Selective disaccharide binding by a macrotetracyclic receptor

Article abstract of DOI:10.1039/b618776e

A new carbohydrate receptor possesses a C₃-symmetric polar cavity capable of encapsulating disaccharides; binding to β-maltosyl is preferred, complementing previous systems which have favoured "all-equatorial" substrates. The Royal Society of C

Full text of DOI:10.1039/b618776e

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Selective disaccharide binding by a macrotetracyclic receptor{
Emmanuel Klein, Yann Ferrand, Elizabeth K. Auty and Anthony P. Davis*
Received (in Cambridge, UK) 2nd January 2007, Accepted 23rd February 2007
First published as an Advance Article on the web 13th March 2007
DOI: 10.1039/b618776e
A new carbohydrate receptor possesses a C -symmetric polar
change in shape is reflected in changes in selectivity, particularly in
a preference for the curved structure of b-maltoside 4.
3
cavity capable of encapsulating disaccharides; binding to
b-maltosyl is preferred, complementing previous systems which
have favoured ‘‘all-equatorial’’ substrates.
The preparation of 3 exploited a side-reaction in our previous
6
a
synthesis of monosaccharide receptor 8. As shown in Scheme 1,
diamine 5 was treated with diester 6 under high dilution to give a
mixture of cyclo-oligomers. After Cbz removal, the major
component 7a was separated and converted into 8. Further
examination of the mixture 7 revealed that higher macrocycles
could also be isolated and, in particular, that the trimeric 7b could
Carbohydrates play key roles in biology, not only as energy
sources and structural building blocks but also as information
carriers. Monosaccharides can be oligomerised to give large
numbers of structures, far more than for peptides or nucleic acids
1
7
of similar molecular weight. Nature exploits this resource, using
be obtained in yields of up to 20%. This hexa-amine might seem
oligosaccharides to label cell surfaces and proteins and thus
mediate processes such as cell–cell recognition, protein trafficking
unpromising as a cage precursor; at first sight, it may appear that
reaction with 6 should yield a complex and intractable mixture of
products. However, free rotation about the biphenyl central C–C
2
and many aspects of the immune response. This importance has
fuelled many efforts to mimic biological carbohydrate recogni-
3
tion. However, most work has targeted monosaccharides, instead
8
bond simplifies the situation. Detailed analysis suggests that, in
principle, a high proportion of the hexa-amine could be converted
to dodeca-amide 3. In practice, treatment of 7b with 6 at high
dilution gave 3 in 15% yield after purification by HPLC.
of the oligosaccharides which are the major focus of biological
4
interest. Of the published biomimetic carbohydrate receptors,
there are few which can span or encapsulate oligosaccharides,
5
selecting for and among these larger substrates.
Macrotetracycle 3 was characterised by MALDI MS and
1
NMR. H NMR spectra in CDCl
3
were broad, but addition of
We have previously described the terphenyl-based macrotricycle
CD OD (25%) caused the lines to sharpen. The resulting spectrum
3
1
, a receptor for the ‘‘all-equatorial’’ b-cellobiosyl disaccharide
showed just 4 absorptions in the aromatic region (d 5 7.2–8.4),
5e
2a. We now report a new tetracyclic system 3, with a ‘‘short, fat’’
consistent with the expected
D
3h time-average symmetry.
architecture which complements the ‘‘long, thin’’ cavity of 1. The
9
Molecular modelling revealed a range of conformations including
˚
open structures with roughly cylindrical cavities of length ca. 16 A
˚
and internal diameters of ca. 7 A. As illustrated in Fig. 1, these
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK
BS8 1TS. E-mail: Anthony.Davis@bristol.ac.uk; Fax: +44 117 9298611;
Tel: +44 117 9546334
conformations were capable of encapsulating disaccharides such as
{
Electronic supplementary information (ESI) available: Experimental
methyl b-D-maltoside 4b and methyl b-D-cellobioside 2b, with
details, including binding curves, and detailed discussion of the cyclisation
giving 3. See DOI: 10.1039/b618776e
formation of at least
6 intermolecular hydrogen bonds.
