSELECT C18 OBD prep column (10 mm × 25 mm). HPLC grade solvents acetonitrile (ACN),
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methanol (MeOH) and Millipore water were used as the mobile phases. After purification,
compounds were freeze-dried (lyophilized) via LABCONCO Freezone 4.5. The nitration steps
and their corresponding purification were conducted in the dark, under red light. The final
products were stored in the dark (covered in Al foil) at -20 °C until further needed.
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Characterization of each product was performed via H NMR and C NMR, using a Bruker 400
MHz NMR spectrometer. NMR solvents (CDCl3, D2O) were purchased from Cambridge Isotope
Laboratories and Acros. Chemical shifts were calcd relative to the solvent residual peaks (CDCl3
= 7.26 ppm, D2O = 4.79 ppm). The UV-vis spectra were obtained with an Agilent 8453 diode
array UV-vis. High-resolution mass spectra (HRMS) were measured using a JEOL DART-AccuTOF
mass spectrometer, while low-resolution mass analyses were conducted on an Agilent LC-MS
with ES+ and 6120 quadrupole. The caged molecules were irradiated during photochemical
studies in a Rayonet photochemical reactor (RPR 100) with an intensity of 24 Wm-2.
tert-Butyl 2-((1H-indol-4-yl)oxy)acetate (5). A 50 mL round-bottomed flask was charged with 4-
hydroxyindole (0.659 g, 4.96 mmol), K2CO3 (3.424 g, 24.78 mmol), freshly distilled acetone (10
mL), and freshly activated molecular sieves. The reaction was allowed to stir for 15 min at 25 °C
under an inert atmosphere (N2). After 15 min tert-butyl bromoacetate (1.29 mL, 8.42 mmol)
was added resulting in a purple colored solution. The reaction mixture was left to stir overnight.
After 18 h, the reaction mixture was filtered through a Büchner funnel. The solvent was
removed under vacuum resulting in the formation of purple oil. The crude product was purified
by column chromatography (3:1 hexane/ethyl acetate) to yield white crystals at 25 °C (0.912 g,
1
3.67 mmol, 80% yield). H NMR: (400 MHz, CDCl3) δ 8.16 (s, 1H), 7.13 (t, J = 4.0 Hz, 1H), 7.10-
7.04 (m, 2H), 6.73 (t, J = 4.0 Hz, 1H), 6.41 (d, J = 4.0 Hz, 1H), 4.67 (s, 2H), 1.50 (s, 9H). 13C NMR:
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(101 MHz, CDCl3) δ 168.36, 151.77, 137.46, 122.79, 122.51, 118.86, 105.18, 100.84, 100.20,
82.09, 66.09, 28.09. HRMS (m/z): calcd for C14H17NO3 [M+H]+ 248.1242, found 248.1191.
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tert-Butyl 2-(indolin-4-yloxy)acetate (6). NaBH3CN (0.525 g, 8.36 mmol) was stirred with
compound 5 (0.762 g, 3.083 mmol) in a 25 mL round-bottomed flask charged with freshly
activated molecular sieves. Then, 7 mL of acetic acid was added, and the reaction mixture
allowed to stir for 2 h at 25 °C. After 2 h, vacuum was used to remove the acetic acid. Ethyl
acetate was added to dilute the mixture, and the mixture was neutralized using NaHCO3
solution. The organic layer was separated and dried over anhydrous Na2SO4. The solvent was
removed under vacuum, and the crude product was purified through column chromatography
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(3:2 hexane/ethyl acetate) to afford product 6 (0.746 g, 2.991 mmol, 97% yield). H NMR: (400
MHz, CDCl3) δ 6.94 (t, J = 8.0 Hz, 1H), 6.32 (d, J = 7.7 Hz, 1H), 6.13 (d, J = 8.0 Hz, 1H), 4.51 (s, 2H),
3.56 (t, J = 8.0 Hz, 2H), 3.06 (t, J = 8.0 Hz, 2H), 1.48 (s, 9H). 13C NMR: (101 MHz, CDCl3) δ 168.34,
154.89, 153.67, 128.46, 116.45, 103.73, 102.37, 82.08, 65.77, 47.49, 28.07, 26.91. HRMS (m/z):
calcd for C15H18N2O4 [M+H]+ 250.1398, found 250.1541.
