Pyridine-based receptors with high affinity for carbohydrates. Influence of the degree of steric hindrance at pyridine nitrogen on the binding mode

Article abstract of DOI:10.1016/j.tetlet.2004.02.087

Remarkable changes in the binding affinity and selectivity of pyridine-based receptors toward monosaccharides have been observed when the degree of steric hindrance at pyridine nitrogen atom decreases.

Full text of DOI:10.1016/j.tetlet.2004.02.087

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 3117–3121
Pyridine-based receptors with high affinity for carbohydrates.
Influence of the degree of steric hindrance at pyridine nitrogen
on the binding mode
Monika Mazika,* and Willi Sickingb
aInstitut f€ur Organische Chemie der Technischen Universit€at Braunschweig, Hagenring 30, 38106 Braunschweig, Germany
bInstitut f€ur Organische Chemie der Universit€at Duisburg-Essen, Universit€atsstraße 5, 45117 Essen, Germany
Received 4 February 2004; revised 16 February 2004; accepted 17 February 2004
Abstract—Remarkable changes in the binding affinity and selectivity of pyridine-based receptors toward monosaccharides have
been observed when the degree of steric hindrance at pyridine nitrogen atom decreases.
ꢀ 2004 Elsevier Ltd. All rights reserved.
The molecular recognition of carbohydrates by synthetic
receptors forms an area of high current interest, arising
from the extreme importance of sugar molecules within
biological systems. Carbohydrates actively control a
whole range of biological processes including cell
growth and differentiation. A range of carbohydrate
binding proteins mediate the interactions of cells with
their environment, primarily via interactions with the
carbohydrates on the surfaces of cell.1 Thus, the
carbohydrates play fundamental roles in cell–cell recog-
nition and other interactions processes. The protein–
carbohydrate interactions involve the hydrogen
bonding, metal coordination, van der Waals forces, and
hydrophobic interactions.1;2 These interactions have
inspired the development of a number of different arti-
ficial receptor structures for the recognition of carbohy-
drates.3 Particularly, many representatives of hydrogen
bonding receptors have been obtained and studied.4;5
However, the selective and effective molecular recogni-
tion of carbohydrates by a synthetic receptor is still a
challenging goal in artificial receptor chemistry.
change from a,a- t o t hea,b- or a,c-disubstituted pyr-
idine groups. The presence of an unsubstituted a-posi-
tion in the pyridine ring is a typical feature of the
receptors 1 and 2. Consequently, the pyridine units are
sterically less hindered at nitrogen and can be involved
much easier and effective in the binding interactions.
These receptors contain neighboring acceptor/donor
groups (pyridine-N/amide-NH), which are able to par-
ticipate in cooperative hydrogen bonds with the sugar
hydroxyls. As shown by our earlier studies, the property
of cooperativity is of particular importance in the area
of synthetic carbohydrate receptors.4i;5 Although, the
structural variation of the receptor structure is minimal,
the binding properties of the new receptors change
dramatically.
R3
R2
R1
O
1: R1 = R2 = H, R3 = CH3
2: R1 = R3 = H, R2 = CH3
3: R1 = CH3, R2 = R3 = H
R3
N
N
O
N
H
R2
R1
In this paper we describe the remarkable changes in the
affinity as well as selectivity of pyridine-based receptors
when the degree of steric hindrance at pyridine nitrogen
atom decreases. The acyclic receptors 1 and 2,6 com-
pared to the previously reported receptor 3,5b reflectthe
O
N
H
R3
R2
N
H
N
R1
CH2OH
CH2OH
O
O
HO
HO
HO
HO
OR
Keywords: Molecular recognition; Carbohydrate receptors; Hydrogen
bonding.
HO
HO
5 : R = C8H17
OR
* Corresponding author. Tel.: +49-531-391-5266; fax: +49-531-391-
8185; e-mail: m.mazik@tu-bs.de
4: R = C8H17, 4a: R = CH3
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.02.087

3118
M. Mazik, W. Sicking / Tetrahedron Letters 45 (2004) 3117–3121
The evidence for the particularly strong complexation of
monosaccharides with the receptors 1 and 2 was
obtained by the NMR spectroscopy and extraction
experiments. The crystal structures of the receptors 1
and 2 are shown in Figure 1.
leads to practically complete complexation of host 1.