2
390 | Chem. Commun., 2007, 2390–2392
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2
Scheme 1 Synthesis of 3. Reagents and conditions: (i) iPr NEt, THF, high dilution; (ii) Pd/C, THF, methanolic ammonia, then flash chromatography; (iii)
6, iPr NEt, THF, high dilution.
2
Monosaccharides were too small to fill the cavity and thus made
fewer binding contacts.
we have used previously for organic-soluble receptors; for example,
terphenyl-based 1 was studied in chloroform–methanol 92 : 8.
b-Cellobioside 2a, b-maltoside 4a, a-maltoside 9, b-glucoside 10
and a-glucoside 11 were used as substrates.
1
The recognition properties of 3 were studied using H NMR
titrations in CDCl
receptor), and by induced CD in the corresponding CHCl3–
CH OH mixture. These solvent systems are more polar than those
3 3
–CD OD (75 : 25, as used for the pure
Initial NMR experiments showed that, despite the competitive
solvent, receptor 3 is indeed effective for 2a and 4a. In both cases
the receptor signals broadened, implying complex formation with
a rate of exchange between ‘‘slow’’ and ‘‘fast’’ on the NMR
timescale. In the case of 4a, broadening was extreme towards
the end of the titration at which point new signals appeared. In the
case of 2a the signals shifted during the titration without the
appearance of new absorptions, suggesting that, to a first
approximation, the exchange could be considered fast. On this
basis, the data for 2a were analysed according to a 1 : 1 binding
3
21
model, giving a good fit to theory with K 5 215 M . In contrast
a
to 2a and 4a, addition of a-maltoside 9 gave minimal signal
motions (, 0.02 ppm) that were linear with concentration, while
glucosides 10 and 11 caused no change whatever to the spectrum
of 3.
To provide more quantitative data we turned to induced circular
dichroism (ICD). This technique is especially useful for achiral
UV-absorbing receptors interacting with chiral UV-silent sub-
1
0
strates such as carbohydrates; the appearance of a CD signal on
mixing is a firm indication of binding. Titrations of receptor 3
a
21
a
Table 1 Association constants (K , M ) for glycosides forming 1 : 1
complexes with receptor 3 in chloroform–methanol 75 : 25, as
determined by CD and H NMR titrations
1
1
( H NMR)
b
Substrate
K
a
(CD)
K
a
c
dodecyl b-D-maltoside 4a
octyl b-D-cellobioside 2a
dodecyl a-D-maltoside 9
octyl a-D-glucoside 11
a
780
310
n.d.
215
d
d
v.s.
—
v.s.
—
e
e
Fig. 1 Model of receptor 3 in an open conformation binding methyl
b-D-maltoside 4b, viewed from two perspectives. Solubilising side-chains
are cyan (stick display), while the maltoside is coloured magenta. The
conformation and position of the maltoside were optimised using
From non-linear least-squares curve-fitting to a 1 : 1 binding
model, implemented within Excel 2002. For binding curves, see
Supplementary Information. CDCl –CD OD. Not determined,
b
c
3
3
d
due to slow exchange. Very small. Effects are minor and linear
with concentration of added glycoside. No effect with either
technique.
e
9
MCMM. Six intermolecular hydrogen bonds are formed between host
and guest.
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against glycosides 2a, 4a, 9 and 11 gave the results gathered in
Table 1. Both 2a and 4a gave substantial ICD effects, with
excellent fits to a 1 : 1 binding model. The binding constant
obtained for 2a was reasonably close to that derived from NMR,
while the value for maltoside 4a was significantly higher at
Notes and references
1
2
R. A. Laine, Glycobiology, 1994, 4, 759.
Leading references: H. J. Gabius, H. C. Siebert, S. Andr e´ , J. Jim e´ nez-
Barbero and H. R u¨ diger, ChemBioChem, 2004, 5, 740; R. A. Dwek and
T. D. Butters, Chem. Rev., 2002, 102, 283; C. R. Bertozzi and
L. L. Kiessling, Science, 2001, 291, 2357; C.-H. Wong, Carbohydrate-
based drug discovery, Wiley-VCH, Weinheim, 2003.
