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sialylated glycoproteins were selectively periodate-oxidized, captured on hydrazide
beads, trypsinized and the released peptides analyzed by mass spectrometry for
protein identification [52]. Next, the remaining glycopeptides were released by acid
hydrolysis of sialic acid glycosidic bonds and the released compounds analyzed by
MS for glycan identification. It is to be expected that this approach will be more
facile when aniline is employed to catalyze the oxime formation.
8.3 AZIDO- AND ALKYNE-BASED STRATEGIES FOR
BIOORTHOGONAL LABELING OF GLYCOCONJUGATES
Azides, which are extremely rare in biological systems, are emerging as a powerful
chemical reporter [30-33, 53, 54]. The azido group is small and therefore only mini-
mally perturbs a substrate. Furthermore, a number of attractive reaction partners for
azides have been developed that are compatible with biological systems.
A number of modified monosaccharides have been employed to introduce azides
into cell surface glycoconjugates (Fig. 8.1) [33]. In this respect, sialic acid-containing
glycans have been labeled by feeding cell
N
-azidoacetylmannosamine (ManNAz)
[44], which is an analog of the biosynthetic precursor
N
-acetylmannosamine
(ManNAc) or by employing
N
-azidoacetylneuraminic acid (SiaNAz) [55] or 9-azido-
N
-acetylneuraminic acid [56]. Mucin-type glycans have been labeled with azido
functions using
N
-azidoacetylgalactosamine (GalNAz) [57]. This unnatural sugar is
OH
O
CO
2
-
O
N
3
OH
NH
HO
HO
N
3
O
O
NH
OH
OH
HO
HO
OH
ManNAz
SiaNAz
OH
HO
N
3
CO
2
-
O
OH
OH
HO
O
OH
NH
AcHN
N
3
HO
OH
O
9-azido-SiaNAc
GalNAz
OH
N
3
O
HO
O
OH
OH
HO
OH
NH
OH
N
3
HO
O
6-azido-Fuc
GlcNAz
FIGURE 8.1
Azide-bearing monosaccharides that as their acetylated derivatives have been
employed for metabolic labeling of glycans.
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