Recognition properties of receptors consisting of imidazole and indole recognition units towards carbohydrates

Article abstract of DOI:10.3762/bjoc.6.9

Compounds 4 and 5, including both 4(5)-substituted imidazole or 3-substituted indole units as the entities used in nature, and 2-aminopyridine group as a heterocyclic analogue of the asparagine/glutamine primary amide side chain, were prepared and their binding properties towards carbohydrates were studied. The design of these receptors was inspired by the binding motifs observed in the crystal structures of protein-carbohydrate complexes. ¹H NMR spectroscopic titrations in competitive and non-competitive media as well as binding studies in two-phase systems, such as dissolution of solid carbohydrates in apolar media, revealed both highly effective recognition of neutral carbohydrates and interesting binding preferences of these acyclic compounds. Compared to the previously described acyclic receptors, compounds 4 and 5 showed significantly increased binding affinity towards β-galactoside. Both receptors display high β-vs. α-anomer binding preferences in the recognition of glycosides. It has been shown that both hydrogen bonding and interactions of the carbohydrate CH units with the aromatic rings of the receptors contribute to the stabilization of the receptor-carbohydrate complexes. The molecular modeling calculations, synthesis and binding properties of 4 and 5 towards selected carbohydrates are described and compared with those of the previously described receptors.

Full text of DOI:10.3762/bjoc.6.9

Recognition properties of receptors consisting
of imidazole and indole recognition units
towards carbohydrates
Monika Mazik* and André Hartmann
Full Research Paper
Open Access
Address:
Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
Institut für Organische Chemie der Technischen Universität
Braunschweig, Hagenring 30, 38106 Braunschweig, Germany, Tel.:
+495313915266, Fax: +495313918185
doi:10.3762/bjoc.6.9
Received: 30 September 2009
Accepted: 19 January 2010
Published: 02 February 2010
Email:
Monika Mazik* - m.mazik@tu-bs.de
Guest Editor: C. A. Schalley
* Corresponding author
© 2010 Mazik and Hartmann; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
carbohydrates; hydrogen bonds; molecular recognition; receptors;
supramolecular chemistry
Abstract
Compounds 4 and 5, including both 4(5)-substituted imidazole or 3-substituted indole units as the entities used in nature, and
2-aminopyridine group as a heterocyclic analogue of the asparagine/glutamine primary amide side chain, were prepared and their
binding properties towards carbohydrates were studied. The design of these receptors was inspired by the binding motifs observed
in the crystal structures of protein–carbohydrate complexes. 1H NMR spectroscopic titrations in competitive and non-competitive
media as well as binding studies in two-phase systems, such as dissolution of solid carbohydrates in apolar media, revealed both
highly effective recognition of neutral carbohydrates and interesting binding preferences of these acyclic compounds. Compared to
the previously described acyclic receptors, compounds 4 and 5 showed significantly increased binding affinity towards
β-galactoside. Both receptors display high β- vs. α-anomer binding preferences in the recognition of glycosides. It has been shown
that both hydrogen bonding and interactions of the carbohydrate CH units with the aromatic rings of the receptors contribute to the
stabilization of the receptor–carbohydrate complexes. The molecular modeling calculations, synthesis and binding properties of 4
and 5 towards selected carbohydrates are described and compared with those of the previously described receptors.
