Page 87 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Carbohydrates 395

corn starch. Now three additional products produced from spectra produced by electron impact seldom show the
starch have entered the market and lowered the consump- molecular ions of saccharides and that other ionization
tion of sucrose. They are fructose syrup, very high fruc- methods (e.g., chemical ionization) must be used to re-
tose syrup, and crystalline fructose. As a result, sucrose veal these ions. The silyl ethers of oligosaccharides or of
can account for only half the sweeteners consumed in the their alditols (obtained by reduction and silylation) are of-
United States. Oligosaccharides are grouped into simple ten used for mass spectrometric measurements, since they
(or true) oligosaccharides, which yield on deploymeriza- are more volatile than free oligosaccharides. Alternatively,
tion monosaccharides only; and conjugate oligosaccha- peracetylated disaccharides or esters of disaccharides or
rides, which are linked to nonsaccharides such as lipids of their glycosides can be used in the chemical ionization
and afford on deploymerization monosaccharides and mode. Figure 12 shows three mass spectra of a disac-
aglycons. The simple oligosaccharides are further classi- charide glycoside obtained by negative chemical ioniza-
fied (1) according to DP, into disaccharides, trisaccharides, tion (a), by electron impact (b), and by positive chemical
tetrasaccharides, and so on; (2) according to whether they ionization (c). Only the chemical ionization mass spectra
are composed of one or more types of monosaccharides, (negative and positive) revealed molecular ions.
into homo- and heterooligosaccharides; and (3) according
to whether they do or do not possess a hemiacetal func- b. Chromatography. It is often possible to deter-
tion at one terminus of the molecule, into reducing and mine the DP of members of a homologous series of poly-
nonreducing oligosaccharides. meric saccharides from their position on chromatograms
Related homooligosaccharides can form homologous relative to a known member of the series. For example,
series; a homologous series of oligosaccharides is a group partial hydrolysis of starch affords D-glucose and a ho-
of similarly linked oligosaccharides that are composed of mologous series of maltooligosaccharides, consisting of
the same monomer and whose DP increases in the se- di-, tri-, tetra-, pentasaccharides, and so on. This mix-
ries one unit at a time. When homopolysaccharides are ture separates on chromatographs according to DP. On pa-
partially hydrolyzed, they often afford homologous series per chromatographs and on high-performance liquid chro-
of oligosaccharides. For example, the maltooligosaccha- matographs, the mobility of the saccharides is inversely
rides obtained by partial hydrolysis of starch comprise proportional to their DP (i.e., the largest oligomers move
dimers, trimers, tetramers, and so on composed of α-D- slowest), and on gel filtration chromatography, it is di-
glucopyranose units linked by 1 → 4 acetal bonds. rectly proportional to DP (i.e., the largest oligomers move
fastest). The fact that the oligomers are eluted in the order
of increasing or decreasing DP, makes it possible to deter-
A. Structure
minetheirDPbydeterminingtheirorderofelutionrelative
The complete structure of oligosaccharides is established to a known member. Thus, by recognizing maltose (and
when the following points are determined: glucose) in chromatograms of partially hydrolyzed starch
(Fig. 13), it is possible to identify the subsequent bands as
1. The DP, that is, the number of monosaccharide units maltotriose, maltotetrose, maltopentose, and so on.
present in the oligomer molecule
2. The nature of the monosaccharide monomer(s) c. Reducing power. The DP of reducing oligosac-
3. In the case of heterooligosaccharides, the monosac- charides, particularly those with low molecular weight,
charide sequence can be determined from their reducing power. For exam-
4. The ring size (pyranose or furanose) and the position ple, the reducing power of a maltooligosaccharide relative
of linkage of the different monosaccharides (1 → ?) to glucose (taken as 100%) is matched with the calculated
5. The anomeric configuration (α or β) values for oligomers having different DPs. Thus, a disac-
6. The conformation of the monosaccharide rings charide would be expected to have about half the reducing
power of glucose (actually 53%), a trisaccharide, a third
(actually 35%), and a tetrasaccharide, a fourth (26%) that
1. Determination of the Degree of Polymerization
of glucose, and so on. Accordingly, if a maltooligosac-
The following procedures are recommended for determin- charide exhibits a reducing power equal to 35.8% that
ing the DP of oligosaccharides: of D-glucose, it can be safely assumed that it is a trisac-
charide. The differences in the reducing powers of suc-
a. Mass spectrometry. If the molecular weight of cessive members of a homologous series of oligosaccha-
the oligosaccharide is less than 1000, mass spectrome- rides decrease with increasing DP, so that beyond a DP
try can be used to determine its molecular weight and of 5, the differences become too small for reliable DP
hence its DP. It must be remembered, however, that mass measurements.