21
780 M . The results for a-glycosides 9 and 11 were also consistent
with the NMR data; the former gave a CD signal which changed
slowly and linearly with concentration, while the latter showed no
ICD whatsoever.
3
Reviews: (a) A. P. Davis and T. D. James, in Functional Synthetic
Receptors, ed. T. Schrader and A. D. Hamilton, Wiley-VCH,
Weinheim, 2005, p. 45; (b) A. P. Davis and R. S. Wareham, Angew.
Chem., Int. Ed., 1999, 38, 2978. Recent examples: (c) M. Mazik and
H. Cavga, J. Org. Chem., 2006, 71, 2957; (d) J. D. Lee, N. T. Greene,
G. T. Rushton, K. D. Shimizu and J. I. Hong, Org. Lett., 2005, 7, 963;
(e) H. Abe, N. Masuda, M. Waki and M. Inouye, J. Am. Chem. Soc.,
2005, 127, 16189; (f) M. Mazik, H. Cavga and P. G. Jones, J. Am.
Chem. Soc., 2005, 127, 9045; (g) H. Abe, Y. Aoyagi and M. Inouye,
Org. Lett., 2005, 7, 59; (h) C. Schmuck and M. Schwegmann, Org. Lett.,
2005, 7, 3517; (i) H. P. Yi, X. B. Shao, J. L. Hou, C. Li, X. K. Jiang and
Z. T. Li, New J. Chem., 2005, 29, 1213; (j) A. Vacca, C. Nativi,
M. Cacciarini, R. Pergoli and S. Roelens, J. Am. Chem. Soc., 2004, 126,
16456; (k) O. Francesconi, A. Ienco, G. Moneti, C. Nativi and
S. Roelens, Angew. Chem., Int. Ed., 2006, 45, 6693.
4
5
We use the term ‘‘biomimetic’’ for receptors which, as a minimum
criterion, operate through non-covalent interactions. Effective but non-
biomimetic carbohydrate recognition is possible through reversible
covalent B–O bond formation. See ref. 3a and: S. Striegler, Curr. Org.
Chem., 2003, 7, 81; T. D. James and S. Shinkai, Top. Curr. Chem., 2002,
218, 159; T. D. James, K. R. A. S. Sandanayake and S. Shinkai, Angew.
Chem., Int. Ed. Engl., 1996, 35, 1911.
Titration of b-glucoside 10 into 3 gave less expected results. In
1
contrast to the H NMR data, quite strong ICD effects were
observed. ‘‘False positive’’ ICD results are unlikely, and we
conclude that binding takes place but does not affect the NMR
spectrum. The ICD data were inconsistent with 1 : 1 binding but
suggested 1 : 2 (host : guest) stoichiometry. Analysis using a 1 : 1 +
(a) U. Neidlein and F. Diederich, Chem. Commun., 1996, 1493; (b)
A. S. Droz and F. Diederich, J. Chem. Soc., Perkin Trans. 1, 2000, 4224;
1
1
21
21
1
: 2 binding model yielded values of y1 M and 1470 M for
(c) A. S. Droz, U. Neidlein, S. Anderson, P. Seiler and F. Diederich,
the successive binding constants Ka1 and Ka2; this implies highly
Helv. Chim. Acta, 2001, 84, 2243; (d) R. D. Hubbard, S. R. Horner and
B. L. Miller, J. Am. Chem. Soc., 2001, 123, 5810; (e) G. Lecollinet,
A. P. Dominey, T. Velasco and A. P. Davis, Angew. Chem., Int. Ed.,
22
a
cooperative 1 : 2 binding with an overall K 5 1470 M .