Introduction
Analysis of the binding motifs found in the crystal structures of tion processes and might serve as a basis for the development of
protein–carbohydrate complexes [1-5] provides much of the new therapeutic agents (for example, anti-infective agents) or
inspiration for the design of artificial carbohydrate receptors saccharide sensors [19-26]. Our previous studies showed that
which use noncovalent interactions for sugar binding [6-18]. mimicking the binding motifs observed in the crystal structures
Such receptors provide valuable model systems to study the of protein–carbohydrate complexes by using natural recogni-
underlying principles of carbohydrate-based molecular recogni- tion groups or their analogues [27-45] represents an effective
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Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
strategy for designing carbohydrate receptors. Among other
things the crystal structures of protein–carbohydrate complexes
revealed that the imidazole and indole groups of His and Trp
respectively are able to participate in both hydrogen bonding
and stacking interactions with the sugar ring. It should be noted
that packing of an aromatic ring of the protein against a sugar is
observed in most carbohydrate–binding proteins [1-5]. Such
packing arrangements and the hydrogen bonding motifs shown
in Figure 1 have inspired the design of receptors 1 and 2 (see
Figure 2), including both 4(5)-substituted imidazole or 3-substi-
tuted indole units as the entities used in nature, and
2-aminopyridine groups as heterocyclic analogues of the aspar-
agine/glutamine primary amide side chains (in analogy to the
binding motif shown in Figure 1a) [31]. The compounds 1 and 2
were established as highly effective receptors for mono- and
disaccharides and shown to display remarkable β- vs. α-anomer
selectivity in the recognition of glucopyranosides, as well as a
binding preference for β-glucopyranoside vs. β-galactopyrano-
side. It has been shown that both hydrogen bonding and interac-
tions of the carbohydrate CH units with the aromatic rings of
the receptors contribute to the stabilization of the
receptor–carbohydrate complexes. Compounds 1 and 2 were
shown to be more powerful carbohydrate receptors than the
symmetrical aminopyridine-based receptor 3.
Figure 2: Structures of receptors 1–5.
methyl α-D-mannopyranoside (10) and dodecyl β-D-maltoside
(11) were selected as substrates for the binding experiments
(see Figure 3). 1H NMR spectroscopic titrations in competitive
and non-competitive media as well as binding studies in two-
phase systems, such as dissolution of solid carbohydrates in
apolar media, revealed highly effective recognition of neutral
carbohydrates and interesting binding preferences of these
acyclic receptors.
Figure 1: Examples of hydrogen bonds in the complex of a) galactose-
binding protein with D-glucose [3], b) Amaranthus caudatus agglutinin
with Galβ3GalNAc [1].
Figure 3: Structures of sugars investigated in this study.
Results and Discussion
We were interested to see whether compounds 4 and 5 (see
Figure 2), which consist of two imidazole or indole groups and Synthesis of the receptors
one 2-aminopyridine unit, would be more effective with mono- The basis for the synthesis of compounds 4 and 5 was 1,3-
and disaccharide substrates. Herein, we describe the synthesis, bis(aminomethyl)-5-[(4,6-dimethylpyridin-2-yl)aminomethyl]-
molecular modeling calculations and the binding properties of 2,4,6- triethylbenzene (17). The synthesis of compound 17 is
the compounds 4 and 5. To compare the binding properties of described in reference [27]. The reaction of 17 with the corres-
the new compounds with those of the previously published ponding carbaldehyde, such as 4(5)-imidazole-carbaldehyde
receptors, octyl β-D-glucopyranoside (6a), methyl β-D-gluco- (18) [46] or 3-indole-carbaldehyde (19), provided the corres-
pyranoside (6b), octyl α-D-glucopyranoside (7a), methyl α-D- ponding imines 20 and 21, which were further reduced with
glucopyranoside (7b), octyl β-D-galactopyranoside (8a), methyl sodium borohydride. The synthesis of receptors 4 and 5 is
β-D-galactopyranoside (8b), methyl α-D-galactopyranoside (9), summarized in Scheme 1.
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Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
case of receptor 5 only about 0.7 equiv of β-glucoside 6b and
β-galactoside 8b could be detected in the solution (see Table 1).
Regarding 4 and 5, the extractability decreased in the sequence
β-glucoside 6b ~ β-galactoside 8b > α-glucoside 7b >
α-galactoside 9 > α-mannoside 10 (see Table 1; control experi-
ments were performed in the absence of the receptor). The pref-
erence of 4 and 5 for β- vs. α-glucoside (6b vs. 7b) as well as
for β- vs. α-galactoside (8b vs. 9) indicated by liquid–solid
extractions was further confirmed by 1H NMR spectroscopic
titrations (see below). Compared to the previously studied
receptors 1–3, the extraction experiments indicated a signifi-
cantly higher level of affinity of 4 and 5 towards β-galactoside.
It should also be noted that the selectivities observed for 4 and 5
are quite different to those of the recently described phenan-
throline/aminopyridine-based receptors 22 and 23 (see Figure 4)
[27,29], which show a strong preference for α-glucoside and
α-galactoside vs. the β-anomers. Thus, depending on the nature
of the recognition units used as building blocks for the acyclic
structures, effective carbohydrate receptors with different
binding selectivities could be obtained. However, the exact
prediction of the binding selectivity still represents an unsolved
problem.
