Supramolecular photochemistry of encapsulated caged: Ortho -nitrobenzyl triggers

Article abstract of DOI:10.1039/c9pp00260j

ortho-Nitrobenzyl (oNB) triggers have been extensively used to release various molecules of interest. However, the toxicity and reactivity of the spent chromophore, o-nitrosobenzaldehyde, remains an unaddressed difficulty. In this study we have applied the well-established supramolecular photochemical concepts to retain the spent trigger o-nitrosobenzaldehyde within the organic capsule after release of water-soluble acids and alcohols. The sequestering power of organic capsules for spent chromophores during photorelease from ortho-nitrobenzyl esters, ethers and alcohols is demonstrated with several examples.

Full text of DOI:10.1039/c9pp00260j

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Supramolecular photochemistry of encapsulated
caged ortho-nitrobenzyl triggers†
Cite this: DOI: 10.1039/c9pp00260j
a
a
b
c
Nareshbabu Kamatham, A. Mohan Raj,
and V. Ramamurthy*
Richard S. Givens,
José P. Da Silva*
a
ortho-Nitrobenzyl (oNB) triggers have been extensively used to release various molecules of interest.
However, the toxicity and reactivity of the spent chromophore, o-nitrosobenzaldehyde, remains an un-
addressed difficulty. In this study we have applied the well-established supramolecular photochemical con-
cepts to retain the spent trigger o-nitrosobenzaldehyde within the organic capsule after release of water-
soluble acids and alcohols. The sequestering power of organic capsules for spent chromophores during
photorelease from ortho-nitrobenzyl esters, ethers and alcohols is demonstrated with several examples.
Received 7th June 2019,
Accepted 16th July 2019
DOI: 10.1039/c9pp00260j
rsc.li/pps
esters and 7-diethylaminocoumaryl-4-methyl esters within OA
Introduction
11–14
capsule.
The advantages and disadvantages of each of these
The protection of molecules of interest and their photorelease triggers prompted us to explore the most well-known and fre-
at chosen locations and times have been an active area of quently applied classical triggering system, ortho-nitrobenzyl
1
–8
15–20
research for several decades.
A commonly adopted strategy (oNB), within OA.
In this study we establish that ortho-nitro-
is the use of ‘phototriggers’ (X-PPG) where a molecule of inter- benzyl systems could be included with the water soluble OA and
est (X) is protected with a photo detachable group PPG and they, depending on their size form either 2 : 1 or 2 : 2 host–guest
4
released at will with the help of a photon. Such techniques capsular complexes. In both cases the guest is encapsulated
are employed to deliver pharmaceuticals, catalysts, reagents, within the capsule formed by two molecules of OA. As foreseen,
pheromones, fragrances, metal ions, signaling agents for inter- irradiation resulted in the release of acid to aqueous medium
1
cell communication etc. Although drug delivery and cell sig- and retainment of the ortho-nitroso compound within the
naling are most effective in aqueous media, most protected capsule. Results presented highlight the value of OA in packa-
molecules are poorly soluble in aqueous media. Water-soluble ging the ortho-nitrobenzyl system, solubilizing the normally in-
9
supramolecular containers help overcome this conundrum.
soluble X-PPG in water and releasing the protected acid upon
We have used octa acid (OA, Scheme 1), a cavitand that forms activation with light. Details are presented below.
capsular host–guest complexes with a wide variety of molecules
and forms a fully closed capsule around X-PPG. The 1 : 1
complex that the other known cavitands such as cyclodextrins,
Experimental
Materials
cucurbiturils and calixarenes form would expose a part of X-PPG
1
0
to the media. We have demonstrated the value of supramole-
cular concepts for the photorelease of organic acids of interest o-Nitrobenzyl (oNB) alcohol, butyric acid, 3,3-dimethylacrylic
into aqueous media by encapsulating and photolyzing mole- acid, hexanoic acid, octanoic acid and decanoic acid (Sigma-
cules protected by well-known PPGs such as p-methoxyphenacyl Aldrich/Alfa Aeser) were used as received. Compounds 1–3 were
2
1,22
esters, p-hydroxyphenacyl esters, 7-methoxy coumaryl-4-methyl synthesized according to reported procedures.
octa acid (OA), was synthesized following literature procedure.
oNB esters 4–8 were synthesized by following the literature
The host,
23
a
Department of Chemistry, University of Miami, Coral Gables, FL 33146, USA.