The signal due to amide NH of 1 shifted downfield by
1.10 ppm and the aromatic phenyl proton peak moved
upfield by 0.40 ppm. Both the stoichiometry observed by
fitting the binding isotherm and the molar ratio plots
verify that host 1 forms 2:1 receptor:sugar complexes
with a-glucopyranoside. A clear break in the molar ratio
plot indicates that the formation of a complex with high
stability constant is taking place.8 Additionally, receptor
1
All H NMR titration experiments were carried out in
the concentration range, in which the receptors do not
self-aggregate ([1] or [2] < 0.0015 mol/L). The concen-
tration range was estimated on the basis of a series of
dilution experiments. The 1H NMR titrations show that
the addition of only 0.5 equiv of a-glucopyranoside 4
1 is able to extract 0.5 equiv of methyl-a-D-glucopyr-
anoside (4a) from the solid state into a CDCl3 solution.
The binding affinity toward a-glucopyranosides 4–4a
obtained with receptor 1 is comparable to that obtained
with the host 2. The complexation between 4 and
receptor 2 is again evidenced by several changes in
the 1H NMR spectra, particularly by the significant
downfield shifts of the receptor amide protons
(Ddmax ¼ 1:10 ppm) and upfield shifted resonances for
the aromatic phenyl protons of 2 (Ddmax ¼ 0:37 ppm), as
shown in Figure 2a. After 0.5 equiv of octyl a-D-gluco-
pyranoside (4) had been added, almostno change was
observed in the NMR spectra.
The plot of the observed and calculated downfield
chemical shifts of the NH resonances of 2 as a function
of added sugar 4 is illustrated in Figure 3a.9 The graph
rises linearly with increasing the sugar concentration
until Ddmax is reached at 1:0.5 receptor:sugar stoichi-
ometry. In the first part of the titration curve the
receptor is the excess component and sugar the minor
component, thus the 1:1 complex is formed followed by
the immediate formation of a strong 2:1 receptor–sugar
complex.
The data from titrations of 1 or 2 versus a-glucopyr-
anoside 4 suggested a very large value of the overall
formation constants and were impossible to follow for
precisely binding constant calculations. In order to
determine the binding constants, additional inverse
titrations were carried out in which the concentration of
glycoside 4 was held constant and that of receptor 1 or 2
varied. In these titrations, the motion of the signal due
to anomeric CH proton of 4, which is shifted signifi-
cantly downfield, was monitored. The typical titration
curves are shown in Figure 3c and d. In the first part of
the titration curves, the sugar is now the excess com-
ponent and the receptor the minor component. The
anomeric CH proton exhibits chemical shift changes of
0.55 and 0.50 ppm (Ddmax) during complexation with 1
and 2 (Fig. 2c), respectively. These shifts are signifi-
cantly larger in comparison to those reported usually for
the anomeric CH in the literature (mostly Ddmax ¼ 0:05–
0.15 ppm), indicating an important contribution of this
unit to the stability of the complexes 4Æ1 and 4Æ2. On
the basis of the inverse titrations the binding constants
of 4 and receptor 1 were found to be 3640 (Ka1) and
82,450 MÀ1 (Ka2), while the binding constants for 4Æ2
amounott 2100 ( Ka1) and 47,600 MÀ1 (Ka2).9 The
stronger binding of 1 in comparison to 2 can be attrib-
uted both to the different basicity of the pyridine rec-
ognition units (the increase in pKa is somewhatgreaetr
for c- than for b-methyl substituted pyridine10) and to
(a)
(b)
Figure 1. Crystal structure of 1 (a) and 2 (b),7 top and side views.

