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Carbohydrates 397

tion, because they possess identical monomers all through
their chains. Heterooligosaccharides, except nonreducing
disaccharides, must have the sequence of their monomers
determined. In the case of reducing heterodisaccharides,
it must be determined because the monomer at the nonre-
ducing end of the molecule is different from the one at
the reducing terminus, and the position of both monomers
must be determined. It was stated above that nonreducing
disaccharides do not require a monosaccharide sequence
determination. This is because the two monomers are lo-
cated at two similar (nonreducing) ends of a chain that
has no beginning (no reducing terminus). On the other
hand, nonreducing heterooligosaccharides with DP > 2
are made up of a monosaccharide linked glycosidically
to the hemiacetal function of a reducing oligosaccharide,
the sequence of which must be determined.
FIGURE 13 Liquid chromatogram of partially hydrolyzed starch The methods available for determining the monomer
showing glucose and the maltooligosaccharides. sequence are presented here, in order of increasing DP of
the oligosaccharides, starting with disaccharides.

2. The Monosaccharide Components
a. Monosaccharide sequence in disaccharides.
The nature of the monomers in an oligosaccharide is es- The sequence of monomers in disaccharides is deter-
tablished by identifying the monosaccharides liberated by mined when the monomer located at the reducing ter-
acid-catalyzed hydrolysis. Homooligosaccharides, which minus of the molecule is identified. This automatically
are composed of one type of monosaccharides, afford only determines the position of the other monomer (it must be
one monosaccharide, which can be isolated from the hy- located at the nonreducing end). To identify the monosac-
drolysate by conversion to crystalline derivatives. On the charide located at the reducing terminus, use is made of
otherhand,heterooligosaccharidesafford,onhydrolysis,a the fact that this saccharide moiety exists in equilibrium
mixture of monosaccharides, which must be separated by with an acyclic form and is therefore much more sus-
chromatography. Paper, thin-layer, or high-performance ceptible to oxidants and reducing agents than the other
liquid chromatography (HPLC) can be used without pre- moiety. Thus, mild oxidation converts reducing disac-
treating the monosaccharides, but a gas-chromatographic charides to aldobionic acids, which on hydrolysis afford
separation requires prior silylation of the saccharides, to aldonic acids (from the reducing termini) and reducing
volatilize them. It should be noted that silylated monosac- monosaccharides (from the nonreducing ends). Reduc-
charides appear as double peaks (one peak for the α tion affords aldobiitols, which on hydrolysis yield alditols,
anomer and one for the β), which tends to crowd the chro- from the reducing moieties and monosaccharides from
matographs. To avoid this complication, the monosaccha- the nonreducing moieties. The use of this method of
rides can be reduced before silylation (silylated alditols structure determination is exemplified by the oxidation
appear as single peaks). It should also be noted that all and reduction of melibiose [6-(α-D-galctopyranosyl)-D-
the above-mentioned chromatographic techniques cannot glucopyranose] to give, in the first case, melibionic acid
differentiate between D and L isomers, which mandates [6-(α-D-galactopyranosyl)-D-gluconic acid] and, in the
that polarimetric measurements (optical rotation, optical second, melibiitol [6-(α-D-galactopyranosyl)-D-glucitol]
rotatory dispersion or circular dichroism) of the monosac- (Scheme 20). Hydrolysis of these affords D-galactose from
charide component(s) or of their derivatives be included the nonreducing end of the molecule and D-gluconic acid
in the structure elucidation of new oligosaccharides. In the in the first case and D-glucitol in the second (both formed
case of heterooligosaccharides, this necessitates prepara- from the reducing half of the molecule). This experiment
tive chromatography of the hydrolysate before the optical indicates that D-galactose is the monomer located at the
measurements are taken. nonreducing end of the dimer and that D-glucose is the
one at the reducing terminus.

3. The Monosaccharide Sequence
b. Monosaccharide sequence in trisaccharides.
Homooligosaccharides (whether reducing or nonreduc- Because it is easier to determine the structure of disaccha-
ing) do not require a monosaccharide sequence determina- rides than it is to investigate the structure of trisaccharides,