tert-Butyl (S)-5-(4-(2-(tert-butoxy)-2-oxoethoxy)indolin-1-yl)-2-((tert-butoxycarbonyl) amino-5-
oxopentanoate (8). A 25 mL flame dried round-bottomed flask was charged with compound 6
(0.272 g, 1.092 mmol), EDC (0.290 g, 1.9 mmol), DMAP (0.230 g, 1.9 mmol), and 5 mL of dry
CH2Cl2. The reaction mixture was allowed to stir for 15 min until complete solubility for all
reaction mixture components was achieved. Then Boc-Glu-Ot-Bu (7) was dissolved in 5 mL of
dry CH2Cl2 and added dropwise to the reaction mixture. The reaction was allowed to stir
overnight (18 h) at 23 °C under Ar. After 18 h, the organic layer was extracted with CH2Cl2,
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neutralized with NaHCO3, and dried over anhydrous Na2SO4. The crude product was purified by
5
6
column chromatography (3:2 hexane/ethyl acetate) to afford compound 8 as brown oil, (0.51 g,
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0.95 mmol, 87% yield). H NMR: (400 MHz, CDCl3) δ 7.86 (d, J = 8.0 Hz, 1H), 7.12 (t, J = 8.0 Hz,
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1H), 6.42 (d, J = 8.0 Hz, 1H), 5.22 (d, J = 8.0 Hz, 1H), 4.53 (s, 1H), 4.24–4.21 (m, 1H), 4.04 (t, J =
8.0 Hz, 2H), 3.18 (t, J= 8.0 Hz, 2H), 2.58–2.42 (m, 2H), 2.30-2.25(m, 1H), 2.04-1.97 (m, 1H), 1.50-
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1.46 (m, 27H). C NMR: (101 MHz, CDCl3) δ 173.54, 171.54, 170.38, 170.27, 167.96, 154.11,
144.54, 128.89, 119.01, 110.78, 106.79, 83.32, 82.27, 65.72, 53.77, 48.40, 32.22, 31.15, 28.05,
25.09. HRMS (m/z): calcd for C28H43N2O8 [M+H]+ 535.3014, found 535.2967.
(S)-2-amino-5-(4-(carboxymethoxy)-5,7-dinitroindolin-1-yl)-5-oxopentanoic acid (1), Caged Glu
(A) . Step 1: A two dram amber vial was charged with compound 8 (0.1104 g, 0.2 mmol) and
trifluoroacetic acid, TFA, (1 mL). The reaction mixture was allowed to stir for 30 min at 23 ͦC.
After 30 min, a TLC was taken (2:1 hexane/ethyl acetate) to verify a single spot for the
formation of the deprotected intermediate, then a nitrogen stream was applied to remove the
1
TFA to give deprotected compound (0.064 g, 0.2 mmol, quantitative yield). H NMR: (400 MHz,
D2O) δ 7.31 (d, J = 8.0 Hz, 1H), 6.85 (t, J = 8.0 Hz, 1H), 6.25 (d, J = 8.0 Hz, 1H), 4.33 (s, 2H), 3.92 (t,
J = 8.0 Hz, 1H), 3.52 (t, J = 8.0 Hz, 2H), 2.61 (t, J = 8.0 Hz, 2H), 2.32-2.29 (m, 2H), 2.01-1.97 (m,
2H). 13C NMR: (101 MHz, D2O) δ 172.68, 171.28, 170.98, 153.48, 143.24, 128.66, 117.35, 110.29,
107.82, 64.68, 55.76, 48.07, 30.98, 29.47, 24.15. HRMS (m/z): calcd for methylated C17H24N2O6
[M+H]+ 351.1551, found 351.1586.