M. Mazik, W. Sicking / Tetrahedron Letters 45 (2004) 3117–3121
3119
10.0 9.8 9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2
9.5 9.4 9.3 9.2 9.1 9.0 8.0 8.8 8.7 8.6 8.5 8.4 8.3
(b)
(a)
(ppm)
(ppm)
5.30
5.20
5.10
(ppm)
5.00
4.90
4.80
4.45 4.40 4.35 4.30 4.25 4.20
(ppm)
(d)
(c)
Figure 2. (a) 1H NMR spectra (CDCl3, 25 ꢁC) of receptor 2 (the shifts of the NH and CHPh resonances are shown) after addition of (from bottom to
top) 0.00, 0.39, 0.59, 0.80, 0.99, 1.19, 1.40, 1.59, 1.99, and 2.39 equiv of a-glucopyranoside 4 ([2]¼1.33 mM). (b) 1H NMR spectra of receptor 2 (the
shifts of the NH and CHPh resonances are shown) after addition of 0.00, 0.25, 0.50, 0.63, 0.75, 0.88, 1.00, 1.13, 1.26, 1.51, 1.76, 2.00, 2.26, and
2.52 equiv of 5 ([2]¼1.03 mM). (c) The downfield chemical shifts of the anomeric CH resonances of a-anomer 4 during the titration with receptor 2
([4]¼0.61 mM, [2]¼0.10–2.50 mM). (d) The downfield chemical shifts of the anomeric CH resonances of b-anomer 5 during the titration with receptor
2 ([5]¼0.61 mM, [2]¼0.10–2.50 mM).
the different sterical effects. The 2:1 receptor:sugar
binding mode is also supported by molecular modeling.
A possible structure of the 2:1 and 1:1 complexes of 2
and 4 is shown in Figure 4.
(Table 1), whereas the binding constants for 5Æ2 amount
to 440 (Ka1) and 22,600 MÀ1 (Ka2). The association
constants obtained from these experiments (typical
titration curve is shown in Fig. 3e) are identical within
the limits of uncertainty to those determined from
titrations where the role of host and guest 5 was
reversed. As shown in Figure 2d, the peak for the ano-
meric CH proton of 5 is affected significant weakly by
complexation (0.18 and 0.15 ppm by complexation with
1 and 2, respectively) than that of 4 (Fig. 2c), indicating
a weaker binding again.
In contrast to a-anomer 4, t heb-anomer 5 showed lower
affinity to 1 and 2. The H NMR titration of 1 or 2
1
versus 5 also produced several spectral changes (Fig.
2b). The largest complexation-induced shifts were
observed for the NH-signal (downfield shift of 0.85 and
0.80 ppm for 1 and 2, respectively,) and for the phenyl
CH-signal of the receptors (upfield shift of 0.35 ppm for
1 and 0.30 ppm for 2). Comparison of Figure 3a and b
clearly shows that the process of complexation is not
reflected by these chemical shifts upon binding with
a- or b-anomer in the same way. Whereas after the
addition of 0.5 equiv of a-anomer almostno change was
observed in the chemical shift of receptor signals, with
the b-anomer chemical shift changes continue to higher
receptor:sugar ratios. Nevertheless, for all systems the
stoichiometry observed by fitting the binding isotherm is
in agreement with that determined by the ratio method,
which indicates the formation of 1:2 sugar:receptor
complexes. The best fit of the titration data was
obtained again with the ꢀmixedꢁ 1:1 and 2:1 recep-
tor:sugar binding models. The association constants of
660 (Ka1) and 24,200 MÀ1 (Ka2) were determined for 5Æ1
Notably, the observed a=b-anomer selectivities of 1 and
2 differ from those observed for the hydrogen-bonding
hostmolecules described so far, which usually show
higher affinity toward b-anomer. This preference has
been ascribed to the hydrogen-bonding abilities of sugar
hydroxyl groups. As discussed in Refs. 4f and 11 the
axial 1-alkoxy group in a-anomer can form intra-
molecular hydrogen bonds with 2-OH group more easily
than equatorial 1-alkoxy substituent in b-anomer can
do. In this context, the 2-OH in b-glucopyranoside 5 is
relatively free from intramolecular hydrogen bonding
and can interact with receptor molecules more strongly.