2
4
procedure as outlined in Scheme 2. In a 100 mL round bot-
tomed flask oNB alcohol (1.3 mmol), the carboxylic acid
(5.2 mmol), triphenylphosphene (5.2 mmol) and 30 mL THF
E-mail: murthy1@miami.edu
b
Department of Chemistry, University of Kansas, Lawrence, KS 66045, USA
c
CCMAR – Centre of Marine Sciences, University of Algarve, Campus de Gambelas,
8005-139 Faro, Portugal
2
were added and stirred under N atm to complete dissolution.
†
Electronic supplementary information (ESI) available: Experimental pro-
The reaction mixture was cooled to 0–5 °C. A separately pre-
pared solution of 1.02 mL of diisopropyl azodicarboxylate
(DIAD) in 10 mL of THF was added dropwise over a 30 min
period at 0–5 °C. The temperature was raised to room tempera-
1
cedures, H NMR, UV and ESI-MS spectra for all new compounds. Irradiation
1
procedures, H NMR titration spectra of host–guest complexes, progress of
1
photoreactions as monitored by
c9pp00260j
H NMR, LC-DAD-MS. See DOI: 10.1039/
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Scheme 1 Structures of water-soluble octa acid (OA) cavitand and oNB triggers (1–8).
5.40 (s, 2H), 7.61–7.68 (m, 2H), 7.78 (t, J = 7.5 Hz, 1H), 8.1 (d,
J = 7.5 Hz, 1H); ESI-HRMS: calculated for C H NO Na
1
3
17
4
+
[
M + Na] 274.1050, observed: 274.0896.
7
1
: H-NMR (500 MHz, DMSO) δ: 0.85 (t, J = 6.5 Hz, 3H),
1
5
.23–1.25 (m, 8H), 1.53–1.55 (m, 2H), 2.38 (t, J = 7.5 Hz, 2H),
.40 (s, 2H), 7.61–7.68 (m, 2H), 7.78 (t, J = 7.5 Hz, 1H), 8.1
(
4
d, J = 7.5 Hz, 1H); ESI-HRMS: calculated for C15H21NO Na
+
[M + Na] 302.1363, observed: 302.1352.
1
8
: H-NMR (500 MHz, DMSO) δ: 0.85 (t, J = 6.5 Hz, 3H),
1.23–1.27 (m, 2H), 1.51–1.55 (m, 2H), 2.38 (t, J = 7.5 Hz, 2H),
.41 (s, 2H), 7.61–7.68 (m, 2H), 7.78 (t, J = 7.5 Hz, 1H), 8.1
d, J = 7.5 Hz, 1H); ESI-HRMS: calculated for C17 Na
24
Scheme 2 Synthetic route for of oNB esters 4–8.
5
(
H25NO
4
+
ture and maintained to complete reaction (12 h). The reaction [M + Na] 330.1676, observed: 330.1675.
was followed by TLC. The solvent was then removed by volatil-
Instrumentation
ization yielding an oily residue. The residue was submitted to
column chromatography using a silica gel column and a
mixture of hexane/EtOAc (60/40) as the mobile phase. As a
colored impurity co-elutes with the product, a preparative TLC
was also performed to obtain the isolate pure product.
NMR studies were performed using a 500 MHz Bruker NMR.
High resolution full scan ESI-MS spectra were obtained using a
Bruker Daltonics microTOF QII mass spectrometer and
ESI-MS/MS spectra were obtained using a Bruker Daltonics
HCT ultra mass spectrometer. GC-MS studies were performed
using a Hewlett Packard 6890N apparatus equipped with a
1
Phototriggers 4–8 were characterized by H NMR and electro-
spray ionization mass spectrometry (ESI-MS and ESI-MS/MS)
(see Fig. S1–S10 in ESI†). The phototriggers were isolated as
5973 series mass selective detector (i.e. 70 eV) and a triple
light brown semi solids, with the following yields. 4: 32%; 5:
quadrupole Bruker SCION TQ 456GC. LC-DAD-MS studies
were performed using an Agilent Technologies 1200 Series LC,
equipped with a diode array detector (DAD), and coupled to a
Bruker Daltonics HCT ultra mass spectrometer (MS). UV
spectra of triggers, products and host–guest complexes were
obtained using Shimadzu UV-3150 spectrophotometer. UV
spectra of isolated products were obtained by LC-DAD.
