Nitrocatechols as tractable surface release systems

Full text of DOI:10.1002/cplu.201200251

DOI: 10.1002/cplu.201200251
Nitrocatechols as Tractable Surface Release Systems
Robin Wehlauch, Johannes Hoecker, and Karl Gademann*[a]
We report the development of a molecular surface modifica-
tion platform based on nitrocatechol derivatives that allows for
small molecule functionalization of TiO2 under mild aqueous
conditions and efficient release triggered by light therefore un-
caging a small molecule cargo on demand. Surface modifica-
tions by using molecular approaches have found applications
in a wide variety of fields, as they allow combining the bulk
properties of a material with the molecular features of the
coating, which can additionally be tailored by synthetic
chemistry.[1] A central challenge remains the nature of the mo-
lecular anchor that links coating and surface. In this respect,
catechols display unique properties owing to their ability to
strongly bind to metal oxides in aqueous media. Originally
found in mussel adhesive proteins,[2] dihydroxyphenylalanine
(DOPA) or dopamine-derived catechols were successfully used
in surface modifications despite their shortcomings of weak
binding and sensitivity to oxidation. We have introduced cate-
chols with electron-withdrawing substituents based on the
iron chelator anachelin that result in stable coatings with non-
polymeric architectures.[3] A second-generation design intro-
duced nitrocatechols such as nitrodopamine owing to the ease
of preparation, improved adhesion properties, and stability to-
wards oxidation.[4] All these anchors have found widespread
applications,[1,5] for example, we have shown that such cate-
chols can be used to functionalize surfaces with complex anti-
biotics by an operationally simple dip-and-rinse procedure to
generate antimicrobial surfaces.[6]
propose a bio-inspired approach that leverages the presence
of a catechol (easily tunable by a variety of parameters such as
pH, ion strength, or temperature) with the versatility of the ni-
trophenyl system (Figure 1).
Figure 1. Concept of the light induced surface release of carrier molecules
for the photolabile NPP protecting group.
Among the most commonly used photocleavable protecting
groups are several nitrophenyl derivatives, including (2-nitro-
phenyl)ethyl (NPE) and (2-nitrophenyl)propyl (NPP) deriva-
tives.[9,10] However, upon inspection of the literature, we were
surprised to find that no free nitrocatechols have been utilized
as photocleavable groups and the surface functionalization
and release properties of such systems have not been investi-
gated.[11] We have targeted both NPE and NPP systems 1 and
2, respectively, and demonstrate in this study that controlled
bonding and release is readily achieved for the latter deriva-
tives.
In addition to immobilization, the release of small molecules
from a modified surface triggered by an external stimulus
would be highly desirable. Potential applications would range
from drug delivery, small molecule microarrays to selective
probes in chemical biology. In particular in the latter area, such
an approach would combine the well-known strategy of tem-
porarily disabling the biological activity of a small molecule
(caging) with the possibility of controlled release on demand.
This would allow for both high spatial control (through immo-
bilization) and temporal control (through caging) of the bio-
logical activity of a small molecule, two key elements in chemi-
cal biology approaches.[7,8] Herein, we report on the develop-
ment of such a molecular platform that allows for the spatio-
temporal control of small molecule release. In particular, we
The first target compound constituted the 2-nitrophenyl
ethyl derivative 1. Its synthesis started with the preparation of
nitrocatechol 4 by nitration of commercially available 3,4-
(methylenedioxy)acetophenone (3) using HNO3 in acetic acid
(Scheme 1). Treatment with AlCl3 at low temperature and sub-
[a] R. Wehlauch,+ J. Hoecker,+ Prof.Dr. K. Gademann
Department of Chemistry
University of Basel
National Centre of Competence in Research “Chemical Biology”
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: karl.gademann@unibas.ch
Scheme 1. Preparation of NPE conjugate 1 for surface modification. Re-
agents and conditions: a) HNO3, AcOH, 08C!RT, 2.5 h, 59%; b) AlCl3, DCE,
ꢀ58C, 1 h, then 48% HBr, RT, 24 h, 84%; c) MOMCl, K2CO3, MeCN, 08C!RT,
3 h, 87%; d) NaBH4, MeOH, 08C!RT, 3.5 h, 99%; e) 6, PPh3, DIAD, THF,
08C!RT, overnight, 41%; f) TFA, H2O. DCE=1,2-dichloroethane, DIAD=di-
isopropyl diazodicarboxylate, MOM=methoxymethyl, TFA=trifluoroacetic
acid, THF=tetrahydrofuran.