Step 2: Following the evaporation of TFA, 2 mL of fresh TFA was added. The reaction mixture
was cooled to 0 °C, and nitronium tetrafluoroborate (0.1056 g, 0.8 mmol) was added and
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stirred for 1.5 h, in the dark, while allowing it to reach room temperature. After TFA removal
with a nitrogen stream, the crude product was dissolved in 1.5 mL of H2O: acetonitrile (75%:
25%) and filtered using a 0.45 micron filter. The product was purified using reverse phase HPLC
chromatography with an isocratic mixture of 60% water, 40 % acetonitrile and 0.1% TFA with an
elution rate of 2 mL min-1. Product 1, caged Glu (A), was eluted at 6 min, collected, and
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lyophilized to dryness. (0.025 g, 0.06 mmol, 30% yield). H NMR: (400 MHz, D2O) δ 8.57 (s, 1H),
4.86 (s, 2H), 4.77 – 4.71 (m, 1H), 4.59 (td, J = 9.8, 2.8 Hz, 1H), 4.42 (q, J = 9.7 Hz, 1H), 3.49 (td, J =
18.4, 9.3 Hz, 1H), 3.38 (ddd, J = 12.4, 9.2, 2.7 Hz, 1H), 2.70 (t, J = 7.2 Hz, 2H), 2.41 – 2.22 (m, 2H).
13C NMR: (101 MHz, D2O) δ 175.97, 172.31, 168.79, 156.07, 155.74, 151.20, 138.73, 132.33,
122.95, 69.74, 51.93, 51.09, 28.39, 26.79, 24.96. Compound 1 is not stable under DART
ionization conditions. Dinitro containing fragments, and methylated intermediates (due to
CH3OH solvent used) can be observed with DART MS (fragments a and b shown in S3, SI).
HRMS (ESI) showed characteristic fragmentation peaks: m/z calcd for methylated aqueous
•+
•+
byproduct (C11H9N3O6 ): 279.0491, found 279.0482; for free glutamate (C5H9NO4 ): 147.0531,
+
found 147.0570; and for the organic byproducts: for a (C11H12N3O7 ): 298.0675, found 298.0617;
•+
+
for b (C11H12N2O5 ): 252.0746, found 252.0764, for (C8H8N3O5 ): 226.0464, found 226.0508, and
+
for (C8H7N2O2 ): 163.0507, found 163.0487. However, the intact caged Glu (A) was observed
with ES+ MS (m/z) calcd for C15H17N4O10 [M+H]+ 413.09, found 413.1.
tert-Butyl(S)-5-(4-(2-(tert-butoxy)-2-oxoethoxy)indolin-1-yl)-4-((tert-butoxycarbonyl)amino)-5-
oxopentanoate (10). A 25 mL flame dried round-bottomed flask was charged with tert-butyl 2-
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(indolin-4-yloxy)acetate (0.36 g, 1.44 mmol), EDC (0.47 g, 2.45 mmol), DMAP (0.30 g, 2.45
mmol), and 7.5 mL of dry CH2Cl2. The reaction mixture was allowed to stir for 15 min until
complete solubility for all reaction mixture components was achieved. Then Boc-D-Glu(Ot-Bu)-
OH (9) (0.43 g, 1.44 mmol) was dissolved in 5 mL of dry CH2Cl2 and added dropwise to the
reaction mixture. The reaction was allowed to stir overnight (18 h) at 23 °C under Ar. The
organic layer was subsequently extracted with CH2Cl2, neutralized with NaHCO3, and dried over
anhydrous Na2SO4. The crude product was purified by column chromatography (3:2
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hexane/ethyl acetate) to get compound 10 as brown oil (0.50 g, 0.95 mmol, 87% yield). H
NMR: (400 MHz, CDCl3) δ 7.78 (d, J = 8.0 Hz, 1H), 7.07 (t, J = 8.0 Hz, 1H), 6.40 (d, J = 8.0 Hz, 1H),
5.40 (d, J = 8.0 Hz, 1H), 4.57 – 4.53 (m. 1H), 4.48 (s, 1H), 4.27-4.19 (m, 2H), 3.15 (t, J = 8.0 Hz,
2H), 2.35 – 2.28 (m, 2H), 2.13 – 2.05 (m, 1H), 1.85 – 1.76 (m, 1H), 1.45 – 1.43 (m, 27H). 13C NMR:
(101 MHz, CDCl3) δ 170.81, 169.18, 166.68, 165.33, 153.25, 143.08, 127.99, 118.82, 109.88,
105.99, 83.82, 81.35, 79.31, 64.84, 50.79, 47.13, 30.42, 29.41, 26.98, 23.72, 21.25. HRMS (m/z):
calcd for C28H43N2O8 [M+H]+ 535.3014, found 535.2965.