3
7%; 6: 22%; 7: 31%; 8: 28%.
1
H NMR and mass spectra (S1–S10) are included as ESI.†
The data are summarized below:
1
4
: H-NMR (500 MHz, DMSO) δ: 0.88 (t, J = 7.5 Hz, 3H),
.54–1.59 (m, 2H), 2.37 (t, J = 7 Hz, 2H), 5.41 (s, 2H), 7.61–7.68
m, 2H), 7.78 (t, J = 7.5 Hz, 1H), 8.1 (d, J = 7.5 Hz, 1H);
1
(
+
4
ESI-HRMS: calculated for C11H13NO Na [M + Na] 246.0742
observed: 246.0752.
1
5
: H-NMR (500 MHz, DMSO) δ: 1.91 (s, 3H), 2.120 (s, 3H),
.42 (s, 2 H), 5.79 (s, 1H), 7.61–7.67 (m, 2H), 7.79 (t, J = 7.5 Hz,
H), 8.11 (d, J = 8 Hz, 1H); ESI-HRMS: calculated for
Methods
Characterization of materials
5
1
C
+
1
12
H
6
13NO
1
4
Na [M + Na] 258.0737, observed: 258.0749.
H NMR spectra of synthesized compounds were collected at
: H-NMR (500 MHz, DMSO) δ: 0.85 (t, J = 6.5 Hz, 3H), 25 °C. For high resolution full scan ESI-MS spectra the syn-
1
.23–1.28 (m, 4H), 1.51–1.57 (m, 2H), 2.37 (t, J = 7.5 Hz, 2H), thesized compounds were solubilized in a mixture methanol–
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chloroform (50 : 50) containing 0.1% formic acid. The solution Determination of trigger conversions yields of photoproducts
−
1
was continuously infused (200 μL h ) into the source, with
the help of a syringe pump (KD Scientific, model 601553,
USA). Typical experimental conditions were: capillary voltage,
The trigger conversions were determined by LC-DAD at
20 nm. The mobile phase comprises acetonitrile (A) and
water (B), both with 0.1% of formic acid, and ethyl acetate (C).
The gradient started with 52% of A, 38% of B and 10% of
C. The mobile phase composition was changed to 2% of A,
3
−
1
4
0
.5 kV; drying gas, 180 °C at 4 L min ; nebulizer gas pressure,
.3 bar; end plate offset −500 V. For ESI-MS/MS spectra the
ions were continuously generated by infusing the compounds
73% of B and 25% of C in 5 minutes and kept at this compo-
−
1
in acetonitrile (50 μM) at 4 μL min into the mass spectro-
meter source with the help of a syringe pump (KD Scientific,
model 781100, USA). Typical experimental conditions were:
capillary voltage, 3.5 kV; capillary exit voltage (CE), 75 V;
sition for an additional 7 minutes. Finally, the system was
allowed to return to the initial mobile phase composition
(
52% of A, 38% of B and 10% of C) in 1 min and then stabil-
ized for an additional 5 minutes before the next run. The flow
−1
skimmer voltage, 40 V; drying gas, 300 °C at 6 L min ; nebuli-
zer gas pressure, 20 psi.
−1
was 0.35 ml min . The column was a Grace C18 reversed
phase LC column (10.0 cm length, 2.1 mm internal diameter,
3
μm), stabilized at 25 °C.