Table 1: Solubilization of sugars in CDCl3 by receptor 4 and 5 (1 mM
solution).
Sugar
Sugar/4a
Sugar/5a
Scheme 1: Reaction conditions: a) AlCl3, CH3CH2Br, 0 °C to r. t., 12 h
(85%) [47]; b) 33% HBr in CH3COOH, ZnBr2, (CH2O)n, 90 °C, 16.5 h
(94%); c) 2 equiv of 2-amino-4,6-dimethylpyridine, CH3CN/THF,
K2CO3, r. t., 3 d (20%); d) potassium phthalimide, dimethyl sulfoxide,
95 °C, 8 h, (57%); e) hydrazine hydrate, ethanol/toluene, reflux, 19.5 h,
KOH (43%) [27]; f) 4 equiv of 4(5)-imidazole-carbaldehyde (18),
CH3OH, 3 d; g) 4 equiv of 3-indole-carbaldehyde (19) CH3OH, 3 d; h)
8 equiv of NaBH4, 0 °C to r. t., 12 h (78% of 4, 92% of 5).
β-D-glucoside 6b
α-D-glucoside 7b
β-D-galactoside 8b
α-D-galactoside 9
α-D-mannoside 10
0.98
0.50
0.95
0.20
0.11
0.72
0.19
0.74
0.09
0.04
aMolar ratios sugar/receptor occurring in solution (the 1H NMR signals of
the corresponding sugar were integrated with respect to the receptor’s
signals to provide the sugar–receptor ratio; control experiments were
performed in the absence of the receptor).
Binding studies in two-phase systems: liquid-
solid extractions
The dissolution of solid carbohydrates in apolar media provides
valuable means of studying carbohydrate recognition by
organic-soluble receptors (for examples of receptors which are
able to dissolve solid carbohydrates in apolar media, see refer-
ences [6,27,41,43,48-50]). Extractions of sugars 6b, 7b, 8b, 9
and 10 from the solid state into a CDCl3 solution of receptor 4
or 5 (1 mM) provided evidence for strong complexation of
β-glucoside 6b and β-galactoside 8b. The extraction of solid
methyl α-glucoside 7b, α-galactoside 9 and α-mannoside 10
into a CDCl3 solution of receptor 4 or 5 indicated a weaker
binding of these sugars than that of 6b and 8b (see Table 1).
The extraction experiments indicated that the imidazole-based
receptor 4 is a more powerful carbohydrate receptor than the
indole-based compound 5. Receptor 4 was able to dissolve
about 1 equiv of β-glucoside 6b and β-galactoside 8b, 0.5 equiv
of α-glucoside 7b and about 0.2 equiv of α-galactoside 9. In the
Figure 4: Structures of the recently described phenanthroline/
aminopyridine-based receptors showing α- vs. β-anomer binding pref-
erences in the recognition of glycosides [27,29].
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Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
Binding studies in homogeneous solution
constant for 1:1 binding and a weaker association constant for
The interactions of the receptors and carbohydrates were inves- the 2:1 receptor–sugar complex (this model was further
tigated by 1H NMR spectroscopic titrations in CDCl3 and supported by the mole ratio plots). The binding constants,
DMSO-d6/CDCl3 mixtures. The stoichiometry of the however, were too large to be accurately determined by the
receptor–sugar complexes was determined by mole ratio plots NMR spectroscopic method (K11 > 105 and K21 ~ 104 M−1; see
[51,52] and by the curve-fitting analysis of the titration data Table 3; for a review discussing the limitations of the NMR
[53].
method, see ref. [55]). After the addition of 5% DMSO-d6 the
binding constants for 4•6a were determined to be 35000 (K11)
The 1H NMR titration experiments [54] with octyl β-glucoside and 1000 M−1 (K12). Thus, the affinity of 4 significantly
6a, α-glucoside 7a, β-galactoside 8a and methyl α-galactoside 9 decreases as solvent polarity increases (the addition of dimethyl
were carried out by adding increasing amounts of sugar to a sulfoxide also caused the change of the binding model; for a
solution of receptor 4 or 5. In addition, inverse titrations were discussion on solvent effects in carbohydrate binding by
performed in which the concentration of the sugar was held synthetic receptors, see ref. [56]).
