Chemistry Reference
In-Depth Information
CO
2
H
CO
2
H
OSO
3
H
OSO
3
H
OSO
3
H
O
O
O
O
O
O
3
O
HCl (aq)
HO
O
HO
O
HO
O
HO
HO
HO
3
SO
HO
HO
3
SO
HO
3
SHN
HO
3
SHN
HO
3
SHN
OR
OR
145
146
OR
147
Scheme 31
small polysaccharides, which have narrower size distribution, compared
to those obtained by classical mild acid hydrolysis or enzymatic deg-
radation. Hu et al. obtained oligoguluronates by microwave-assisted
degradation of polyguluronates without any addition of organic or in-
organic acids in high yield (71%).
91
The process is clean, fast (15 min),
economical, environmental-friendly and eliminates the desalting pro-
cedure required in classical acid hydrolysis processes. Oligosaccharides
with DP 1-10 were obtained after Biogel fractionation as single com-
pounds as shown by ESI-MS analysis. In 2009, another acid-free protocol
was reported.
92
The solvolytic depolymerization of CS and DS using a
mixture of DMSO/MeOH 9 : 1 led to DP 4-20 oligosaccharides after frac-
tionation on Biogel. Nevertheless, this method was accompanied by
desulphation, preventing the preservation of the original substitution
pattern. However, this can be an advantage for the purification of the
obtained molecules. Subcritical water microreaction systems are also a
way to avoid acids. Indeed, subcritical water is a fluid that shows higher
proton concentration than liquid water, and so it is able to promote acid-
catalyzed reactions. This method was for the first time applied to
the depolymerisation of sulphated polysaccharides such as CS by
Yamada et al.
93
Interestingly, CS oligosaccharides of DP 4 to 12 were
obtained without desulphation. Other chemical reagents can also be
used for the degradation of polysaccharides. For example, free radical
depolymerisation, already used for the degradation of xyloglucan, fucan
and HA, was applied to polygalacturonic acid (PGA) using H
2
O
2
/cop-
per(II) system (Fenton reaction). This method led to oligogalacturonic
acids (OGA) with DP up to 6 after fractionation by Biogel.
94
This work
highlighted the possibility of operating on a large scale keeping the
polymer sequence intact without the drawbacks generally observed for
harsh processes. On the other hand, oligosaccharides obtained by
chemical or enzymatic b-elimination generally contain a C4-C5 double
bond in the non-reducing terminal residue which contributes to strong
antigenicity. Ozonolysis and subsequent acid hydrolysis (HCl) were ap-
plied to anionic and particularly sulphated oligosaccharides for the re-
moval of unsaturated residues without modification of the core
structure leading to the corresponding oligosaccharides of DP (n-1), see
Scheme 31. Especially, neither hydrolysis of the other glycosidic linkages
nor desulphation was observed.
95
3.2 Enzymatic depolymerisation
A wide variety of enzymes can be used to degrade anionic polysaccharides
such as bacterial heparinases and tumoral heparanases, which degrade
Search WWH ::
Custom Search