(S)-4-Amino-5-(4-(carboxymethoxy)-5,7-dinitroindolin-1-yl)-5-oxopentanoic acid (2), Caged Glu
(B). A two dram amber vial was charged with compound 10 (0.115 g, 0.22 mmol) and TFA (1
mL). The reaction mixture was allowed to stir for 30 min at 23 °C. After 30 min, a nitrogen
stream was applied to remove the TFA. Then 2 mL of fresh TFA was added and the reaction
was cooled to 0 °C. Then nitronium tetrafluoroborate (0.12 g, 0.9 mmol) was added and the
reaction mixture was stirred for 1.5 h, in the dark, while allowing it to reach room temperature.
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The TFA was removed using a nitrogen stream, and the residue was dissolved in 1.5 mL of
H2O:acetonitrile (75%: 25%) and filtered using a 0.45 micron filter. The crude product was
purified by reverse phase HPLC chromatography using 60% water, 40 % acetonitrile, and 0.1%
TFA with an isocratic elution at 2 mL min-1. Caged Glu (B), compound 2, was eluted at 6 min,
collected, and then lyophilized to dryness (0.027 g, 0.066 mmol, 27% yield). 1H NMR: (400 MHz,
D2O) δ 8.55 (s, 1H), 4.83 (s, 2H), 4.59-4.56 (m, 1H), 4.40 (q, J = 10.0 Hz, 1H), 3.49-3.35 (m, 1H),
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3.33-3.25 (m, 1H). C NMR: (101 MHz, D2O) δ 175.98, 172.54, 168.77, 162.79, 151.26, 139.47,
138.69, 134.91, 132.19, 122.91, 69.86, 51.91, 51.07, 28.39, 26.80, 24.96. LCMS (m/z) calcd for
C15H17N4O10 [M+H]+ 413.09, found 413.0.
tert-Butyl 2-((1-(chlorocarbonyl)indolin-4-yl)oxy)acetate (11). To a flame dried round-bottom
flask triphosgene (0.46 equiv, 0.08 g, 0.27 mmol) was added. Then 3 mL of CH2Cl2 was added,
and the reaction mixture was cooled to 0° C. Afterwards, pyridine (1 equiv) 0.09 mL was added
and the reaction mixture was allowed to stir for 5 min. Then compound 6 (0.13 g, 0.52 mmol)
was dissolved in 1 mL of CH2Cl2 and added dropwise to the reaction mixture. The reaction
mixture was stirred for 3 h while allowing warm up to room temperature. After 3 h it was
quenched with HCl (1 M) and extracted using CH2Cl2. The organic layer was washed with
saturated NaHCO3 and dried with anhydrous Na2SO4. The product was then purified using
1
column chromatography (3:2 hexane: ethyl acetate) (0.115 g, 0.37 mmol, 71% yield). H NMR:
(400 MHz, CDCl3) δ 7.48(d, J = 8.0 Hz, 1H), 7.14(t, J = 8.0 Hz, 1H), 6.48(d, J = 8.0 Hz, 1H), 4.53(s,
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167.69, 154.37, 144.89, 142.65, 129.08, 128.78, 120.23, 109.74, 108.05, 82.46, 65.71, 52.11,
27.67, 24.20. HRMS (m/z): calcd for C15H19ClNO4 [M+H] + is 312.0997, found 312.0994.