constant and that of the receptor was varied. The complexation
between receptors 4 or 5 and the monosaccharides was evi- The interactions between the β-glucoside 6a and the indole-
denced by several changes in the NMR spectra (for examples, based receptor 5 in CDCl3 were shown to be strong but less
see Table 2 and Figure 5a and Figure 5b). The addition of the favorable than those with the receptor 4. The best fit of the titra-
monosaccharides 6a, 7a or 8a to a CDCl3 solution of receptors tion data was obtained with the “mixed” 1:1 and 1:2
4 or 5 caused significant downfield shift of the amine NHA receptor–sugar binding model. The association constants for
signal (for labeling, see Figure 2), downfield shift and strong 5•6a were found to be 45900 (K11) and 730 M−1 (K12).
broadening of the NHD signal as well as changes of the chem-
ical shifts of the CH3F,G, CH2B,C,E, pyridine CH and imidazole The interactions between β-glucopyranoside 6a and receptors 4
or indole CH resonances of 4 or 5 (see Table 2). The signal due and 5 were also investigated on the basis of inverse titrations in
to the indole NH of 5 shifted downfield by 0.20–0.40 ppm. The which the concentration of sugar 6a was held constant and that
complexation-induced chemical shifts of the NHA, indole-NH, of receptor 4 or 5 was varied. During the titration of 6a with 4
CH2B, CH3F,G and the aromatic CH protons were monitored for or 5 the signals due to the OH protons of 6a shifted downfield
the determination of the binding constants, which are summar- with strong broadening and became almost indistinguishable
ized in Table 3. Binding studies with β-glucoside 6a and from the base line after the addition of only 0.1 equiv of the
β-galactoside 8a showed the interactions of receptors 4 and 5 receptor, indicating important contribution of the OH groups of
with these monosaccharides to be much more favorable than 6a to the complex formation. Furthermore, the addition of 4 or
those with the α-anomers 7a and 9.
5 to a CDCl3 solution of β-glucoside 6a caused significant
upfield shift of the CH signals of 6a, indicating the participa-
The curve fitting of the titration data for 4 and β-glucoside 6a tion of the sugar CH units in the formation of the CH···π inter-
suggested the existence of 1:1 and 2:1 receptor–sugar actions with the aromatic rings of the receptor (for discussions
complexes in CDCl3 solutions with a stronger association on the importance of carbohydrate–aromatic interactions, see
Table 2: Change in chemical shifta observed during 1H NMR titrations of receptor 4 or 5 with sugar 6a, 7a, 8a or 11 in CDCl3.
Receptor– Δδa [ppm]
sugar
complex
4•6a
4•7a
4•8a
4•11
5•6a
5•7a
5•8a
5•11
NHA: 2.01; CH2B: −0.17; imidazole-CH’s: 0.06, 0.08; CH3F: −0.07
NHA: 1.17; CH2B: −0.15; imidazole-CH’s: 0.05, 0.06; CH3F: −0.05
NHA: 0.79; CH2B: −0.12; CH2E: −0.11; imidazole-CH’s: 0.11, 0.08; CH3F: −0.10; CH3G: 0.05
NHA: 0.80; CH2B: −0.19; CH2C: −0.09; imidazole-CH’s: 0.09, 0.04; CH3F: −0.06; CH3G: 0.03
NHA: 2.06; indole-NH: 0.20; CH2B: −0.18; CH2E: −0.06; CH3F: −0.07; CH3G: 0.04
NHA: 1.50; indole-NH: 0.17; CH2B: −0.18; CH2C: −0.06; CH3F: −0.06
NHA: 1.15; indole-NH: 0.27; CH2B: −0.15; CH2C: 0.06; CH2E: −0.11; pyr-CH’s: −0.01, 0.11; CH3F: −0.09; CH3G: 0.06
NHA: 1.80; indole-NH: 0.40; CH2B: −0.20; CH3F: −0.06; CH3G: 0.04
aLargest change in chemical shift observed during the titration for receptor signals (the concentration of receptor was kept constant and that of sugar
varied).
b(−) Δδ = upfield shift.
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Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
Table 3: Association constantsa,b for receptors 1–6 and carbohydrates 6a, 7a, 8a, 9 and 11.