The yields of acid released from 4 were determined by H
NMR. A known amount of internal standard, methyl viologen
the same equivalent of the guest), was added to the complex
@(OA) solution and irradiated to complete conversion. The
Preparation of host–guest complexes
1
Preparation of host–guest complexes for guest binding
studies probed by NMR. A D O stock solution (600 μL) of host
2
(
4
OA (1 mM) and sodium borate buffer (10 mM) taken in a NMR
tube was titrated with the guest by sequential addition of 0.25
equiv. of guest (2.5 μL of a 60 mM solution in DMSOd ). The
2
product yields were calculated by comparison with the inte-
gration value of the methyl viologen peak as the reference. The
yields of acid released for triggers 5–8 were determined by
GC-MS. The samples were prepared in the following way: one
mL of irradiated solution was transferred to a closed vessel.
One mL of aqueous HCl with 1.0 M concentration, was added
to lower the pH and protonate the acids. Then 0.5 mL of di-
chloromethane was added and the mixture shaken for
6
complexation was achieved by shaking the NMR tube for about
1
five minutes. H NMR spectra were recorded at room tempera-
ture. 2 : 2 complex was achieved by 10 μL of guest solution to
6
00 μL of 1 mM OA host in 10 mM buffer.
Preparation of host–guest complexes for absorption studies.
A 60 mM stock solution of each guest was prepared in DMSO,
and 12 mL of 5 × 10−5 M of host (OA) solution was prepared at
2
minutes. The mixture was then centrifuged to separate the
2 4 7 2
pH 8.7 using 10 mM Na B O buffer/H O. The solutions of the
phases. The dichloromethane phase was then analyzed. For
quantification the sample was spiked with known amounts of
acid and the signal increase was used to determine the concen-
tration of acids before spiking. A ZB-5MS (Phenomenex) capil-
lary column with 30 m length, 0.25 mm internal diameter and
complex were prepared by adding 5 μL of the 60 mM guest
6
solution in DMSO-d which resulted in a final guest concen-
tration of 2.5 × 10− M for the host solution. After shaking the
5
mixtures manually for 2 min, the UV-vis absorption spectra
were recorded (Fig. S11 and 12 in ESI†).
Preparation of host–guest complexes for LC-DAD-MS
studies. A 1 mM stock solution of host (OA) was prepared in
0
.25 μm film thickness was used. The oven temperature
−
1
program was 70 °C for 1.0 min, 10 °C min increased until a
final temperature of 280 °C. The injector and the transfer line
were set to 280 °C and the injection volume was 1 μL. The
acids were detected in the single ion mode by selecting the m/z
values of the main fragments obtained by electron impact. For
hexanoic acid a Grace AT-WAXMS column with 30 m length,
1
2 4 7
0 mM borate (Na B O ) buffer aqueous solution. Stock solu-
tions of guests were prepared in DMSO at 10 mM concen-
tration. The solutions of complexes contain 100 μM of guest
and 200 μM of OA.
Irradiations
0.25 mm internal diameter and 0.25 μm film thickness was
Photochemical studies with 1–3. The NMR tube containing used. The oven temperature program was 80 °C for 2.0 min,
mM host–guest complex borate buffer solution was placed in a 10 °C min− increased until a final temperature of 260 °C. The
1
1
Rayonet reactor fitted with 360 nm lamps and a cooling fan. injector was set to 260 °C and the injection volume was 1 μL.
Absorption spectra were recorded for compounds 1–3 (5 × 10− M)
5
The identification of major products, namely o-nitrosobenzal-
in water and also in the presence of OA. Progress of the dehyde (o-nitroso-BA), was performed using a triple quadrupole
reaction was monitored by recording absorption spectra at – GC-MS by comparison of the experimental spectra with those
th
various times during irradiation of the samples.
of the library NIST 2014, 10 edition. A ZB-5MS (Phenomenex)
Photochemical studies with 4–8. A 600 µL solution of 1 mM capillary column with 30 m length, 0.25 mm internal diameter
OA (10 mM Na B O in D O, pH = 8.7) was placed in an NMR and 0.25 μm film thickness was used. The oven temperature
2
4
7
2
tube. Then 0.5 equivalents of guest (5 µL of a 60 mM solution program was 45 °C for 1.0 min, 25 °C min− increased until a
1
6
in DMSO-D ) were added. After shaking the NMR tube for final temperature of 250 °C. The final temperature was keep for
1
5
min, the H NMR was recorded to confirm the complex for- 4.8 minutes. The injector and the transfer line were set to 250 °C
mation. The sample was irradiated with a 450 W medium and 255 °C, respectively, and the injection volume was 1 μL.
pressure mercury vapor lamp (Pyrex containers, λ ≥ 300 nm)
and the progress of the reaction was monitored by H NMR.
