Glucosylated tris-bipyridine ferrous complexes: Construction of hexavalent saccharide clusteres via self-assembly and their recognition by lectin

Article abstract of DOI:10.1246/cl.2000.466

Hexavalent glycoconjugates were prepared via self-assembly of divalent glycosylated 2,2′-bipyridine derivatives with ferrous chloride. The Λ- and Δ-stereoisomeric complexes were preferentially formed from α- and β-glucoside derivatives, respectively. The

Full text of DOI:10.1246/cl.2000.466

466
Chemistry Letters 2000
Glucosylated Tris-bipyridine Ferrous Complexes: Construction of
Hexavalent Saccharide Clusteres via Self-Assembly and Their Recognition by Lectin
Teruaki Hasegawa, Kazunori Matsuura, and Kazukiyo Kobayashi*
Department of Molecular Design and Department of Biotechnology, Graduate School of Engineering, Nagoya University,
Chikusa-ku, Nagoya 464-8603
(Received January 31, 2000; CL-000103)
Hexavalent glycoconjugates were prepared via self-assem-
bipyridine ferrous complex (Chart 1) displayed high diastereo-
selectivity and acquired a strong affinity to lectin.
bly of divalent glycosylated 2,2'-bipyridine derivatives with fer-
rous chloride. The Λ- and ∆-stereoisomeric complexes were
preferentially formed from α- and β-glucoside derivatives,
respectively. The α-glucoside cluster exhibited an enhanced
affinity for concanavalin A.
The tris-bipyridine ferrous complex bearing hexavalent α-
glucoside was prepared from p-nitrophenyl α-D-glucopyra-
noside (α-Glc-pNP) as follows. The glucopyranoside moiety
was acetylated and the nitro function was hydrogenated and
amidated with 4-azido-n-butyryl chloride. The azido group was
hydrogenated and amidated with 2,2'-bipyridyl-4,4'dicarboxyl
chloride, followed by deacetylation to afford divalent α-gluco-
sylated 2,2'-bipyridyl-ligand (α-Glc-3-bpy) with a flexible C3
spacer. When α-Glc-3-bpy was mixed with FeCl2 in
water/methanol (1/1 v/v) at room temperature, the solution
turned red immediately. The formation of tris-complex ([Fe(α-
Glc-3-bpy)3]Cl2) was confirmed by the characteristic shift of
1H-NMR signals of bipyridyl moieties, the appearance of
metal-to-ligand charge transfer (MLCT) absorption band at 544
nm, and the stoichiometry demonstrated by Job method. The
complex bearing hexavalent saccharide terminals was well sol-
uble in water. The corresponding β-O-glucosylated 2,2'-bipyri-
dine (β-Glc-3-bpy) and ferrous tris-complex ([Fe(β-Glc-3-
bpy)3]Cl2) were also prepared from p-nitrophenyl β-D-glucopy-
ranoside (β-Glc-pNP).
Multivalent or clustered saccharide chains of glycoproteins
and glycolipids on cell surfaces are key substances in various cel-
lular recognition events and signal transductions.1,2 Recently,
artificial glycoconjugate polymers, dendorimers, calixarenes,
cyclodextrins, and porphyrins have been developed as clustered
saccharide models3-5 to investigate their high affinities to
lectins, toxins, viruses, and cells. In the series of our research
for functional glycoconjugate materials,6-9 we are now interest-
ed in clustered glycoconjugate assemblies on metal templates.
These asemblies can be simply prepared only by mixing of gly-
cosylated bipyridine ligands with metal ions. In addition, tran-
sition metal complexes are redox-active and some are fluores-
cent (e.g., [Ru(bpy)3]2+).10 In this respect, transition metal
complex-based glycoconjugates will be useful for sensing vari-
ous saccharide recognition phenomena. However, little has
been reported on coordinate-bonded glycoconjugates, except of
those by Sakai et al.11,12 They prepared trivalent glycoconju-
gate clusters based on tris-bipyridine metal complexes, which
exhibited unique CD spectral change on binding to lectins. We
here report that hexavalent glycoconjugates based on tris-
[Fe(α-Glc-3-bpy)3]Cl2 and [Fe(β-Glc-3-bpy)3]Cl2 gave
almost symmetrical CD spectra assignable to Λ- and ∆-
[Fe(bpy)3]2+, respectively, as shown in Figure 1. The opposite