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di-tert-Butyl (4-(2-(tert-butoxy)-2-oxoethoxy)indoline-1-carbonyl)glutamate (13). Compound 12
and triethylamine (0.1 mL) were transferred to a flame dried round-bottomed flask and 5 mL of
CH2Cl2 was added. The reaction mixture was stirred at 0 °C. Then compound 11 (0.10 g, 0.32
mmol) was dissolved in 1 mL of CH2Cl2 and added dropwise. The reaction mixture was allowed
to stir and reach room temperature overnight. Then the reaction mixture was washed with
saturated NH4Cl solution and brine. Afterwards, it was dried with anhydrous Na2SO4. Finally,
the product was purified using column chromatography (3:2 hexane: ethyl acetate) (0.15 g, 0.29
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mmol, 90% yield). H NMR: (400 MHz, CDCl3) δ 7.52(d, J = 8.0 Hz, 1H), 7.06(t, J = 8.0 Hz, 1H),
6.30(d, J = 8.0 Hz, 1H), 5.45(d, J = 8.0 Hz, 1H), 4.50(s, 2H), 4.46(t, J = 8.0 Hz, 1H), 3.97(t, J = 8.0
Hz, 2H), 3.15(t, J = 8.0 Hz, 2H), 1.45-1.42 (m, 27H). 13C NMR: (101 MHz, CDCl3) δ 172.63, 172.09,
168.00, 154.38, 145.13, 128.80, 118.07, 108.68, 105.22, 82.15, 82.09, 80.60, 65.69, 53.30, 47.41,
31.67, 27.99, 24.84. HRMS (m/z): calcd for C28H23N2O8 [M+H] + is 535.3014, found 535.2236.
(4-(Carboxymethoxy)-5,7-dinitroindoline-1-carbonyl)glutamic acid (3), Caged Glu (C). A 2 dram
vial was charged with compound 13 (0.04 g, 0.064 mmol) and TFA and stirred for 30 min at
room temperature. After completion, a nitrogen stream was applied to remove the TFA. Then
1 mL of fresh TFA and sodium nitrate (0.04 g, 0.44 mmol) were added and the reaction mixture
was stirred for 3 days, in the dark. The TFA was removed using a nitrogen stream. Then, the
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compound was dissolved in 1.5 mL of H2O:acetonitrile (75%: 25%) and filtered using a 0.45
micron filter. The product was purified by reverse phase HPLC chromatography using 60%
water, 40 % acetonitrile and 0.1% TFA with an isocratic elution at 2 mL min-1. Compound 3 was
eluted at 6 min, collected, and then lyophilized to dryness (0.0225 g, 0.049 mmol, 77% yield). 1H
NMR: (400 MHz, D2O) δ 8.45(s, 1H), 4.75(s, 1H), 4.31-4.29(m, 3H), 3.35(t, J = 8.0 Hz, 2H), 2.53(t,
J = 8.0 Hz, 2H), 2.26-2.22(m, 1H), 2.04-2.01(m, 1H). 13C NMR: (101 MHz, D2O) δ 174.52, 172.44,
170.07, 153.60, 148.39, 140.12, 133.41, 129.85, 128.04, 120.85, 67.06, 50.52, 49.21, 27.55,
23.43, 22.63. LCMS (m/z) calcd for C16H16N4NaO12 [M+Na]+ 479.1, found 479.1.