Host–guest complex
Solvent
K11 [M −1]
K21c or K12d [M−1]
β21 = K11K21 or
β12 = K11K12 [M−2]
g
4•6a
CDCl3
>105; g
35000
7450
5% DMSO-d6/CDCl3
CDCl3
1000; d
3.50×107
8.56×106
4•7a
4•8a
1150; d
g
CDCl3
>105; g
40700
700
5% DMSO-d6/CDCl3
5% DMSO-d6/CDCl3
CDCl3
800; d
3.25×107
3.60×107
4•9
g
4•11
>105; g
12000
5% DMSO-d6/CDCl3
3000; c
5•6a
5•7a
5•8a
5•11
CDCl3
45900
1280
730; d
250; d
3.35×107
3.20×105
4.18×107
CDCl3
CDCl3
38000
>105; g
42000
191730
3160
1100; d
g
CDCl3
5% DMSO-d6/CDCl3
CDCl3
1•6ae
1•7ae
1•8ae
1•11e
8560; c
1540; d
300; d
1.64×109
4.86×106
9.96×105
1.78×109
CDCl3
CDCl3
3320
CDCl3
205760
8670; c
2•6ae
2•7ae
2•8ae
2•11e
CDCl3
CDCl3
CDCl3
CDCl3
156100
2820
10360; c
350; d
1.62×109
9.87×105
8.25×106
2.71×109
7470
1100; d
14840; c
182690
3•6af
3•7af
3•8af
CDCl3
CDCl3
CDCl3
48630
1310
3070
1320; d
470; d
6.42×107
1.35×106
aAverage Ka values from multiple titrations in CDCl3.
bErrors in Ka are less than 10%.
cK21 corresponds to 2:1 receptor–sugar association constant.
dK12 corresponds to 1:2 receptor–sugar association constant.
eResults from ref. [31].
fResults from ref. [41].
gHostest program indicated “mixed” 1:1 and 2:1 receptor-sugar binding model with K11>105 and K21 ~ 104; however, the binding constants were too
large to be accurately determined by the NMR method.
refs. [57-63]; for examples of CH-π interactions in the crystal Similar to 4•6a, the best fit of the titration data for receptor 4
structures of the complexes formed between artificial receptors and β-galactoside 8a was obtained with the “mixed” 1:1 and 2:1
and carbohydrates, see ref. [40]). Among the CH signals, the receptor–sugar binding model. However, the binding constants
signal due to the 2-CH proton of 6a showed the largest shift were again too large to be accurately determined by the NMR
(1.78 and 1.62 ppm for the titration with 4 and 5, respectively). spectroscopic method (see Table 3). Studies performed in 5%
In both cases, 6a•4 and 6a•5, the best fit of the titration data was DMSO-d6 in CDCl3 revealed that K11 = 40700 M−1 and K12 =
obtained with the “mixed” 1:1 and 1:2 sugar–receptor binding 800 M−1. The titration experiments with β-galactoside 8a
model. Thus, the inverse titrations fully confirmed the binding clearly showed that receptor 5 is less effective towards this
model determined through the titrations of 4 or 5 with sugar 6a. monosaccharide than the imidazole-based receptor 4 but much
The association constants obtained on the basis of these titra- more effective than the previously described receptors 1–3. The
tions are identical within the limits of uncertainty to those deter- motions of the signals of 5 were consistent with 1:1 and 1:2
mined from titrations where the role of receptor and substrate receptor–sugar binding and could be analyzed to give associ-
was reversed.
ation constants of 38000 (K11) and 1100 M−1 (K12). Compared
to receptors 1–3 [31,41], receptors 4 and 5 showed a significant-
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Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
ly higher binding affinity towards the β-galactoside 8a. The determined by the NMR spectroscopic method (see Table 3).
differences in the complexation abilities of receptors 1/3 and 4/ After the addition of DMSO-d6 a substantial fall in the binding
5 towards β-galactoside 8a are clearly visible in the comparison affinity was observed. Studies that were performed with 4 and
of the chemical shifts of the signals of the four receptors after 11 in 5% DMSO-d6 in CDCl3 revealed K11 = 12000 M–1 and
the addition of β-galactoside 8a (illustrated in parts a–d of K21 = 3000 M–1, those performed with 5 and 11 indicated the
Figure 5 for the pyridine CH3 signals).
formation of complexes with 1:1 receptor–sugar stoichiometry
with K11 = 42000 M–1.