The strong binding of 4 with 1 or 2 indicates that the
intermolecular host–guest H-bonding compete effec-
tively with the intramolecular H-bonding network in

3120
M. Mazik, W. Sicking / Tetrahedron Letters 45 (2004) 3117–3121
Table 1. Association constants Ka (MÀ1) and corresponding free
a
energy changes DGꢁ (kJ molÀ1) for receptors 1–2 and glucopyranosides
4–5
b
Host–guest Ka1 (DGꢁ)
complex
Ka2 (DGꢁ)
Ddmax (ppm)
1Æ4
2Æ4
1Æ5
2Æ5
3640 (À20.3)
2100 (À18.9)
660 (À16.1)
440 (À15.1)
82,450 (À28.0)
47,600 (À26.7)
24,200 (À25.0)
22,600 (À24.8)
0.55c/1.10d
0.50c/1.10d
0.18c/0.85d
0.15c/0.80d
a Average Ka values from multiple titrations (CDCl3, stored over
activated molecular sieves and deacidified with Al2O3), values pro-
9
vided by HOSTEST
.
The reproducibility of the Ka values was 10–
15%. Uncertainty in a single Ka estimation was 2–10%. Dilution
experiments show that receptors do not self-aggregate in the used
concentration range.
b 2:1 Receptor:sugar complex.
c Complexation-induced shifts observed for the anomeric CH of sugar
(titrations of sugar vs receptor, inverse titrations).
d Complexation-induced shifts observed for the NH of receptor
(titrations of receptor vs sugar).
tence of hydrogen bonds between OHs of 4 and the
amide-NH/pyridine-N of 1 or 2, including cooperative
hydrogen bonds (OHÁ Á ÁN-pyr, HOÁ Á ÁHN-amide), in
which the hydroxyl group acts simultaneously as a
hydrogen bond donor and acceptor. The hydrogen
bonds are supplemented by interactions of sugar CH
moieties with the phenyl and pyridine groups of the
receptors. Both sides of the pyranoside ring (a- and
b-face) are involved in the stacking interactions with
aromatic residues of the two receptor molecules in 2:1
receptor:sugar complexes. Furthermore the complex of
4 is also stabilized by weak CHÁ Á ÁN-pyr and CHÁ Á ÁO@C
hydrogen bonds. These intermolecular interactions were
also evidenced by the observed changes of chemical
shifts in the NMR spectra of the complexes. In com-
parison with 4, the CHs of sugar 5 participate in less
intermolecular interactions (as indicated by molecular
modeling, 5 is less favorable positioned in the cavity
between the two receptors). The 2:1 receptor:sugar
complexes display also hydrogen bonding between the
two receptor molecules (two amide-NHÁ Á ÁN-pyr
hydrogen bonds).
Figure 3. Plotof hte observed ( ·) and calculated (––) downfield
chemical shifts of the NH resonances of 2 as a function of added
a-glucopyranoside 4 (a) or b-glucopyranoside 5 (b). The [receptor]/
[sugar] ratio is marked (a–b). Plot of the downfield chemical shifts of
the anomeric CH resonances of sugar 4 as a function of added 1 (c) or
2 (d), respectively. Plot of the downfield chemical shifts of the ano-
meric CH resonances of sugar 5 as a function of added 2 (e). The
[sugar]/[receptor] ratio is marked (c–e).
In contrast to receptors 1 and 2, receptor 3, in which
both pyridine a-positions are blocked, shows signifi-
cantly lower affinity to both anomers and the formation
of 1:1 complexes. The binding constant for a-anomer 4
and receptor 3 was found to be 4000 MÀ1, the one for
b-anomer 5 and host 3 amounts to 8700 MÀ1.5 Thus, the
binding properties of the new receptors 1 and 2 appar-
ently indicate that the steric interactions involving the
pyridine substituents significantly affect the binding
properties. These results reflect clearly the great impor-
tance of hydrogen-bonding interactions with the pyr-
idine nitrogen atom for the binding affinity of the
receptors. According to molecular modeling the steric
effects from the a-position of the pyridine rings of 3
hinder the formation of intermolecular interactions,
which are typical for 2:1 receptor:sugar complexes
between 1/2 and glucopyranosides. The decreased degree
of steric hindrance at the pyridine nitrogen allows for
Figure 4. Energy-minimized structure of the 2:1 (left) and 1:1 (right)
complexes formed between receptor 2 and glucopyranoside 4 (Mac-
roModel V.6.5, Amberà force field, Monte Carlo conformational
searches, 50,000 steps).
