Synthesis of the first water-soluble hemicryptophane host: Selective recognition of choline in aqueous medium

Article abstract of DOI:10.1021/ol500706z

The first water-soluble hemicryptophane cage compound, 4, was synthesized in seven steps from commercially available products, and its complexation properties were studied. NMR and ITC experiments indicate that 4 is an efficient receptor for choline in wa

Full text of DOI:10.1021/ol500706z

Letter
pubs.acs.org/OrgLett
Synthesis of the First Water-Soluble Hemicryptophane Host:
Selective Recognition of Choline in Aqueous Medium
Aline Schmitt,† Vincent Robert,‡ Jean-Pierre Dutasta,*,† and Alexandre Martinez*,†
†
́
Laboratoire de Chimie, Ecole Normale Super
́
ieure de Lyon, CNRS, UCBL, 46, Allee
́
d’Italie, F-69364 Lyon, France
‡Laboratoire de Chimie Quantique Institut de Chimie, UMR CNRS 7177, Universite
Strasbourg, France
́
de Strasbourg, 4, rue Blaise Pascal, F-67070
S
* Supporting Information
ABSTRACT: The first water-soluble hemicryptophane cage com-
pound, 4, was synthesized in seven steps from commercially available
products, and its complexation properties were studied. NMR and
ITC experiments indicate that 4 is an efficient receptor for choline in
water (ΔG° = −5.2 kcal mol−1). High substrate selectivity was
achieved since no complexation was observed for its closely related
substrates: glycine betaine and betaine aldehyde. Density functional
theory calculations were performed to analyze the interactions that
are involved in the formation of the inclusion complex.
he design of artificial molecular receptors is very attractive
choline concentration. Although selective complexation of
choline over glycine betaine or closely related substrates has
been reported,11 to the best of our knowledge, there is no host
capable of distinguishing choline from its closely related
metabolite betaine aldehyde.
1−3
Tas they can mimic biological systems such as enzymes.
A large number of host compounds have been designed, and in
most cases, their complexation properties have been studied in
organic solvents.4 Nevertheless, most of the recognition events
in nature take place in aqueous medium, and to obtain a better
understanding and control of biological phenomena, synthetic
receptors should be able to selectively complex guests in water.5
Supramolecular recognition in water is a difficult challenge
since, on one hand, the host must be water-soluble, and on the
other hand, specific interactions are required to overcome the
competitive influence of water. Indeed, the formation of
complexes involves interchange between guest−water and
guest−host interactions, which is not obviously a favorable
process.6 A prerequisite is that the water-soluble host
compound should be able to efficiently discriminate closely
related substrates to mimic more accurately the selectivity of
biological systems and thus to lead to potential applications.
Choline oxidase is an important enzyme catalyzing the
oxidation of choline to glycine betaine via betaine aldehyde as
intermediate.7,8 This enzyme is of great interest for medicinal
and biotechnological applications. For instance, intracellular
accumulation of glycine betaine improves resistance in
pathogenic bacteria and transgenic plants, allowing normal
cell function under conditions of hyperosmotic and temper-
ature stress.9 Furthermore, choline oxidase controls the level of
water-soluble organic amines (choline and glycine betaine) in
cells, therefore regulating their osmotic equilibrium.10 Thus,
host molecules with the ability to selectively complex choline
over its glycine betaine and betaine aldehyde metabolites may
find applications as inhibitors or for the selective monitoring of
Among different classes of host molecules soluble in water,
cryptophanes,12 adequately substituted with water-solubilizing
groups, have been identified as remarkable complexation agents
for cesium ion,13 xenon,14−19 or ammonium ions. For instance,
water-soluble cryptophane-O binds choline efficiently (ΔG° =
−5.3 kcal/mol) but without significant selectivity when
compared to acetylcholine or trimethylpropylammonium
guests.20 The related hemicryptophane compounds represent
a very interesting family of host molecules for the recognition
of various substrates.21,22 Over the past decade, several
examples have been reported in the fields of molecular
recognition and supramolecular catalysis.23−37 However, their
solubilization in aqueous media is far from trivial, and no water-
soluble hemicryptophanes or their recognition properties in
water have been reported so far. Herein, we describe the
unprecedented synthesis of a water-soluble hemicryptophane
host and its use as an efficient receptor for the complexation of
choline neurotransmitter in water. The lack of complexation of
both glycine betaine and betaine aldehyde under the same
conditions highlights the high substrate specificity of this
heteroditopic host compound.
A few examples of recognition of ammonium and
zwitterionic neurotransmitters by hemicryptophane hosts are
Received: March 6, 2014
Published: April 15, 2014
© 2014 American Chemical Society
2374
dx.doi.org/10.1021/ol500706z | Org. Lett. 2014, 16, 2374−2377