The concentrations of product o-nitroso-BA were estimated
by LC-DAD using the calibration curves obtained for the
1
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1
trigger. The areas measured for o-nitroso-BA in the LC trace at triggers 1–8 within OA was confirmed by the H NMR spectra
1
3
20 nm (signal at 2.3 minutes) were multiplied by the ratio of the complexes. Partial H NMR spectra of guests included
of the absorbance coefficients of trigger and product at within OA (represented as guest@OA for 1 : 2 and guest @OA
20 nm. The resulting value was used to estimate the concen- for 2 : 2 complexes respectively) are presented in Fig. 1 and 2.
tration of o-nitroso-BA using the above mentioned calibration These confirm that the signals due to aliphatic hydrogens are
curve. upfield shifted to appear between δ 1 and −3.5 ppm, indicat-
The absorbance coefficients of triggers at 320 nm were ing the inclusion of guests within OA.
2
2
2
3
2
5,26 1
H NMR titration
measure by UV-Vis absorbance and the extinction experiments (Fig. S13–S18, S20 and S22†) suggested that the
coefficient of o-nitroso-BA at 320 nm was obtained from guest to host stoichiometry of 1 : 1 (or 2 : 2) for 1–3 and 1 : 2 for
2
2
Gaplovsky et al.
4–8. The 2 : 2 complex imply each OA capsule contains two
molecules of the guest while 1 : 2 indicates each capsule to
contain one molecule of the guest. The reason for this differ-
ence in stoichiometry has to do with the size of the guest;
smaller guests 1–3 form 2 : 2 complexes while larger 4–8 form
Results and discussion
The study consisted of two aspects, (a) inclusion of the triggers 1 : 2 complexes. To confirm that all guests form a capsule the
–8 with the host OA and (b) photochemical study of the host– diffusion constants were measured by DOSY experiments.
1
guest complexes. The first part required us to determine the Diffusion constant will help one to distinguish between 1 : 1
inclusion of the guests within OA and the nature of the host– cavitandplex and 2 : 2 capsuleplex, although both have the
guest complexes by spectral means. The inclusion of the PPG same stoichiometry. Smaller sized cavitandplexes would be
expected to have higher diffusion constants while larger
capsuleplexes would have lower diffusion constants. The
diffusion constants measure for all seven complexes indepen-
dent of whether they are 1 : 2 or 2 : 2 had closely similar con-
stants (Table 1). The diffusion constants of all eight complexes
−
6
2
−1
close to 1.4 × 10 cm s is lower than those for free OA
−
6
2
−1
−6
(
∼1.9 × 10 cm s ) and 1 : 1 open cavitandplex (∼1.65 × 10
2
−1
27,28
cm s ) confirming their capsular nature.
Having confirmed that the triggers 1–8 form capsular com-
plexes we proceeded to irradiate these either in a UV-cuvette or
an NMR tube. Results of the photolysis (>340 nm) of oNB-
ethers and alcohol 1–3 are presented first. Wirz group’s
detailed studies on 1–3 in solution are valuable in interpreting
2
1,22,29
the photobehavior of 1–3@OA.
and encapsulated 1–3 in water were clean and complete within
0 min. The absorption spectra recorded at regular intervals of
Photoreactions of free
3
irradiation are provided in Fig. 3. Appearance of a new band
1
with a maximum around 320 nm corresponding to o-nitroso-
Fig. 1 Partial H NMR spectra of the OA complexes o-nitrobenzyl
ethers 1 and 3 and oNB alcohol 2. (2 : 2 complexes): (i) 1, (ii) 2, (iii) 3. “*” BA and ortho-nitroso acetophenone is consistent with literature
2
1,22,29
indicates the OA bound guest aliphatic proton signals.
reported values during the photolysis of 1–3 in solution.