[+] These authors contributed equally to this work.
Supporting information for this article, including details for the cleavage
along with characterization and spectroscopic data of all compounds, is
available on the WWW under http://dx.doi.org/10.1002/cplu.201200251.
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sequent hydrolysis with concentrated HBr gave nitrocatechol 4
in good yield. Catechol 4 was then further protected as the
MOM ether and the keto group was reduced using NaBH4
leading to alcohol 5. In the context of this synthesis, it was
found necessary to protect the catecholate OH groups, to pre-
vent attachment of these compounds to silica gel during chro-
matographic purification. The alcohol 5 was then attached to
the fluorophore[12] 6 through Mitsunobu reaction in moderate
yields. When alcohol 5 was coupled, reactions were performed
under exclusion of light to prevent decomposition of the
formed product. During isolation of the photolabile compound
from the reaction mixtures, it became already apparent that
the uncaging process could be induced, when TLC plates were
exposed to UV light at 366 nm (see video and Supporting In-
formation). The MOM protecting group could be finally re-
moved using aqueous TFA, leading to the NPE derivative 1.
With the NPE derivative in hand, we sought to prepare the
related unsubstituted compound 7 (X=H) as well as the fluori-
nated compound 8 (X=F, for detailed synthesis see Support-
ing Information) to investigate the role of the nitro substituent
on the cleavage mechanism (Scheme 2). There are at least two
stallment of the nitro group (see Supporting Information).[14]
The reverse synthetic order, nitration followed by protection,
failed as no acetonide formation could be detected. The crucial
elongation reaction of 10 with paraformaldehyde and Triton B
gave the desired product 11,[15] although only moderate con-
version was achieved (with 40% starting material 10 recov-
ered). With the catechol protected as the acetonide, carbonate
formation using triphosgene and triethylamine in THF led to
the desired chloroformate in quantitative yield within 25 mi-
nutes at 08C.[16] To prevent decomposition, the chloroformate
Scheme 3. Preparation of NPP conjugate 2 for surface modification. Re-
agents and conditions: a) 2,2-dimethoxypropane, cat. TsOH, benzene, reflux,
overnight, b) HNO3/H2O (1:1), 08C!RT, 1.5 h, 86% (over 2 steps); c) CH2O,
Triton B in MeOH, 858C, 65 h, 80% (brsm); d) i) triphosgene, Et3N, THF; 08C,
25 min, ii) 3b, pyridine, CH2Cl2, 08C!RT, 1.5 h; e) TFA, H2O, RT, overnight,
68% (over 2 steps). brsm=based on recovery of starting material, Ts=4-tol-
uenesulfonyl.
Scheme 2. Deprotection studies of fluorinated and unsubstituted catechols.
fundamentally different pathways for cleavage of
such catechols from titania possible, either via photo-
catalytic oxidation to the quinone[7] or via nitroaryl
mediated bond cleavage.[8,9] To investigate these two
options and to delineate the role of the nitro sub-
stituent, we have prepared control compounds 7 and
8 lacking this substituent, where only photocatalytic
oxidation to the quinone would be possible. We
were surprised to discover that the deprotection
under a variety of conditions of these compounds
proved to be impossible, as only decomposition of
the starting material and the free fluorophore could
be detected. This could be explained by the forma-
tion of reactive quinone methides by cleavage of the
fluorophore at the benzylic position.[13]
To circumvent the decomposition via the quinone
methide pathway, we wanted to investigate the cor-
responding homologated propyl substituted nitro-
phenols 2. Therefore, acetonide 10 was successfully
prepared from catechol 9 by using 2,2-dimethoxypro-
pane and catalytic amounts of TsOH in benzene at
reflux (Scheme 3). Subsequent nitration of the aceto-
nide with half-concentrated HNO3 afforded 10 after
1.5 hours in very good yield. To our delight, these
Figure 2. A) Fluorescence spectra of 12 and 2 after certain irradiation times. B) Fluores-
cence emission intensity at 454 nm depending on the irradiation time at 366 nm of 2 as
a 5·10ꢀ5 m solution in MOPS buffer. C) UV spectra of 2 and 12. D) Decay of 2 upon UV ir-
strongly acidic, aqueous conditions did not affect the
acetonide protection group. A crystal structure of 10
could be obtained, which confirmed the correct in- radiation at 366 nm, determined by HPLC-MS analysis.