carbohydrates. The intramolecular bond is replaced by
enough intermolecular H-bonds during the binding
process. Molecular modeling studies suggest the exis-

M. Mazik, W. Sicking / Tetrahedron Letters 45 (2004) 3117–3121
3121
Acad. Sci. U.S.A. 2002, 99, 4972–4976; (g) Ladomenou,
K.; Bonar-Law, R. P. Chem. Commun. 2002, 2108–2109;
(h) Liao, J.-H.; Chen, C.-T.; Chou, H.-C.; Cheng, C.-C.;
Chou, P.-T.; Fang, J.-M.; Slanina, Z.; Chow, T. J. Org.
Lett. 2002, 4, 3107–3110; (i) Mazik, M.; Radunz, W.;
Sicking, W. Org. Lett. 2002, 4, 4579–4582, and references
cited therein.
the development of much stronger host:guest hydrogen-
bonding interactions in the complexes. This knowledge
may also enable the design of further hydrogen-bonding
receptors, which may lead to the development of new
chemosensors. Structures of type 1 or 2 can be incor-
porated into more sophisticated architectures to create a
range of receptors, which should be able to form ener-
getically favorable 1:1 complexes with sugar derivatives.
Studies to synthesis of new receptors of this type are in
progress.
5. (a) Mazik, M.; Sicking, W. Chem. Eur. J. 2001, 7, 664–670;
(b) Mazik, M.; Bandmann, H.; Sicking, W. Angew. Chem.,
Int. Ed. 2000, 39, 551–554.
6. Compounds 1 and 2 were synthesized from benzene-1,3,5-
tricarbonyl chloride and 2-amino-4-methyl-pyridine or
2-amino-5-methyl-pyridine, respectively (CH2Cl2 or THF,
NEt3, room temperature). N,N0,N00-Tris-(4-methylpyridin-
1
Acknowledgements
2-yl)benzene-1,3,5-tricarbonamide (1): H NMR (CDCl3):
d ¼ 2:40 (s, 9H, 3 · CH3), 6.93 (d, 3Hpyr, J ¼ 5:2 Hz), 8.17
(d, 3Hpyr, J ¼ 5:2 Hz), 8.20 (s, 3Hpyr), 8.68 (s, 3HPh), 8.82
(s, 3H, 3 · NH). 13C NMR (CDCl3): d ¼ 21:35, 114.98,
121.51, 129.38, 135.70, 147.58, 150.07, 151.27, 163.94. HR-
MS, calcd for C27H24N6O3: 480.1910. Found: 480.1903.
N,N0,N00-Tris-(5-methyl-pyridin-2-yl)benzene-1,3,5-tricar-
bon-amide (2): 1H NMR (200, CDCl3): d ¼ 2:32 (s, 9H,
This work was supported by the Deutsche Forschungs-
gemeinschaft(SFB 452). We ht ank Prof. Roland Boese
€
and Dieter Blaser for performing the X-ray measure-
ments, and Prof. Craig Wilcox for giving access to the
HOSTEST program.
3 · CH3), 7.58 (dd, 3Hpyr, J ¼ 8:6=2:20 Hz), 8.13 (d, 3Hpyr
,
J ¼ 2:2 Hz), 8.25 (d, 3Hpyr, J ¼ 8:6 Hz), 8.67 (s, 3HPh),
8.73 (s, 3H, 3 · NH). 13C NMR (CDCl3): d ¼ 17:87,
113.84, 129.17, 129.83, 135.73, 139.07, 147.96, 148.97,
163.38. HR-MS, calcd for C27H24N6O3: 480.1910. Found:
480.1907.
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