Organic Letters
Letter
known in the literature.29−32 We have recently reported the
efficient binding properties of hemicryptophane 1 toward
alkylammonium derivatives (Scheme 1).27 Accordingly, its
water-soluble counterpart is likely the best suitable host to
investigate the recognition of choline guest in aqueous media.
The strategy used to synthesize the water-soluble variant of
hemicryptophane 1 is presented in Scheme 1 and relies on the
deprotection of the methoxy groups of the cyclotribenzylene
moiety to restore the phenol functions allowing the
solubilization in basic aqueous solution.12 The deprotection
of the methoxy groups in 1 follows the strategy developed for
the cryptophane derivatives using Ph2PLi.20,37 To avoid the
concomitant presence of amine and phenol units in the
resulting crude reaction mixture that requires tedious and
difficult purification steps, we first Boc-protected the benzyl-
amine groups in 1 giving rise to compound 2. Then the
methoxy groups of 2 were removed using Ph2PLi to afford the
trihydroxy-N-Boc protected derivative 3. Finally, deprotection
of the amine functions in 3, using triflic acid, gave the water-
soluble hemicryptophane 4. Following this synthetic pathway,
host 4 was obtained in seven steps from the commercially
available products with an overall yield of 8% (Scheme S-1,
Supporting Information). We should notice that the use of
BBr3 or TMSI as reactants to obtain compound 4 directly from
1 did not lead to the desired product.
1
Figure 1. H NMR spectrum (500 MHz, D2O, 298 K, pd =12) of
■
▲
hemicryptophane 4 ( , water; , ethanol).
M−1 (Figure S-2, Supporting Information). However, the guest
displays broad NMR signals, preventing an accurate estimation
of the host/guest ratio, hence of the binding constant. At lower
temperature, slow exchange conditions could be reached and at
270 K we observed a new signal in the high-field region,
highlighting the formation of an inclusion complex with the
ammonium moiety of the choline located in the shielding
region of the cyclotribenzylene unit (Figure 2).
1
The H NMR spectrum of hemicryptophane 4 in D2O/
NaOD confirms the absence of the methoxy groups and
displays the expected signals for the cyclotribenzylene unit
(Figure 1): two singlets for the aromatic protons and the
characteristic AB system for the ArCH2 bridges, two doublets
for the H3−H4 aromatic protons of the linkers and multiplets
for the OCH2 and NCH2 groups. At pH = 12, the hydroxy
phenol groups in 4 are deprotonated to generate the
triphenolate species.
The complexation of choline neurotransmitter (as its
chloride salt) by host 4 was first investigated in deuterated
water (pD = 12) at 298 K through 1H NMR titration (Figure S-
1, Supporting Information). During the experiments, only
averaged signals were observed for 4 and choline, indicating fast
exchange conditions. Increasing the host/guest ratio induced a
significant high-field shift of the guest’s protons due to the
shielding effect of the aromatic cavity (Figure S-1, Supporting
Information). The determination of the binding constant by
modeling the titration curve afforded a Ka value of 6.4 × 103
Figure 2. 1H NMR spectra of host 4 with an excess of choline chloride
in D2O/NaOD at different temperatures.
We then used isothermal titration calorimetry (ITC) to
determine more accurately the association constant for the
choline@4 complex in water (pH = 12). Addition of successive
aliquots of choline chloride (24.03 mM) to 4 (1.12 mM)
Scheme 1. Synthesis of Water-Soluble Hemicryptophane 4
2375
dx.doi.org/10.1021/ol500706z | Org. Lett. 2014, 16, 2374−2377

