Chemistry Reference
In-Depth Information
21.4.2
2 D /3 D Polyvalent Model Systems
To generate polyvalent 2D model systems neoglycoconjugates of the natural epi-
topes are deposited on gold surfaces to form self - assembled monolayer s ( SAM s)
[19] that can mimic, for instance, the multivalent arrays of GSL patches in the cell
membrane [20]. Polyvalent 3D model systems are mainly based on carbohydrate-
modifi ed gold nanoparticles [19, 21]. Such glyconanoparticles (GNPs) provide a
glycocalix-like surface with globular carbohydrate display and chemically well-
defi ned composition optimally suited to study cooperative carbohydrate interac-
tions [22]. GNPs can be easily prepared by
in situ
reduction of a gold salt in the
presence of an excess of the corresponding thiol- derivatized carbohydrate [21] .
These GNPs can have a core down to only 2 nm across, and are highly water-
soluble and stable for months without fl occulation under physiological conditions
[20]. Manipulation of the ratio of the ligands permits preparation of GNPs with
differing carbohydrate density at the surface, providing a versatile model for
investigating the effect on molecular recognition events of epitope clustering and
presentation.
Functionalized liposomal nanoparticles have also been applied to the analysis
of carbohydrate-carbohydrate associations. Lipid vesicle micromanipulation with
the micropipette aspiration technique was used to study Le
x
interactions (see
Section 21.2.3) [23]. The results obtained revealed a Ca
2+
- dependent self - binding
of monomeric Le
x
, whereas dimeric Le
x
exhibited a repulsive behavior in the pres-
ence of Ca
2+
. The lactose-GM3 interaction involved in B16 melanoma cell adhe-
sion and signaling processes (see Section 21.2.2) was investigated using micelles
of a lactosyl lipid and monolayers of GM3 [24]. In the absence of divalent cations
the lactose-GM3 binding was strengthened at higher NaCl concentrations in
the subphase of the monolayer, whereas when divalent cations were present the
process was not as sensitive to ionic strength. These results suggested a role for
both cation- dependent as well as - independent lactose - GM3 interactions.
21.4.2.1
Surface Plasmon Resonance (SPR )
SPR spectroscopy is an evanescent-wave biosensor technology that monitors the
interaction of two or more molecules or molecular assemblies in real- time [25] . In
SPR, a ligand is immobilized on the surface of a gold-coated sensor chip, whereas
the corresponding binding partner, or analyte, is carried in a fl ow of buffer solu-
tion along a miniature fl ow cell. Light from a laser source arriving through a prism
at the angle of total internal refl ection induces a nonpropagative evanescent wave
that penetrates into the cell opposite the prism. At a given angle dependent upon
the refractive index of the solution, resonance between the evanescent wave and
free electrons in the gold layer results in a reduction in the intensity of refl ected
light. The change in angle of reduced intensity refl ects changes in the refractive
index of the solution in the fl ow cell immediately adjacent to the gold layer. A
dextran layer coupled to the gold surface allows immobilization of ligands within
the evanescent fi eld. Any binding event on the surface of the sensor chip leads to