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was coupled immediately to coumarin 12 to afford the desired
carbonate, which was deprotected to give the target catechol
2 using neat TFA. In total the preparation of 2 was achieved in
five steps using only two purification steps rendering 2 an
easily accessible and attractive candidate for further investiga-
tions. In particular, we expect that the alcohol 11 could serve
as an ideal starting point for the attachment of various cargo
compounds by coupling chemistry.
The stability of the different caged compounds was then in-
vestigated using a variety of methods and assays. Gratifyingly,
the NPP linker 2 was fully stable to hydrolysis in aqueous
medium (MOPS buffer) at pH 5.5 in the dark for at least
72 hours. This is contrast to the ethyl derivative 1, which dis-
plays only limited stability. In order to determine the half-life
time of 2 in aqueous solution (MOPS buffer) under near UV-ir-
radiation, photocleavage of catechol 2 was investigated by de-
tection of the evolving fluorescence during release of
coumarin 12.[17] As expected, a rapid increase in fluo-
rescence was detected during the first minutes of ir-
radiation and the fluorescence maximum was identi-
fied at 454 nm (Figure 2A). However, after approxi-
mately 15 minutes, the detected fluorescence intensi-
ty was decreasing, thus indicating that photobleach-
ing of coumarin 12 occurred, which could be
confirmed by control experiments (Figure 2B, and
Supporting Information). Quantification by HPLC-MS
determined the half-life time of 2 to be around
12 minutes at a concentration of 5ꢁ10ꢀ5 m in buffer
(Figure 2D). This is an attractively short period for
cleavage, since the light intensity of a simple labora-
tory UV-lamp is rather low (ꢁ20 mWcmꢀ2).
Scheme 4. Immobilization of nitrocatechol 2 on TiO2 microparticles to deliv-
er Ti-2 and release of the cargo upon irradiation.
To evaluate the surface release properties, TiO2 par-
ticles (1.0–2.0 mm) were functionalized with nitrocate-
chol 2 in MOPS buffer at 558C according to previous-
ly developed conditions (Scheme 4).[4,18] After the in-
cubation, the functionalized particles Ti-2 were
washed three times with CH3CN, and HPLC analysis
of the washing solutions determined only small
amounts of fluorophore 12 and no free 2 present.
The release of the fluorescent cargo from the func-
tionalized beads was investigated next. The function-
Figure 3. A) TLC plate coated with nitrocatechol 2 (Si-2) and irradiated for 60 sec at
366 nm under a printed foil, after 10 sec exposure to UV light without foil (1), after
60 sec (2), and after 180 sec (3) UV irradiation at 366 nm (see videos and the Supporting
Information). B) Ti-2 (left) and negative control (uncoated TiO2 particles, right) in MOPS
buffer upon UV irradiation. C) Release kinetics of 12 from Ti-2 upon UV irradiation at
366 nm in MOPS buffer.
alized TiO2 beads Ti-2 were suspended in MOPS
buffer and the mixture was irradiated. Fluorescence
became immediately visible (Figure 3B). Aliquots
were taken after certain irradiation time intervals and
analyzed by HPLC at 366 nm. The obtained data
demonstrates that no catechol 2 is present and the
amount of 12 was increasing over the irradiation
which could be unmasked by release of the fluorophore
time (Figure 3C), providing evidence that photocleavage of
surface-adsorbed nitrocatechol 2 and concomitant release of
coumarin 12 has occurred. In this reaction setup, a half-life
time of approximately 19 minutes was determined for the re-
lease of 12. Qualitatively, these observations could be corrobo-
rated by the immobilization of 2 on silica (TLC plate) and con-
trolled release under UV light (Figure 3A, see Supporting Infor-
mation for videos). In this simple application, a printed foil was
used as template to generate the desired surface pattern,
through irradiation.
In conclusion, we report in this Communication the develop-
ment of a bio-inspired surface modification platform based on
nitrocatechols that allows for the controlled release of small
molecules under an external stimulus. Salient features of this
method involve 1) ease of functionalization of TiO2 particles
under an operationally simple dip-and-rinse procedure, 2) sta-
bility of the resulting functionalized particles to repeated wash-
ing, and 3) rapid release of the small molecule cargo under an
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K.G. is a European Young Investigator (EURYI). We thank Dr M.
Neuburger for X-ray analyses and J. Gomes for useful discussions.
We gratefully acknowledge financial support by the Swiss Nation-
al Science Foundation (SNF; PE002–117136/1). Part of this work
is supported by the NCCR Chemical Biology.
Keywords: adhesion · biomimetic synthesis · photochemistry ·
release on demand · synthesis design
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Received: September 20, 2012
Published online on October 22, 2012
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