Biomedical Engineering Reference
In-Depth Information
However, the generation of reactive dendrimer is limited owing to steric hindrance. On the
other hand, method B, with binding of chitosan to the dendrimer surface, allows the use of
commercially available amino-dendrimers such as PAMAM or poly(ethylene imine) den-
drimers; and binding is possible even for high generations. One weak point in method B is
that it has two or more binding points that may sometimes cause cross-linking. A typical
example of a hybrid obtained by methods A and B is shown in
Figure 2.44
[318,320]. In this
case, tetraethylene glycol was modified in 5-7 steps, to synthesize the scaffold of the den-
drimer. PAMAM dendrimers of generations (G) from 1 to 3 bearing tetraethylene glycol
spacers were attached to sialic acid by reductive N-alkylation, and finally attached to chi-
tosan. The DS of dendrimer per sugar unit decreased with increasing generation [0.08
(
G
= 1), 0.04 (
G
= 2), and 0.02 (
G
= 3)] owing to steric hindrance of the dendrimers. Figure
2.45 shows a different type of chitosan-dendrimer hybrid [319]. A sialic acid dendron bear-
ing a focal aldehyde end group was synthesized by a reiterative amide bond strategy.
Trivalent (
G
= 1) and nanovalent (
G
= 2) dendrons having gallic acid as the branching unit
and triethylene glycol as the spacer arm were prepared and initially attached to a sialic
acid
p
-phenylisothiocyanate derivative. The focal aldehyde sialodendrons were then con-
vergently attached to chitosan. The DS values of sialodendrimer were 0.13 (
G
= 1) and 0.06
(
G
= 2). The water solubility of these novel hybrids was further improved by N-succinylation
of the remaining amine functionality.
OH
OH
OH
O
O
O
O
O
O
HO
HO
HO
NHR
RHN
NHAc
0.74
0.2
0.06
NH
O
O
O
O
O
O
O
O
O
O
O
NH
O
HN
O
O
NH
S
O
O
O
O
O
S
O
O
O
O
N
H
OH
HN
NHAc
HN
N
H
O
O
O
O
O
O
O
OH
OH
O
O
NaO
2
C
O
O
O
S
CO
2
Na
NHO
O
O
O
H
O
O
S
O
NH
HO
O
HN
O
OH
NHAc
HN
HN
HO
HO
O
S
HN
HN
HN
O
NaO
2
C
OH
S
S
O
HN
NaO
2
C
HN
NHAc
HN
O
S
S
HN
O
HN
NaO
2
C
OH
HO
HO
OH
O
HN
HO
O
HO
O
OH
O
O
NHAc
O
NaO
2
C
NaO
2
C
NHAc
HO
OH
O
NaO
2
C
HO
NaO
2
C
O
HO
HO
O
OH
O
NHAc
OH
HO
OH
OH
NHAc
HO
HO
HO
HO
NHAc
HO
NHAc
HO
HO
OH
HO
HO
Figure 2.45
Hybridization of chitosan with sialodendrimer, composed of gallic acid as junction point.
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