1
Fig. 2 Selected guest region of the H NMR spectra of the OA complexes o-nitrobenzyl esters 4–8. (2 : 1 complexes): (i) 4, (ii) 5, (iii) 6, (iv) 7, (v) 8.
“*” indicates the OA bound guest aliphatic proton signals.
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Table 1 Diffusion constants of OA complexes
The similarity between the absorption spectra observed in the
presence and absence of OA suggested the photoreaction
within OA to be identical to that in solution.
Diffusion constant (cm s−1)
2
Compound
Only OA
1.88 × 10−
6
1
H NMR spectra of the irradiated samples confirmed the
−
6
6
6
6
6
6
6
2
1 @OA
2
2 @OA
2
3 @OA
4
5
6
7
2
2
2
1.30 × 10
formation of methanol in the case of 1 and 3 (Fig. 4).
Appearance of a signal at δ 3.25 and disappearance of the
signal at δ −1.0 with 1 and 3 suggested the aqueous residence
1.46 × 10−
−
1.32 × 10
@OA
@OA
@OA
@OA
2
2
2
2
1.39 × 10−
−
1.38 × 10
of the released methanol (Fig. 5). As expected, release of water
1.42 × 10−
1
1.43 × 10−
in the case of 2
2
@OA
2
could not be detected by H NMR.
Fig. 3 (a) Progress of reaction as followed by absorption spectra upon photolysis of compounds 1–3 in water. Irradiation was done using Rayonet
2 2 2 2 2 2
reactor (365 nm) UV lamps. (b) Progress of reaction as followed by absorption spectra upon photolysis of (a) 1 @OA ; (b) 2 @OA ; (c) 3 @OA in
buffer. Irradiation was done using Rayonet reactor (365 nm) UV lamps.
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1
1
Fig. 4 (a) H NMR (500 MHz) spectra of (i) 1
2
@OA
2
before irradiation; (ii) 1
2
@OA
2
after 30 min irradiation. (b) H NMR (500 MHz) spectra of (i)
3
2
@OA
2
before irradiation; (ii) 3
2
@OA
2
after 30 min irradiation. “*” represents the bound protons of guest 1 and 3.
1
Fig. 5 Progress of reaction as followed by H NMR (500 MHz upon photolysis of 1
2
@OA
2
(a) disappearance of methyl proton; (b) formation of
photoproduct (methanol) with time.
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Fig. 6 (a) Partial H NMR (500 MHz) spectra of photoirradiation of 2
2
@OA
2
monitored vs. time. (b) LC-DAD (320 nm) and LC-MS single ion (m/z
1
2 2
50) traces of 2 @OA . (i) LC-DAD trace before irradiation; (ii) LC-DAD trace after 5 minutes irradiation (λ > 300 nm); (iii) single ion trace at m/z 150,
the expected value for the oNBAP under positive polarity ionization. The insert show the absorption spectrum taken at 6.67 minutes.
Locating the signals from o-nitroso-AP released by 2 and 3 was released o-nitroso-AP from 2 and 3 was straight forward.
1
1
easier by H NMR spectra. Formation of o-nitroso product Fig. 6a provides partial H NMR spectra (δ 1 to −2.5) of the
upon release of water and methanol from 1 –3 @OA was con- irradiated 2 @OA for various time intervals. We attribute the
2
2
2
2
2
firmed by a combination of ESI-MS, LC-DAD-MS and GC-MS decrease in signal at δ 0.5 due to the methyl in the reactant
(Fig. S31–S35†).
