Carbohydrate Analysis
267
O
CH
2
OH
HO
OH
OH
O
O
CH
2
OH
OH
OH
O
O
CH
2
OH
OH
OH
OH
n
Dimsyl anion/CH
3
I
(1)
O
CH
2
OMe
MeO
OMe
OMe
O
O
CH
2
OMe
OMe
OMe
O
O
CH
2
OMe
OMe
OMe
OMe
n
Hydrolysis (2)
O
CH
2
OMe
HO
OMe
OMe
OH
NaBD
4
Acetic anhydride
(3)
D
C
OAc
H
C
H
OMe
C
MeO
H
C
H
AcO
C
H
OAc
C
H
OMe
H
GC-MS
(4)
D
C
OAc
H
C
H
OMe
C
MeO
H
+
C
H
OMe
H
+
D
C
OAc
H
C
H
OMe
C
MeO
H
C
+
AcO
H
+
C
H
OAc
C
H
OMe
H
m
/
z
233.1
m
/
z
118.1
m
/
z
45.0
m
/
z
162.1
Fig. 8
Methylation analysis. In this technique all
the free hydroxyl groups in the intact glycan are
methylated (1). The fully methylated glycan is
then hydrolyzed (2), reduced, and acetylated (3).
The linkage positions may be deduced from the
positions of the methyl and acetyl groups in the
resultant partially methylated alditol acetates, as
determined using GC-MS. The ring hydrogen
atoms are omitted for the sake of clarity. The
breakdown of such compounds in EI-MS occurs
primarily between neighboring methyl ethers.
Secondary fragmentation then occurs by the
elimination of the neutral molecules acetic acid
(CH
3
COOH,
60 Da), methanol (CH
3
OH,
32 Da), ketene (CH
2
CO,
42 Da), and
formaldehyde (CH
2
O,
30 Da) from these
primary fragments. Thus, the
m
/
z
162 fragment
gives rise to an
m
/
z
102 peak. The deuterium
atoms in these structures distinguish the C-1
atoms from the C-6 atoms.
of water, the DMSO is converted into the
dimsyl anion, which is then able to con-
vert all the free hydroxyl, carboxyl, and
amino groups to their anions. Methylation
of the polyanion is then carried out using
methyl iodide. At the end of the reaction,
the methylated product is hydrolyzed with
acid to its component partially methylated
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