Chemistry Reference
In-Depth Information
110
[Hex
2
Li]
+
349
100
YZ
88
O
B
1
[Hex-H
2
O+Li]
+
O
75
O
63
%
Y
1
[Hex+Li]
+
50
Y
1
[Hex+H
2
O+Li]
+
BC
38
25
169
187
205
13
222
0
150
180
200
220
240
260
280
300
320
340
360
380
400
m/z
FIGURE 5.6
The mass spectrum of sucrose showing a lithiated pseudo-molecular ion and
subsequent fragment ions.
glucose/fructose moieties, respectively. Other peaks represent small fragments of the
monomer: ion peaks with mass m/z 89 and m/z 118 in the sucrose spectrum originate
from further fragmentation of glucose; and m/z 101 and m/z 143 originate as frag-
ment ions derived from fructose.
5.5 POLYSACCHARIDES
Monosaccharides are linked to other sugars through glycosidic linkages to form
polysaccharides (carbohydrates). Glycosides can be hydrolyzed to simple building
blocks by rupturing the glycosidic bonds. One important class of polysaccharide is
represented by cellulose. Cellulose has the formula (C
6
H
10
O
5
)
n
, where n ranges from
500 to 5000, depending on the source of the polymer. Over half of the total organic
carbon in the earth's biosphere is in cellulose, for example, cotton and fibers. Wood
of bushes and trees contains roughly 50% cellulose.
5.6 GALACTOMANNAN
An important group of polysaccharides is represented by the galactomannans.
These polymers contain the monosaccharides mannose and galactose. These mono-
mers are 1-4-β-D-mannopyranose linked and they also have branch points from
their six positions linked to α-D-galactose. A representative structure indicating
the linkages is shown in Figure 5.7.
Galactomannans are found in guar gum where
the mannose:galactose ratio is approximately 2:1. They are often added into food
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