with time, accompanied by a corresponding increase of the
Since one of the goals of this study is to sequester the toxic new signal at δ −1.1 to the acetyl methyl of o-nitroso-AP, which
3
0–33
photo reactive nitroso release byproduct
following release is confirmed by LC-DAD-MS (Fig. 6b). The significant upfield
of the acid, establishing the location of o-nitroso-BA and shift of the methyl signal suggests the photoproduct o-nitroso-
1
o-nitroso-BAP was critical. Locating the H NMR signals of the AP is within the OA capsule; a signal near ∼δ 2 would be
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Table 2 Photoconversion of oNB phototriggers and corresponding
yields of acids released in aqueous solutions containing OA (200 μM
host: 100 μM guest)
The above study with ethers was extended to oNB esters, a
system studied extensively in solution. Since molecules of
interest are generally protected as esters, our main goal was
a
15
to establish the generality of the triggering process within the
water-soluble OA capsule. Simultaneously follow the nitroso
moiety formation (within or outside the capsule) along with
the release of the caged acid. Thus, we investigated the photo-
release from encapsulated oNB esters 4–8@OA2 that release
acids of different hydrophilicities (for example compare pro-
pionic acid and decanoic acid in Fig. 8 and 9). As shown in
Compound
no
Yield
of acid
Yield of nitroso
benzaldehyde
% converted
b
b
4
5
6
7
8
67
91
83
46
81
82
92
c
55
35
33
28
65
c
83
c
79
c
81
a
After 90 minutes, Pyrex glass filter, water filter, air equilibrated. Fig. S12† the absorption spectra of OA and oNB esters fully
b
c
Determined by NMR. Estimated by GC-MS. The errors are ∼15%.
overlap with a maximum at 280 nm and the latter molecules
insoluble in water are solubilized by OA. The complexes were
irradiated (>300 nm) in Pyrex tubes using a 450 W medium
expected had it been in aqueous solution. A similar observation pressure mercury lamp, a condition in which both OA and 4–8
was made in the case of 3 (Fig. S25†). These findings indicate would absorb the incident light. Since OA in the excited state
that photolysis of the OA encapsulated nitrobenzyl triggers has been established to transfer excitation energy to the guest
3
4
results in the release of alcohols from the capsule and retain- of lower energy, we believed regardless of the light absorbing
ment of the toxic nitroso byproduct within OA. The ability to entity the reaction would occur from the excited state of oNB
retain the unwanted and toxic nitroso product within OA pro- esters. The same triggering process is established to occur
3
5–38
vides a solution to a long-standing problem with the byproduct from both S and T of o-nitrobenzyl esters.
when using oNB triggers for delivery of a molecule of interest.
We are aware
1
1
39
the product o-nitroso-BA is photochemically active. To over-
2
Fig. 7 LC-DAD traces (320 nm) of 4@OA , before and after irradiation (λ > 300 nm) in aqueous medium, (a), and aqueous medium with OA, (b). The
inserts show the absorption spectra taken at retention times of observed signals.
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come this problem all photolysis were conducted only up to
3
0% conversion. We did not undertake photochemical and
toxicological studies of o-nitroso-BA.
1
Progress of the irradiation was followed by H NMR, GC-MS
and LC-DAD-MS. The photoreaction was clean and the corres-
ponding acids were released in >80% yield for most triggers,
1
as monitored by H NMR and GC-MS (Table 2). Unfortunately,
we could not clearly identify the peaks due to o-nitroso-BA by
1
H NMR. However, formation of o-nitroso-BA was detected by
its characteristic absorption using a diode array detector
during LC-DAD-MS analysis of the irradiated sample and
further confirmed by GC-MS. HPLC traces of 4 irradiated as a
free molecule and as 4@OA
in Fig. 7. Assuming the absorption spectrum of the signal at
.3 min with close resemblance to that of o-nitroso-BA
2
and their absorptions are shown
2
1
Fig. 9 H-NMR spectra (500 MHz, 10 mM Na
.7) of (i) 8 in DMSO-d (ii) 8@(OA) ([OA] = 1 mM and [8] = 0.5 mM); (iii)
mated its yield (Table 2). There is almost 1 : 1 correspondence 5 h irradiation of 8@(OA)2 at (λ ≥ 300 nm); (iv) decanoic acid@OA ([OA] =
between o-nitroso-BA and released acid. Based on the results 1 mM, [decanoic acid] = 0.25 mM); (v) decanoic acid in Na buffer/
O. Symbols ■ and ● indicate the residual solvent peaks of water and
DMSO-d , respectively. “a–i” indicate the OA bound guest aliphatic
2 4 7 2
B O buffer/D O, pH =
2
2
reported in the literature to be o-nitroso-BA, we have esti-
8
6
2
2 4 7
B O
D
2
discussed above with oNB ethers we believe it must be retained
within OA. In addition to major amounts of o-nitroso-BA the
two minor products detected by LC-DAD (signals at 7.4 and
6
proton peaks.
