between epoxides and DNA is of nucleophilic substitution type (S N 2 reaction) and
occurs at the nucleophilic sites in DNA, mainly at the N7 position of guanine. DNA
hydrogels are responsive systems that exhibit drastic volume changes in response to
external stimuli, such as acetone [115], poly(ethylene glycol) [112], inorganic salts
[116, 117], polyamines [117], cationic macromolecules [118], or surfactants
[117]. Responsive DNA hydrogels with a wide range of tunable properties, such
as the conformation of the network strands, viscoelasticity, and nonlinear elasticity
(strain hardening), have been prepared in the past few years [111, 112, 119].
Recently, DNA cryogels with excellent mechanical properties have been
obtained by conducting gelation reactions at subzero temperatures [47, 48]. DNA
cryogels were prepared at T prep ¼ À18
C from frozen aqueous solutions of
ds-DNA (about 2,000 base pairs long) containing BDDE crosslinker and TEMED
catalyst. Infrared (IR) and fluorescence measurements show the predominantly
ds-DNA conformation of the network chains of DNA cryogels. An almost complete
conversion of soluble to crosslinked DNA required 3 % DNA and 0.39 % BDDE,
corresponding to the presence of an equimolar ratio of epoxy to DNA base pair in
the gelation solutions [48]. DNA cryogels exhibited weight swelling ratios (q w )
between 40 and 130, indicating that they are in a highly swollen state with 98–99 %
water in the cryogel structure. However, their volume swelling ratios q v are about
one order of magnitude smaller than their weight swelling ratios q w . The total
porosity P s of the cryogels estimated from their q w and q v values using (4) is 93–
99 % for all the cryogels. The total volume of the pores V p in the cryogels, estimated
from the uptake of methanol, is between 3 and 20 mL/g, indicating that the total
pore volume of the cryogel samples is much larger than the bulk gel volume. This
suggests that most of the water in swollen cryogels is within the pores, while the
crosslinked DNA forming the pore walls is less swollen. Mechanical tests show that
the cryogels formed at 5 % DNA are very tough and can be compressed up to about
80 % strain without any crack development [48]. They also exhibit completely
reversible swelling–deswelling cycles in response to solvent changes.
Carcinogenic agents having planar aromatic groups can intercalate into DNA
double helices, which means that DNA can be used to prepare selective and
efficient adsorbents of such specific toxic agents. It was shown that DNA cryogels
can be used for the removal of such toxic agents from aqueous solutions
[47, 48]. This is exampled by use of the classical intercalator ethidium bromide
(EtBr), which is a four-ringed aromatic molecule with three of the rings conjugated.
The nature of the interaction of EtBr with DNA has been examined by a variety of
techniques over the past 50 years [120–123]. It is generally recognized that the
strong mode of binding of EtBr to ds-DNA results in the intercalation of the planar
phenanthridium ring between adjacent base pairs on the double helix. The intercalation of EtBr both increases the distance between base pairs by 0.3 nm and
unwinds the double helix by 26
o , which causes an increase in length of the DNA
[124, 125]. Moreover, at high concentrations, EtBr also interacts with DNA by
electrostatic interactions.
Figure 16a shows the EtBr concentration in the external solution (C f, t ) plotted
against the contact time t with DNA cryogels [48]. For comparison, the behavior of
Synthesis and Structure–Property Relationships of Cryogels
137
occurs at the nucleophilic sites in DNA, mainly at the N7 position of guanine. DNA
hydrogels are responsive systems that exhibit drastic volume changes in response to
external stimuli, such as acetone [115], poly(ethylene glycol) [112], inorganic salts
[116, 117], polyamines [117], cationic macromolecules [118], or surfactants
[117]. Responsive DNA hydrogels with a wide range of tunable properties, such
as the conformation of the network strands, viscoelasticity, and nonlinear elasticity
(strain hardening), have been prepared in the past few years [111, 112, 119].
Recently, DNA cryogels with excellent mechanical properties have been
obtained by conducting gelation reactions at subzero temperatures [47, 48]. DNA
cryogels were prepared at T prep ¼ À18
C from frozen aqueous solutions of
ds-DNA (about 2,000 base pairs long) containing BDDE crosslinker and TEMED
catalyst. Infrared (IR) and fluorescence measurements show the predominantly
ds-DNA conformation of the network chains of DNA cryogels. An almost complete
conversion of soluble to crosslinked DNA required 3 % DNA and 0.39 % BDDE,
corresponding to the presence of an equimolar ratio of epoxy to DNA base pair in
the gelation solutions [48]. DNA cryogels exhibited weight swelling ratios (q w )
between 40 and 130, indicating that they are in a highly swollen state with 98–99 %
water in the cryogel structure. However, their volume swelling ratios q v are about
one order of magnitude smaller than their weight swelling ratios q w . The total
porosity P s of the cryogels estimated from their q w and q v values using (4) is 93–
99 % for all the cryogels. The total volume of the pores V p in the cryogels, estimated
from the uptake of methanol, is between 3 and 20 mL/g, indicating that the total
pore volume of the cryogel samples is much larger than the bulk gel volume. This
suggests that most of the water in swollen cryogels is within the pores, while the
crosslinked DNA forming the pore walls is less swollen. Mechanical tests show that
the cryogels formed at 5 % DNA are very tough and can be compressed up to about
80 % strain without any crack development [48]. They also exhibit completely
reversible swelling–deswelling cycles in response to solvent changes.
Carcinogenic agents having planar aromatic groups can intercalate into DNA
double helices, which means that DNA can be used to prepare selective and
efficient adsorbents of such specific toxic agents. It was shown that DNA cryogels
can be used for the removal of such toxic agents from aqueous solutions
[47, 48]. This is exampled by use of the classical intercalator ethidium bromide
(EtBr), which is a four-ringed aromatic molecule with three of the rings conjugated.
The nature of the interaction of EtBr with DNA has been examined by a variety of
techniques over the past 50 years [120–123]. It is generally recognized that the
strong mode of binding of EtBr to ds-DNA results in the intercalation of the planar
phenanthridium ring between adjacent base pairs on the double helix. The intercalation of EtBr both increases the distance between base pairs by 0.3 nm and
unwinds the double helix by 26
o , which causes an increase in length of the DNA
[124, 125]. Moreover, at high concentrations, EtBr also interacts with DNA by
electrostatic interactions.
Figure 16a shows the EtBr concentration in the external solution (C f, t ) plotted
against the contact time t with DNA cryogels [48]. For comparison, the behavior of
Synthesis and Structure–Property Relationships of Cryogels
137