The results may be summarised as follows. Firstly macrotetra-
2002, 41, 4093; (f) O. Rusin, K. Lang and V. Kr a´ l, Chem.–Eur. J., 2002,
cycle 3 is a powerful carbohydrate receptor, comparing well with
8, 655; (g) M. Dukh, D. Saman, K. Lang, V. Pouzar, I. Cerny, P. Drasar
terphenyl-based 1. Although higher K
a
values were measured for 1
up to 7000 M ), the solvent mixture was less polar (containing
and V. Kr a´ l, Org. Biomol. Chem., 2003, 1, 3458; (h) M. Mazik and
A. K o¨ nig, J. Org. Chem., 2006, 71, 7854. For other systems with notable
oligosaccharide binding properties, see ref. 3c and: (i) J. Otsuki,
K. Kobayashi, H. Toi and Y. Aoyama, Tetrahedron Lett., 1993, 34,
1945; (j) J. L. Hou, X. B. Shao, G. J. Chen, Y. X. Zhou, X. K. Jiang and
Z. T. Li, J. Am. Chem. Soc., 2004, 126, 12386; (k) Y. Q. Chen,
X. Z. Wang, X. B. Shao, J. L. Hou, X. Z. Chen, X. K. Jiang and Z. T. Li,
Tetrahedron, 2004, 60, 10253; (l) J. Billing, H. Grundberg and
U. J. Nilsson, Supramol. Chem., 2002, 14, 367.
6 (a) E. Klein, M. P. Crump and A. P. Davis, Angew. Chem., Int. Ed.,
2005, 44, 298. For related systems see (b) A. P. Davis and
R. S. Wareham, Angew. Chem., Int. Ed., 1998, 37, 2270; (c) T. J. Ryan,
G. Lecollinet, T. Velasco and A. P. Davis, Proc. Natl. Acad. Sci. U. S. A.,
21
(
only 8% methanol). Secondly, 3 is strongly selective for b-glyco-
sides 4a/2a/10 as against a-glycosides 9/11. Thirdly, it is selective
for paired monosaccharide units, forming 1 : 1 complexes with
disaccharides 4a/2a and a 1 : 2 complex with glucoside 10. Of
these, it prefers the disaccharides under the conditions of the
1
2
titrations. Finally receptor 3 is selective for maltoside 4a
vs. cellobioside 2a, thus reversing the preference of the earlier
system 1.
In conclusion, we report a carbohydrate receptor with a novel
tetracyclic architecture, and the rare ability to distinguish between
disaccharide stereoisomers. The new receptor is the first to select
for maltoside vs. cellobioside, complementing the cellobiose-
selective 1. The results confirm that selectivity-tuning is possible
in these macropolycyclic polyamide hosts, encouraging the hope
that ‘‘synthetic lectins’’ with a range of specificities may ultimately
be possible.
2
002, 99, 4863. Like 1, these receptors prefer all-equatorial substrates.
7 For details of the synthesis and binding experiments, see Supplementary
Information.
For further discussion see Supplementary Information.
Monte Carlo Molecular Mechanics (MCMM) searches were performed
using MacroModel 9.0 (Maestro 7.0 interface), employing the MMFFs
force field and GB/SA chloroform solvation.
8
9
1
0 Y. Kikuchi, K. Kobayashi and Y. Aoyama, J. Am. Chem. Soc., 1992,
14, 1351.
1 The programme EQNMR was employed; see M. J. Hynes, J. Chem.
1
1
Financial support from the EU and EPSRC is gratefully
acknowledged. Mass spectra were provided by the EPSRC
National MS Service Centre at the University of Swansea. We
thank Prof. M. J. Hynes for access to the EQNMR binding
analysis programme, and Drs P. Gale and G. Bates for assistance
with its use.
Soc., Dalton Trans., 1993, 311.
12 Comparisons are difficult as the stoichiometries, and therefore the units
of the binding constants, are different. However, an estimate can be
obtained by comparing the concentrations at which guests occupied 50%
of receptor binding sites during the titrations. For 4a and 10 the figures
are 1.4 mM and 25 mM respectively. By this measure, the receptor
favours disaccharide 4a by a factor of 18.
2
392 | Chem. Commun., 2007, 2390–2392
This journal is ß The Royal Society of Chemistry 2007