8
.5 min) have absorption characteristic of OA and o-nitroso-
BA. We suspect these are derived via reaction between OA and
acid (Fig. S29†) upon release shuttles between inside and
outside the OA container.
the intermediates or products formed after intramolecular
1
6,17,36,40
hydrogen abstraction.
1
We recorded H NMR spectra of the free guest, host–guest
complex, irradiated sample and the free acid to ascertain the
location of the released acid in each case. The spectra for
Summary
4
@OA and 8@OA are displayed in Fig. 8 and 9 and for the
2 2
others in ESI (Fig. S26–S30†). Comparison of the spectra in Several of our recent studies have demonstrated the value of
Fig. 8(iii), (iv) and 9(iii), (iv) and (v) clearly show the released encapsulating X-PPG molecules where PPGs are derived from
1
1–14
butanoic acid from 4@OA2 is in water while decanoic acid substituted acetophenones and coumarins.
The current
from 8@OA stays within OA. From the figures presented in study has explored a fifth and the most popular PPG, the
2
ESI (Fig. S27 and S28†) it should be clear that 3,3-dimethyl- ortho-nitrobenzyl system. The major advantage of the supra-
acrylic acid from 5@OA2 and hexanoic acid from 6@OA2 molecular encapsulation approach is that the water insoluble
following release exit into the aqueous solution while octanoic PPG protected substrates could be solubilized and the mole-
cules of interest be released into water. An additional advan-
tage is that the main reaction of photorelease from the excited
PPG occurs within the capsule, which minimizes the potential
of quenching by exogenous quenchers such as oxygen as well
as reaction of any very reactive and short-lived intermediates
that might be initially generated. With certain PPG’s, we have
observed reactions of OA with reactive intermediates that
remain within the OA capsule, thus encapsulating additional
1
2,14
unwanted byproducts of the photorelease process.
These
attributes along with the demonstrated ease of synthesis of the
oNB protecting group are distinct advantages in the release of
drugs at the required place. We recognize that these advan-
tageous properties are significantly limited by the capacity of
the OA when the size of guest exceeds the size that the OA can
contain. None the less, the established ‘proof of principle’ of
the supramolecular photorelease strategy has potential for
delivering hydrophobic, reactive reagents of interest in
aqueous media. This underexplored supramolecular strategy
for reagent delivery to a remote, desired location where the
spent, unwanted, often times toxic trigger retains within the
1
Fig. 8 H-NMR spectra (500 MHz, 10 mM Na
.7) of (i) 4 in DMSO-d , (ii) 4@(OA) ([OA] = 1 mM and [4] = 0.5 mM), (iii)
.5 h irradiation of 4@(OA) at (λ ≥ 300 nm), (iv) butyric acid in Na
buffer/D
O. Symbols ■ and ● indicates the residual solvent peaks of
water and DMSO-d , respectively. “a–c” indicate the OA bound guest ali-
phatic proton peaks.
2 4 7 2
B O buffer/D O, pH =
8
2
6
2
2
2 4 7
B O
2
6
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Photochemical & Photobiological Sciences
delivering capsule will have far reaching applications toward 16 J. E. T. Corrie and D. R. Trentham, in Biological Applications
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Conflicts of interest
The authors declare no competing financial interest.
3
93–417.
9 J. A. Barltrop, P. J. Plant and P. Schofield, Chem. Commun.,
966, 822–823.
1
Acknowledgements
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1 Y. V. Il’ichev, M. A. Schworer and J. Wirz, J. Am. Chem. Soc.,
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University Endowment Association respectively are thanked by
VR and RSG for financial support. JPDS thanks Portuguese
national funds from FCT – Foundation for Science and
Technology through project UID/Multi/04326/2019, and from
the operational programs CRESC Algarve 2020 and COMPETE
2004, 126, 4581–4595.
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