Figure 6: Partial 1H NMR spectra (400 MHz, CDCl3) of 5 after addi-
tion of (from bottom to top) 0.00–1.63 equiv of β-maltoside 11 ([5] =
0.96 mM). Shown are chemical shifts of the pyridine CH3 and indole
NH signals of receptor 5.
Molecular modeling
The formation of hydrogen bonds and CH···π interactions
between the binding partners was also suggested by molecular
modeling calculations. For example, molecular modeling
suggested that all OH groups and the ring oxygen atom of the
bound β-galactoside 8b in the complex 4•8b are involved in the
formation of hydrogen bonds (see Table 4 and Figure 7a and
Figure 8). In addition, interactions of sugar C-H units with the
central phenyl ring of 4 (see Table 4) were shown to provide
Figure 5: Partial 1H NMR spectra (400 MHz; CDCl3) of receptor 4 (a),
5 (b), 1 (c), and 3 (d) before (bottom) and after the addition of
β-galactoside 8a. Shown are chemical shifts of the pyridine CH3 reson-
ances of the corresponding receptor. [4] = 0.89 mM, equiv of 8a:
0.00–4.65; [5] = 0.90 mM, equiv of 8a: 00–4.52; [1] = 0.95 mM, equiv
of 8a: 0.00–4.26; [3] = 0.90 mM, equiv of 8a: 0.00–5.20.
Our previous studies showed compounds 1–3 to be highly additional stabilization of the complex. Furthermore, the
effective receptors for β-maltoside 11 [28,31]. This disac- molecular modeling calculations indicated that within the 2:1
charide [64] is almost insoluble in CDCl3 but could be solubil- receptor–sugar complex the two receptor molecules almost
ized in this solvent in the presence of the corresponding completely enclose the sugar, leading to involvement of all
receptor. Similar solubility behavior of 11, indicating favorable sugar hydroxyl groups in interactions with the two receptor
interactions between the binding partners, could be observed in molecules (see Table 4 and Figure 7b). The OH groups are
the presence of compounds 4 and 5. Thus, the receptor in involved in the formation of cooperative hydrogen bonds which
CDCl3 was titrated with a solution of maltoside dissolved in the result from the simultaneous participation of a sugar OH as
same receptor solution. The complexation between 4 or 5 and donor and acceptor of hydrogen bonds. The phenyl units of the
the disaccharide 11 was evidenced by several changes in the both receptors stack on the sugar ring and both sides of the
NMR spectra (for example, see Table 2 and Figure 6). The pyranose ring are involved in CH···π interactions (see Table 4
saturation occurred after the addition of about 0.7 equiv of 11. and Figure 7b).
Both the curve fitting of the titration data and and the mole ratio
plots suggested the existence of 1:1 and 2:1 receptor–sugar Conclusion
complexes in the chloroform solution (with stronger associ- The analysis of the binding motifs which are observed in the
ation constant for 1:1 binding and a weaker association constant crystal structures of protein-carbohydrate complexes has influ-
for 2:1 receptor–sugar complex). In both cases, 4•11 and 5•11, enced the design of receptors 4 and 5, including two 4(5)-
the binding constants in CDCl3 were too large to be accurately substituted imidazole or 3-substituted indole units as well as an
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Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
Table 4: Examples of noncovalent interactions indicated by molecular modeling calculationsa for the complexes formed between receptor 4 and sugar
8a or 8b.
1:1 receptor–sugar complexb
2:1 receptor–sugar complexb,c
1:2 receptor–sugar complexd
imidazole-NH···OH-2
HND···HO-2
(I) imidazole-NH···OH-2
(I) HND···HO-2
imidazole-NH···OH-6
HND···HO-6
NHD···O-CH3
(I) NHD···O-CH3
NHD···OH-4
imidazole-NH···OH-3
HND···HO-3
(I) imidazole-NH···OH-3
(I) HND···HO-3
imidazole-NH···OH-4
pyridine-N···HO-2
NHA···OC8H17
NHD···OH-4
(I) NHD···OH-4
phenyl···HO-4
pyridine-N···HO-6
NHA···O-ring
(I) pyridine-N···HO-6
(I) NHA···O-ring
phenyl···HO-4
phenyl···HCH-6
pyridine-N···HC-2e
(I) phenyl···HO-4; (I) phenyl···HC-2
phenyl···HC-2
(II) imidazole-NH···OH-6; (II) NHD···OH-6 pyridine-CH3···OH-4e
(II) NHA···OH-3
3-HO···HO-2f
3-OH···OH-3f
(II) phenyl···HC-1
(II) phenyl···HC-3; (II) phenyl···HC-5
aMacroModel V.8.5, OPLS-AA force field, MCMM, 50000 steps.
