Biomedical Engineering Reference
In-Depth Information
Nanovesicles were prepared thereof and cytotoxic agents such as doxorubicin were
internalized for controlled drug release. (iv) Sorbi
et al.
introduced the coupling of
methotrexate to the primary alcohol of HyA for the therapy of autoimmune dis-
eases such as rheumatoid arthritis [162].
7.11
Pullulan
Pullulan is a water soluble, neutral polysaccharide consisting of maltotriose mono-
meric units interconnected by
4) glycosidic bonds, whereas maltrotriose
sequences are combined to yield a polymeric chain by
α
- (1
→
α
- (1
→
6) glycosidic linkages.
The structural formula corresponds to [
α
- d - Glc
p
- (1
→
4) -
α
- d - Glc
p
- (1
→
4) -
α
- d -
Glc
p
(1
6)]
n
. The assembled polysaccharide strain may display some minor struc-
tural abnormalities like maltotetraose; however, chemical properties are not
affected. The regular alteration of the glycosidic bonds results in two distinctive
properties: structural fl exibility and enhanced solubility. The fi rst description of
pullulan has been published by Wallenfels
et al.
in the 1960s, based on the ana-
lyzed fermentation medium of
Aureobasidium pullulans
[163] (Scheme 7.11 ).
Because of the coexistence of
→
6) -bonds in the chain, the
overall structure is an intermediate between amylose and dextran. Native water-
soluble pullulan is produced extracellularly by several strains such as
A. pullulans
,
Pullularia pullulans
, or
Dematium pullulans
. However, the bacterial cultures gener-
ate two different exopolysaccharides. Besides pullulan, a water-insoluble jelly-like
α
- (1
→
4) - and
α
- (1
→
β
3) -glucan is also produced [164]. The detailed mechanism of the biosynthesis
is not known yet, accordingly there is no explanation of the
- (1
→
- glucan formation.
Some indications adumbrate the dependency on the individual genetic type of
A.
pullulans
[165] . Local peculiarities of the biosynthesis with various
A. pullulans
strains have been investigated by Kondratyeva
et al
[166]. Pullulan can be synthe-
sized from sucrose by cell-free enzymes of
A. pullulans
when both ATP and UDPG
are added to a reaction mixture. In the case of glucose, mannose, galactose, fruc-
tose, or other carbon sources, the pathway is not clear [167]. The average molecular
β
Scheme 7.11
Chemical structure of pullulan.
