Chemistry Reference
In-Depth Information
approaches is that the tagged glycans can be enriched for further analysis by, for
example, mass spectrometry, detected by flow cytometry for quantitative studies, or
visualized by confocal microscopy to study localization and trafficking.
It has been found that hydrazones hydrolyze when internalized into vesicular
compartments where the pH is relatively low. This chemical lability can be exploited
for drug [47], gene [48], or toxin delivery [45]. It can also be employed for loading
cell with an MRI contrast agent [49]. On the other hand, oximes are much more acid
resistant and can be recycled after internalization [47].
The use of ketones as chemical reporter has a number of drawbacks such as the
requirement to perform the ligation reaction under mild acidic conditions and a limited
possibility to extend the technology to a wide range of sugar derivatives. Paulson
and coworkers have pioneered an alternative approach for introducing aldehydes
into cell-surface sialic acid residues by mild periodate oxidation followed by an
aniline catalyzed reaction with aminooxy-biotin to give biotinylated glycoconjugates
(Scheme 8.1). Importantly, it has been found that aniline accelerates oxime formation
by approximately 10-fold, and furthermore can be performed under neutral conditions
[50, 51]. Reagents to implement the periodate oxidation aniline catalyzed oxime
ligation (PAL) are commercially available. Furthermore, the use of aniline makes it
possible to reduce the amount of aminooxy reagent by 10- to 50-fold compared to
what typically is employed in uncatalyzed reactions.
Selective oxidation of sialic acids has also been employed for enrichment of
glycoproteins for analysis by mass spectrometry. In the most advanced approach,
OH
CO
2
-
OH
OH
HO
O
O
AcHN
O
O
OH
HO
OH
Mild NaIO
4
CO
2
-
O
OH
HO
O
O
AcHN
O
O
OH
OH
O
O
TAG
H
2
N
H
O
aniline
TAG
O
CO
2
-
H
N
OH
HO
O
O
AcHN
O
O
OH
OH
SCHEME 8.1
Covalent labeling of sialylated glycoconjugates by mild periodate cleavage
followed by aniline mediated oxime ligation. The tag can be biotin or a fluorescent probe.
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