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Two-photon CDNI-Glu uncaging in acute hippocampal slices. Whole-cell patch clamp
recordings were performed from somata of hippocampal CA1 pyramidal neurons in acute
hippocampal slices prepared from C57BL/6 mice as described previously.32 Briefly, slices were
perfused with artificial cerebrospinal fluid at 32–33 °C and containing: NaCl 125 mM, NaHCO3
25 mM, KCl 2.5 mM, NaH2PO4 1.25 mM, MgCl2 1.0 mM, CaCl2 2.0 mM, glucose 22.5 mM, Na-
pyruvate 3.0 mM, ascorbate 1.0 mM, and saturated with 95% O2 and 5% CO2. Slices were
visualized with a Zeiss Examiner.Z1 with a 63x objective (NA 1.0) and a two-photon scanning
microscope (Bruker). Somatic whole-cell patch clamp recordings were performed from visually-
identified CA1 pyramidal neurons with a Dagan BVC-700A amplifier (filtered at 1–10 kHz and
digitized at 50 kHz). Recording pipettes were pulled from borosilicate glass to tip resistances of
~4–8 MΩ and contained: K-gluconate 130 mM, KCl 8.0 mM, NaCl 4.0 mM, HEPES 10 mM,
Mg2ATP 4.0 mM, Tris2GTP 0.3 mM, phosphocreatine 14 mM, Alexa 594 0.05 mM, pH 7.25. Two-
photon Glu uncaging was performed as described previously.33 Briefly, a dual galvanometer
based scanning system was used to photo-release glutamate at multiple dendritic spines. Ultra-
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fast, pulsed laser light (Chameleon Ultra II; Coherent) at 920–930 nm was used to excite Alexa
594, while 720 nm was used to photolyze CDNI-Glu (2 mM applied via broken pipette above
slice). The uncaging laser power was kept below 8 mW at slice surface.
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ASSOCIATED CONTENT
The supporting Information is available free of charge on the ACS Publication website at DOI:
10.1021/acschemneuro.7b0xxxx
NMR spectra of caged Glu A, B, and C, and all intermediates leading up to these target
structures; Kinetic data; LC MS data, and Direct Analysis in Real Time (DART)-MS data.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nesnas@fit.edu
ORCID
Nasri Nesnas: 0000-0003-3511-940X
Attila Losonczy: 0000-0002-7064-0252
Charitha Guruge: 0000-0003-4405-2905
Author Contributions
C.G. performed the synthesis of all compounds reported; Y.P.O. and N.N. developed the
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improved synthesis of CDNI-Glu, and R.L.C. and Y.P.O. developed the NMR technique as an
unambiguous means to determine quantum efficiencies. C.G. and N.N. conceived the
development of α-CDNI-Glu, and C.G. originated all work on N-CDNI-Glu independently. J.M.
and C.G. performed the photophysical and NMR studies. A.L. is our neuroscience collaborator
and conducted brain slice electrophysiological studies reported. All authors contributed to and
approved the final manuscript.
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Funding
The study was support by NIH Award R15-GM112119-01A1.
Notes
The authors declare no competing financial interest.
ABBREVIATIONS
CDNI-Glu, 4-carboxymethoxy-5,7-dinitroindolinyl glutamate; MNI-Glu, 4-methoxy-7-
nitroindolinylglutamate; GABA, gamma aminobutyric acid; RT, room temperature; h, hour(s);
min, minute(s); s, seconds; DMAP, 4-dimethylaminopyridine; Boc, tert-butyloxycarbonyl; Boc-
Glu-OtBu, Boc-L-glutamic acid 1-tert-butyl ester; NMR, nuclear magnetic resonance; TFA,
trifluoroacetic acid; EDC, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide; AcOH, acetic acid;
Boc-D-Glu(OtBu)-OH, Boc-D-glutamic acid 5-tert-butyl ester; UV, ultraviolet; vis, visible; ppm,
parts per million; s, seconds; TLC, thin layer chromatography; HPLC, high performance
(pressure) liquid chromatography; DART, Direct Analysis in Real Time; LCMS, liquid
chromatography–mass spectrometry; HRMS, High Resolution Mass Spectrometry; ESI, electron
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spray ionization; HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid); ATP, adenosine
triphosphate; GTP, guanosine-5'-triphosphate.
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ACKNOWLEDGMENTS
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The authors wish to thank NIH for their financial support for N.N., C.G., and supplies (R15-
GM112119-01A1). N.N. thanks Boris V. Zemelman (UT Austin) for helpful discussions and
initiating collaboration with A.L.
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