Organic Letters
Letter
revealed the formation of a complex with a Ka value of 2300
M−1, which is of the same order of magnitude as that obtained
by NMR titration (Figure 3). Furthermore, ITC thermogram is
Figure 4. DFT-optimized structure of the choline@4 complex.
hydrogen bonds occur between the OH unit of the guest and
two nitrogen atoms of the tren unit (OH···N distance is 2.7 Å
and NH···O distance is 3.0 Å). Thus, both the cyclo-
tribenzylene and tren units contribute to the efficient and
selective binding of choline, emphasizing the heteroditopic
character of the host.
In summary, we have described the synthesis and
charaterization of the first water-soluble hemicryptophane
host compound. Efficient complexation of choline in water
was then achieved with this receptor. NMR experiments and
DFT calculations highlight the formation of an inclusion
complex, the ammonium part of the guest being located inside
the cavity in the vicinity of the cyclotriveratrylene unit. ITC
measurements allowed the determination of the binding
constant and the thermodynamic parameters. The combination
of such techniques and analysis suggests that 4 is an efficient
and selective artificial receptor for the choline neurotransmitter
in water since neither betaine aldehyde nor glycine betaine were
complexed by this host. This opens the way for the use of
hemicryptophane structures for the selective recognition of
compounds of biological interest in biological media.
Figure 3. Enthalpogram of an aqueous solution of hemicryptophane 4
(1.12 mM) titrated by choline chloride (24.03 mM) at pH 12 and 298
K.
exothermic, characterized by both a favorable enthalpic term
(ΔH = −5.22 kcal·mol−1) and a negative entropy (ΔS = −2.12
cal mol−1 K−1, −TΔS = +0.63 kcal mol−1), suggesting that the
complexation is mainly enthalpy-driven. Favorable interactions
of choline with the host cavity of 4 can account for the
efficiency of the recognition process.
More information on the selectivity of the host−guest
recognition process is accessible by a comparison of the
complexation properties of 4 toward choline and its related
metabolites betaine aldehyde and glycine betaine. No complex-
ation was observed by ITC for these latter two guests (Figure
S-4, Supporting Information), emphasizing the high selectivity
of hemicryptophane 4 for choline. This is all most striking given
that betaine aldehyde is predominantly under its gem-diol
hydrate form in aqueous solution (99% at pH 12).8 Thus, host
4 is able to discriminate choline from its aldehyde, although
they only differ by one OH function. It can be noticed that the
selectivity observed for choline may be related to the
zwitterionic character of betaine aldehyde and choline betaine
at pH 12.
Further insight into this recognition process was obtained
from density functional theory (DFT) calculations.38 In the
optimized geometry of the complex, choline is encapsulated in
the hemicryptophane cavity (Figure 4). The ammonium unit of
the guest interacts with both (i) the aromatic rings of the host
with several CH···π distances in the range of 3.3 Å and (ii) the
negatively charged oxygen atoms (the N+···O− distances are 4.4
Å). This is consistent with the high-field shift observed for the
methyl protons of the guest in the NMR spectra. Interestingly,
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures, scheme of the synthesis, and
spectral data for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jean-pierre.dutasta@ens-lyon.fr.
*E-mail: alexandre.martinez@ens-lyon.fr.
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) Kirby, A. J. Angew. Chem., Int. Ed. 1996, 35, 707−724.
(2) Sanders, J. K. M. Chem.Eur. J. 1998, 4, 1378−1383.
(3) Lehn, J.-M. Rep. Prog. Phys. 2004, 67, 245−249.
2376
dx.doi.org/10.1021/ol500706z | Org. Lett. 2014, 16, 2374−2377