stereoisomers of the bipyridyl array should arise from the oppo-
site chirality of their anomeric positions of the saccharide moi-
eties. Both molar ellipticities due to π-π* transition (ca. 300
nm) and MLCT (ca. 540 nm) were smaller than those of enan-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
467
tio-pure [Fe(bpy)3]2+,11,13 which indicated that the glycoconju-
gate metal complexes were mixtures of Λ- and ∆-stereoisomers.
The two stereoisomers in each glycoconjugate were sepa-
rated by reverse-phase HPLC on a CrestPak C18T-5 analytical
column using a linear gradient of acetonitrile and an aqueous
0.1 M ammonium acetate solution. The major and minor peaks
of [Fe(α-Glc-3-bpy)3]Cl2 were assigned, respectively, to Λ- and
∆-isomer on the basis of their CD spectra. The Λ-∆ ratio esti-
mated from their integration was 73:27. The rather high
diastereo-excess (46% de) was in accordance with that estimat-
1
ed by H-NMR spectrum. Nuclear Overhauser effect (NOE)
1
was detected between the phenyl and bipyridyl protons in H-
NMR spectrum of [Fe(α-Glc-3-bpy)3]Cl2 in D2O. We assume
that the hydrophobic interaction between the phenyl and
bipyridyl moieties separating with the flexible alkyl spacer may
result in a compactly packed conformation of the complex in
water. The resultant proximity of the chiral saccharide units to
the complex center may account for the high diastereo-selectiv-
ity of the complexes.
(ICmin ≥ 1 × 10-1 M). The enhanced specific interaction of
[Fe(α-Glc-3-bpy)3]Cl2 for ConA may arise from hexavalent α-
glucoside assembly and hydrophobic phenyl aglycon. In addi-
tion, their saccharide moieties may be induced-fit to the binding
site of ConA, because of the flexibility of alkyl spacer.
In conclusion, hexavalency, hydrophobicity, and flexibility
of the glycosignals on metal complexes play a substantial role
in enhancement of diastereo-selectivities and affinities for
lectins. These properties will be advantageous for high-sensi-
tive monitoring of various saccharide recognition phenomena.
References and Notes
1
R. A. Dwek, Chem. Rev., 96, 683 (1996).
2
M. Mammen, S.-K. Choi, and J. M. Whitesides, Angew.
Chem., Int. Ed. Engl., 37, 2754 (1998).
3
4
L. L. Kiessling and N. L. Pohl, Chem. Biol., 3, 71 (1996).
S. Nishimura and Y. C. Lee, “Polysaccharide : Structural
Diversity and Functional Versatility,” ed by S. Dumutriu,
Ed.; Marcel Dekker, Inc., New York (1998), p. 523.
N. V. Bovin and H.-J. Gabius, Chem. Soc. Rev., 24, 413
(1995).
T. Akasaka, K. Matsuura, N. Emi, and K. Kobayashi,
Biochem. Biophys. Res. Commun., 260, 323 (1999).
H. Dohi, Y. Nishida, M. Mizuno, M. Shinkai, T.
Kobayashi, T. Takeda, H. Uzawa, and K. Kobayashi,
Bioorg. Med. Chem., 7, 2053 (1999).
5
6
7
8
9
T. Hasegawa, S. Kondoh, K. Matsuura, and K. Kobayashi,
Macromolecules, 32, 6595 (1999).
A. Yashiro, Y. Nishida, M. Ohno, S. Eguchi, and K.
Kobayashi, Tetrahedron Lett., 39, 9031 (1998).
Binding affinity of the conjugates was investigated by inhi-
bition of lectin-induced hemagglutination14 using ConA (con-
canavalin A from jack bean, α-Glc specific) and RCA120
(Ricinus communis agglutinin from castor bean, β-Gal specific).
Table 1 summarizes the minimum inhibition concentrations per
saccharide residue (ICmin). [Fe(α-Glc-3-bpy)3]Cl2 was a
stronger inhibitor for ConA-induced hemagglutination than D-
glucose and α−Glc-pNP by about 1000- and 100-fold, respec-
10 P. D. Beer, Acc. Chem. Res., 31, 71 (1998).
11 S. Sakai and T. Sasaki, J. Am. Chem. Soc., 116, 1587
(1994).
12 S. Sakai, Y. Shigemasa, and T. Sasaki, Bull. Chem. Soc.
Jpn., 72, 1313 (1999).
13 S. J. Milder, J. S. Gold, and D. S. Kliger, J. Am. Chem.
Soc., 108, 8295 (1986).
tively. [Fe(β-Glc-3-bpy)3]Cl2 was less potent inhibitor. RCA120
-
14 I. J. Goldstein and C. H. Hayes, Adv. Carbohydr. Chem.
Biochem., 35, 127 (1978).
induced hemagglutination was not inhibited by these conjugates