bComplex with sugar 8b.
cI and II: two receptors in the 2:1 receptor–sugar complex; for labeling see Figure 2.
dComplex with sugar 8a.
eInteraction with the second sugar.
fSugar–sugar interaction.
Figure 8: Examples of hydrogen bonding motifs indicated by
molecular modeling studies in the 1:1 complex between receptor 4 and
β-galactoside 8b (MacroModel V.8.5, OPLS-AA force field, MCMM,
50000 steps).
studied on the base of 1H NMR spectroscopic titrations in
CDCl3 and DMSO-d6/CDCl3 mixtures as well as binding
studies in two-phase systems, such as dissolution of solid carbo-
hydrates in apolar media. The imidazole-based receptor 4 was
found to be a more powerful monosaccharide receptor than the
indole-based compound 5 and the previously described
receptors 1–3. Compared to 1 and 2, incorporating only one
imidazole or indole recognition unit, receptor 5 showed
increased affinity to β-galactoside but decreased affinity to
β-glucoside. The binding affinity of 1–5 towards β-galactoside
8a and β-glucoside 6a increases in the sequence 3 ~ 1 < 2 < 5 <
Figure 7: Energy-minimized structure of the 1:1 a) and 2:1 complex b)
formed between receptor 4 and β-galactoside 8b (different representa-
tions). MacroModel V.8.5, OPLS-AA force field, MCMM, 50000 steps.
Color code: receptor C, grey; receptor N, blue; sugar molecule, yellow.
aminopyridine-based recognition group. The compounds 4 and 4 and 3 ~ 5 < 1 ~ 2 < 4, respectively. It is remarkable that the
5 were established as highly effective receptors for neutral strong enhancement of the binding affinity of 4 and 5 towards
carbohydrates and were shown to display a significantly higher β-galactoside was achieved through a relatively simple vari-
level of affinity towards β-galactoside than the previously ation of the receptor structure. In contrast to 4 and 5, the previ-
described acyclic receptors. Both receptors were shown to ously described phenanthroline/aminopyridine-based receptors
display high β- vs. α-anomer binding preferences in the recogni- 22 and 23 were shown to display a high binding affinity
tion of glycosides. The binding properties of 4 and 5 were towards α-galactoside as well as a strong α- vs. β-anomer
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Beilstein Journal of Organic Chemistry 2010, 6, No. 9.
binding preference. Thus, depending on the nature of the recog- Hz, 6H), 1.06 (t, J = 7.5 Hz, 3H) ppm; 13C NMR (100 MHz,
nition units incorporated into the acyclic receptor structure, CDCl3): δ = 158.28, 156.64, 148.55, 142.87, 142.49, 136.37,
effective carbohydrate receptors with different binding prefer- 134.51, 132.41, 127.16, 122.48, 122.02, 119.46, 119.00, 115.21,
ences can be generated. However, the exact prediction of the 113.60, 111.03, 103.55, 47.28, 45.51, 40.59, 24.20, 22.59,
binding preference still represents an unsolved problem and 22.52, 21.05, 16.77 ppm; HR-MS (ESI) calcd for C40H49N6 [M
remains an important goal for future research.
+ H]+: 613.4018, found: 613.4012; Rf = 0.12 [CHCl3/CH3OH
(incl. 1% 7 M NH3 in CH3OH) 3:1 v/v].
Experimental section
Analytical TLC was carried out on silica gel 60 F254 plates Acknowledgements
employing chloroform/methanol mixtures as the mobile phase. Financial support by the Deutsche Forschungsgemeinschaft
Melting points are uncorrected. Sugars 6–11, 4(5)-imidazole- (German Research Foundation) is gratefully acknowledged.
carbaldehyde (18) and 3-indole-carbaldehyde (19) are commer-
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