Saccharide-coated M12L24 molecular spheres that form aggregates by multi-interaction with proteins

Article abstract of DOI:10.1021/ja0693082

The self-assembly of saccharide-coated nanometer-sized molecular spheres is reported, where 24 (oligo)saccharides are precisely arrayed at the periphery of the spherical core. When combined with lectins, they form aggregates because of the cluster effect of the saccharides on the spheres. Copyright

Full text of DOI:10.1021/ja0693082

Published on Web 03/08/2007
Saccharide-Coated M12L24 Molecular Spheres That Form Aggregates by
Multi-interaction with Proteins
Nozomi Kamiya, Masahide Tominaga, Sota Sato, and Makoto Fujita*
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, and CREST, Japan Science and
Technology Agency (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received December 29, 2006; E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Scheme 1. Self-Assembly of M12L24 Complexes with 24
Saccharide Moieties at the Periphery
Saccharide-protein interactions play essential roles in vivo,
particularly in cellular adhesion and infection.1 Such biomolecular
interactions take place via the recognition of not a monosaccharide
but a saccharide cluster by the binding site of protein surfaces.2
Many efforts have been made for synthesizing saccharide clusters
to study how saccharides work on cellular recognition and to add
their functions to synthetic molecules. Previously, saccharide
clusters have been prepared by the covalent or noncovalent
immobilization of saccharides on artificial scaffolds, such as
polymers,3 dendrimers,2b,4 cyclodextrins,5,6 calixarenes,6,7 gold
nanoparticles,8 vesicles,9 and so on.10 However, the facile library
preparation of structurally well-defined saccharide clusters has been
still unexplored. Here, we report a self-assembly approach to the
precise clusterization of saccharides at the periphery of ca. 4-nm-
sized M12L24 molecular sphere (where M and L denote metal ions
and bridging ligands, respectively). We designed mono-, di-, or
trisaccharide-substituted bridging ligands 1a-f. On complexation
with Pd(II) ions, the bridging ligands are assembled into an M12L24
spherical core (Scheme 1).11 Since each ligand carries one (oligo)-
saccharide unit, the periphery of the self-assembled sphere is
covered precisely with 24 (oligo)saccharides. We show the sac-
charide-dependent interaction of a series of spherical complexes
with lectins to form colloidal aggregates.
assembled into the corresponding M12L24 complexes 2b-f. The
estimated diameters of the mono-, di-, and trisaccharide-appended
spheres are 5.2, 6.1, and 7.0 nm, respectively (Figure 3). All of the
six complexes have 24 saccharide units equivalently aligned at the
R-Mannopyranoside derivative 1a, synthesized in three steps
from 2-bromoethyl 2,3,4,6-tetra-O-acetyl-â-D-mannopyranoside,
was treated with Pd(NO3)2 in DMSO-d6 for 12 h at 70 °C. The
quantitative formation of spherical complex 2a was indicated by
1H NMR (Figure 1). Only one set of signals for the ligand was
observed, in good agreement with the highly symmetric structure
of 2a. A large downfield shift of the signals of pyridine protons
(e.g., ∆δPyHR ) 0.77 ppm, ∆δPyHâ ) 0.48 ppm) is ascribed to the
metal-pyridine coordination. The signals of aromatic ring protons
involved in the spherical core broadened because of the slow
dynamic motion of the rigid large core.
Diffusion-ordered NMR spectroscopy (DOSY) confirmed the
selective formation of a single species. All proton signals showed
the same diffusion coefficient at (3.7 ( 0.3) × 10-11 m2 s-1
,
consistent with the estimated diameter of 5.2 nm (Supporting
Information).12 A large spherical structure was also ascertained by
direct AFM observation (Supporting Information). Cold-spray
ionization mass spectrometry (CSI-MS) confirmed an M12L24
composition with the molecular weight of 15762 Da after exchange
of counterions from nitrate to triflate ions (Figure 2). A series of
[2a-(OTf-)n]n+ (n ) 8-14) peaks was observed. For example, a
peak at m/z 1283.5 was assigned to [2a-(OTf-)11]11+ (calculated
as 1283.8).
Figure 1. 1H NMR spectra of (a) ligand 1a and (b) complex 2a.
Using the self-assembly method, we easily established the library
of saccharide-appended spherical complexes. In addition to monosac-
charide-appended ligands 1b and 1c, di- and trisaccharide deriva-
tives 1d-f were easily prepared, all of which were successfully
Figure 2. CSI-MS spectrum of 2a (CH3CN:DMSO ) 20:1, OTf- salt).
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10.1021/ja0693082 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
the spherical complexes depended on the kind of terminal saccharide
unit, indicating that ConA recognizes the terminal saccharide unit
of 2a-f.
When peanut agglutinin (PNA), a galactose-binding lectin from
Arachis hypogaea,15 was used, 2b and 2d bearing â-galactopyra-
noside formed aggregates as indicated by the turbidity measurement.
However, the other spherical complexes (2a, 2c, 2e, and 2f) did
not (Figure 4b).
In conclusion, we established the facile preparation of the library
of spherical complexes having 24 saccharide units at the periphery
by metal-directed self-assembly. Clustered saccharides on the
spherical platform are efficiently bound to ConA or PNA recogniz-
ing the conformation of the saccharides. We expect that the
combination of the protein recognition at the periphery and
molecular encapsulation at the interior of the spheres will lead to
many biomedical applications.
Supporting Information Available: Preparation and physical
properties of 1a-f and 2a-f, and detailed methods of turbidity
experiments. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 3. Optimized molecular models and estimated diameters of (a) 2a,
(b) 2d, and (c) 2f.
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Figure 4. Turbidity changes, monitored by the absorbance at 500 nm, on
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The aggregate formation was also observed on the addition of
2e and 2f bearing R-glucopyranoside. In contrast, aggregates were
not obtained in the case of 2b, 2c, and 2d bearing R-galactopyra-
noside or â-glucopyranoside (Figure 4a). Aggregation of ConA with
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