Organic Letters
Letter
(4) Steed, J. W.; Atwood, J. L. In Supramolecular Chemistry, 2nd ed.;
Wiley-VCH: Weinheim, 2009.
(5) Oshovky, G. V.; Reinhoudt, D. N.; Verboom, W. Angew. Chem.,
(34) Perraud, O.; Sorokin, A. B.; Dutasta, J.-P.; Martinez, A. Chem.
Commun. 2013, 49, 1288−1290.
(35) Makita, Y.; Sugimoto, K.; Furuyosho, K.; Ikeda, K.; Fujiwara, S.-
i.; Shin-ike, T.; Ogawa, A. Inorg. Chem. 2010, 49, 7220−7222.
(36) Makita, Y.; Ikeda, K.; Sugimoto, K.; Fujita, T.; Danno, T.;
Bobuatong, K.; Ehara, M.; Jujiwara, S. I.; Ogawa, A. J. Organomet.
Chem. 2012, 706, 26−29.
Int. Ed. 2007, 46, 2366−2393.
(6) Klein, E.; Ferrand, Y.; Barwell, N. P.; Davis, A. P. Angew. Chem.,
Int. Ed. 2008, 47, 2693−2696.
(7) Fan, F.; Gadda, G. J. Am. Chem. Soc. 2005, 127, 2067−2074.
(8) Fan, F.; German, M. W.; Gadda, G. Biochemistry 2006, 45, 1979−
1986.
(37) Canceill, J.; Lacombe, L.; Collet, A. J. Chem. Soc., Chem.
Commun. 1987, 219−221.
(38) Gaussian 03: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone,
V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J.
L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J.
J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.;
Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.;
Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.;
Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A.
D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian, Inc.: Wallingford, CT, 2009.
(9) (a) Ko, R.; Tombras Smith, L.; Smith, G. M. J. Bacteriol. 1994,
176, 426−431. (b) Chen, T. H.; Murata, N. Plant Cell Environ. 2011,
34, 1−20. (c) Sakamoto, A.; Valverde, R.; Alia; Chen, H. H.; Murata,
N. Plant J. 2000, 22, 449−453.
(10) (a) Peddie, B. A.; Lever, M.; Hayan, C. M.; Randall, K.;
Chambers, S. T. FEMS Microbiol. Lett. 1994, 120, 125−131. (b) Boch,
J.; Kempf, B.; Schmid, R.; Bremer, E. J. Bacteriol. 1996, 178, 5121−
5129.
(11) (a) Guo, D.-S.; Uzunova, V. D.; Su, X.; Liu, Y.; Nau, W. M.
Chem. Sci. 2011, 2, 1722−1734. (b) Hof, F.; Trembleau, L.; Ullrich, E.
C.; Rebeck, J. Angew. Chem., Int. Ed. 2003, 42, 3150−3153.
(c) Ballester, P.; Sarmentero, M. A. Org. Lett. 2006, 8, 3477−3480.
(d) Ballester, P.; Shivanyuk, A.; Far, A. R.; Rebek, J. J. Am. Chem. Soc.
2002, 124, 14014−14016.
(12) Brotin, T.; Dutasta, J.-P. Chem. Rev. 2009, 109, 88−130.
(13) Brotin, T.; Montserret, R.; Bouchet, A.; Cavagnat, D.; Linares,
M.; Buffeteau, T. J. Org. Chem. 2012, 77, 1198−1201.
(14) Huber, J. G.; Brotin, T.; Dubois, L.; Desvaux, H.; Dutasta, J.-P.;
Berthault, P. J. Am. Chem. Soc. 2006, 128, 6239−6246.
(15) Aru Hill, P.; Wei, Q.; Troxler, T.; Dmochowski, I. J. J. Am.
Chem. Soc. 2009, 131, 3069−3077.
(16) Delacour, L.; Kotera, N.; Traore,
C.; Leteurtre, F.; Gravel, E.; Tassali, N.; Boutin, C.; Leo
́
T.; Garcia-Argote, S.; Puente,
nce, E.;
́
Boulard, Y.; Berthault, P.; Rousseau, B. Chem.Eur. J. 2013, 19,
6089−6093.
(17) Fairchild, R. M.; Joseph, A. I.; Holman, K. T.; Fogarty, H. A.;
Brotin, T.; Dutasta, J.-P.; Boutin, C.; Huber, G.; Berthault, P. J. Am.
Chem. Soc. 2010, 132, 15505−15507.
(18) Jacobson, D. R.; Khan, N. S.; Colle,
Perez, L.; Bai, Y.; Dmochowski, I. J. Proc. Natl. Acad. Sci. U.S.A. 2011,
108, 10969−10973.
́
R.; Fitzgerald, R.; Laureano-
́
(19) Hill, P. A.; Wei, Q.; Eckenhoff, R.; Dmochowski, I. J. J. Am.
Chem. Soc. 2007, 129, 9262−9263.
(20) Garel, L.; Lozach, B.; Dutasta, J.-P.; Collet, A. J. Am. Chem. Soc.
1993, 115, 11652−11653.
(21) Gosse, I.; Dutasta, J.-P.; Perrin, M.; Thozet, A. New J. Chem.
1999, 23, 545−548.
(22) Canceill, J.; Collet, A.; Gabard, J.; Kotzyba-Hibert, F.; Lehn, J.-
M. Helv. Chim. Acta 1982, 65, 1894−1897.
(23) Martinez, A.; Robert, V.; Gornitzka, H.; Dutasta, J.-P. Chem.
Eur. J. 2010, 16, 520−527.
(24) Le Gac, S.; Jabin, I. Chem.Eur. J. 2008, 14, 548−557.
(25) Perraud, O.; Robert, V.; Martinez, A.; Dutasta, J.-P. Chem.Eur.
J. 2011, 17, 4177−4182.
(26) Perraud, O.; Martinez, A.; Dutasta, J.-P. Chem. Commun. 2011,
47, 5861−5863.
(27) Perraud, O.; Lefevre, S.; Robert, V.; Martinez, A.; Dutasta, J.-P.
Org. Biomol. Chem. 2012, 10, 1056−1059.
(28) Wang, L.; Wang, G.-T.; Zhao, X.; Jiang, X.-K.; Li, Z.-T. J. Org.
Chem. 2011, 76, 3531−3535.
(29) Perraud, O.; Robert, V.; Gornitzka, H.; Martinez, A.; Dutasta, J.-
P. Angew. Chem., Int. Ed. 2012, 51, 504−508.
(30) Schmitt, A.; Chatelet, B.; Collin, S.; Dutasta, J.-P.; Martinez, A.
Chirality 2013, 8, 475−479.
(31) Cochrane, J. R.; Schmitt, A.; Wille, U.; Hutton, C. A. Chem.
Commun. 2013, 49, 8504−8506.
(32) Perraud, O.; Robert, V.; Martinez, A.; Dutasta, J.-P. Chem.Eur.
J. 2011, 17, 13405−13408.
(33) Martinez, A.; Dutasta, J.-P. J. Catal. 2009, 267, 188−192.
2377
dx.doi.org/10.1021/ol500706z | Org. Lett. 2014, 16, 2374−2377