A study by Kataoka and colleagues of PDMAEMA-b-PEG (Fig. 12a), which had
around the same ratio of DMAEMA:PEG as in the previous study but was twice as
long [189], also showed the formation of micellar structures. There was a steep
decrease in the size of these micelles with increasing N:P ratio, with a leveling off
to approximately 95 nm in diameter at a N:P ratio of about 3, which was not only
ascribed to DNA condensation (according to EtBr quenching, a fully condensed
state occurred at N:P ¼ 1.2) but also to a reduction in the association number to
form non-stoichiometric micellar structures similarly to polyplexes based on
PLL-b-PEG [190]. The presence of PEG in the polyplexes both increased the
stability of pDNA against DNAse I and the affinity of the PDMAEMA segment
for pDNA (exchange reaction with polyanions). Indeed, PEG provides a protecting
layer by decreasing the permittivity of the microenvironment. The authors studied
the transfection efficiency of this copolymer compared to PDMAEMA on HEK293
cells. In contrast to Stolnik and colleagues [182], they found a slightly better
transfection efficiency of PDMAEMA-b-PEG compared to PDMAEMA, even at
N:P ratio of 5 (and higher), possibly due to the longer chain length of the copolymer
(or to the cell line). The authors also reported an unusual association of excess block
copolymers on the micelles above the charge stoichiometric point, leading to an
increase in zeta potential that finally leveled off at N:P ¼ 15 (zeta potential of
17 mV), which could also be an explanation for the better transfection efficiency.
Indeed, positively charged micellar polyplexes may benefit from a facilitated
association with the cellular surface but, unfortunately, the comparison is difficult
because the zeta potential was not reported in the studies of Stolnik and colleagues
[182, 186]. Unfortunately, this improved performance was also associated with an
increased toxicity compared to PDMAEMA.
For a similar PDMAEMA-b-PEG (Fig. 12a), Tam et al. took into consideration
the critical micelle concentration (CMC; determined previously to be in the range
2 μg mL
À1 ) of the polymer to explain the changes in the size and shape of the
polyplexes (Fig. 14) [191]. The results corresponded to those of Kataoka, with a
maximum condensation at N:P ¼ 1.2 (EtBr quenching and zeta potential). As the
polymer solution was added to the naked DNA (R h ¼ 56 nm), at concentration
below its CMC (shifted to higher values due to the presence of DNA), the polymer
existed as a free cationic unimer that bound to the DNA to form complexes with a
worm-like Gaussian structure. With the addition of more polymer, the polyplexes
grew in size to around 90 nm (R h ) probably due to secondary aggregation of the
neutral nanoparticles (zeta potential). Above a N:P ratio of 1, the size of the
polyplexes decreased and excess unbound polymeric unimers started to appear in
the solution (DNA + 0.02 mg mL
À1 polymer, i.e., N:P ratio of 2). At a polymer
concentration above 0.06 mg mL
À1 , the polyplexes underwent significant structural
rearrangements to form spherical aggregates of R h % 35 nm, which was probably
due to polymer aggregation above its CMC, accompanied by a coil–globule transition of the DNA molecules. The difference in the CMC with and without DNA can
be attributed to two reasons. First, the DNA forms strong hydrogen bonds with
water, which can “break” the water structure and consequently increase the CMC.
Second, since binding between the polymer and DNA took place once the polymer
158
A. Bertin
around the same ratio of DMAEMA:PEG as in the previous study but was twice as
long [189], also showed the formation of micellar structures. There was a steep
decrease in the size of these micelles with increasing N:P ratio, with a leveling off
to approximately 95 nm in diameter at a N:P ratio of about 3, which was not only
ascribed to DNA condensation (according to EtBr quenching, a fully condensed
state occurred at N:P ¼ 1.2) but also to a reduction in the association number to
form non-stoichiometric micellar structures similarly to polyplexes based on
PLL-b-PEG [190]. The presence of PEG in the polyplexes both increased the
stability of pDNA against DNAse I and the affinity of the PDMAEMA segment
for pDNA (exchange reaction with polyanions). Indeed, PEG provides a protecting
layer by decreasing the permittivity of the microenvironment. The authors studied
the transfection efficiency of this copolymer compared to PDMAEMA on HEK293
cells. In contrast to Stolnik and colleagues [182], they found a slightly better
transfection efficiency of PDMAEMA-b-PEG compared to PDMAEMA, even at
N:P ratio of 5 (and higher), possibly due to the longer chain length of the copolymer
(or to the cell line). The authors also reported an unusual association of excess block
copolymers on the micelles above the charge stoichiometric point, leading to an
increase in zeta potential that finally leveled off at N:P ¼ 15 (zeta potential of
17 mV), which could also be an explanation for the better transfection efficiency.
Indeed, positively charged micellar polyplexes may benefit from a facilitated
association with the cellular surface but, unfortunately, the comparison is difficult
because the zeta potential was not reported in the studies of Stolnik and colleagues
[182, 186]. Unfortunately, this improved performance was also associated with an
increased toxicity compared to PDMAEMA.
For a similar PDMAEMA-b-PEG (Fig. 12a), Tam et al. took into consideration
the critical micelle concentration (CMC; determined previously to be in the range
2 μg mL
À1 ) of the polymer to explain the changes in the size and shape of the
polyplexes (Fig. 14) [191]. The results corresponded to those of Kataoka, with a
maximum condensation at N:P ¼ 1.2 (EtBr quenching and zeta potential). As the
polymer solution was added to the naked DNA (R h ¼ 56 nm), at concentration
below its CMC (shifted to higher values due to the presence of DNA), the polymer
existed as a free cationic unimer that bound to the DNA to form complexes with a
worm-like Gaussian structure. With the addition of more polymer, the polyplexes
grew in size to around 90 nm (R h ) probably due to secondary aggregation of the
neutral nanoparticles (zeta potential). Above a N:P ratio of 1, the size of the
polyplexes decreased and excess unbound polymeric unimers started to appear in
the solution (DNA + 0.02 mg mL
À1 polymer, i.e., N:P ratio of 2). At a polymer
concentration above 0.06 mg mL
À1 , the polyplexes underwent significant structural
rearrangements to form spherical aggregates of R h % 35 nm, which was probably
due to polymer aggregation above its CMC, accompanied by a coil–globule transition of the DNA molecules. The difference in the CMC with and without DNA can
be attributed to two reasons. First, the DNA forms strong hydrogen bonds with
water, which can “break” the water structure and consequently increase the CMC.
Second, since binding between the polymer and DNA took place once the polymer
158
A. Bertin