of the polycation, the stability of the polyplex will be more limited in an aqueous
environment.
If the concentration of the amphiphilic polymer in the sample is above its CMC,
there are two cases to consider: the micellar solution is added to the DNA solution
or the DNA solution is added to a micellar solution. In the first case, where the
micellar solution of polycation is added to DNA, the micelles are immediately
diluted in the DNA solution, which on one hand can mean that the polymer
concentration is under its CMC and therefore that the polymer is only present as
individual chains in solution (unimers), on the other hand, that there is concurrence
between self-assembly of micelles versus electrostatic interactions with a large
quantity of negatively charged material. In this case, the micelles are usually
destabilized as soon as they reach the DNA solution (Scheme 11, case 1).
In the case of DNA added to a micellar solution of polycation, the micelles can
stay stable in some cases: (it depends on the hydrophobicity of the micellar core and
strength of the electrostatic interactions). This can be proven if pyrene or other
hydrophobic molecules entrapped in the hydrophobic interior are not released even
after addition of DNA [77]. It has to be noted that micelles are in thermodynamic
equilibrium with unimers and that both species can form electrostatic interactions
with DNA. When the micelles do not undergo a structural change, no rearrangement into a “scrambled eggs” structure takes place between the amphiphilic polycation and DNA, and because DNA is in minority, it adds to the positive shell of the
structure until neutralization (Scheme 11, case 2).
Influence of Salts
After changes in ionic strength (due to the addition of salt), swelling or deswelling
of IPECs occurs immediately, whereas coagulation (i.e., destabilization of colloids
by neutralizing the electric charge of the dispersed nanoparticles, which results in
aggregation of the colloidal particles) is a much slower process and is dependent on
the concentration of the colloidal particles [74]. Two major effects on the formation
of IPECs in the presence of salt were found by Dautzenberg [77]. On the one hand,
the presence of a very small amount of salt during formation dramatically decreased
the level of aggregation, probably due to the less stiff and more coiled structure that
the polymers can adopt. On the other hand, a higher ionic strength resulted in
macroscopic flocculation, explained by the contribution of two factors: particle
swelling because of charge screening of the stabilizing outer shell and particle
aggregation due to colloidal instability. However, the internal structure of most
IPECs is marginally affected by salt [77]. With a further increase in ionic strength,
the point is reached where charges are screened at the level of the polymers, and
polycations and DNA are dissolved as individual polymers.
As already mentioned, counterions seem to be important for the interaction
between polyelectrolytes (uni- or multivalent) and the specific ions involved
(size, chaotropic/kosmotropic) [78, 79].
122
A. Bertin
environment.
If the concentration of the amphiphilic polymer in the sample is above its CMC,
there are two cases to consider: the micellar solution is added to the DNA solution
or the DNA solution is added to a micellar solution. In the first case, where the
micellar solution of polycation is added to DNA, the micelles are immediately
diluted in the DNA solution, which on one hand can mean that the polymer
concentration is under its CMC and therefore that the polymer is only present as
individual chains in solution (unimers), on the other hand, that there is concurrence
between self-assembly of micelles versus electrostatic interactions with a large
quantity of negatively charged material. In this case, the micelles are usually
destabilized as soon as they reach the DNA solution (Scheme 11, case 1).
In the case of DNA added to a micellar solution of polycation, the micelles can
stay stable in some cases: (it depends on the hydrophobicity of the micellar core and
strength of the electrostatic interactions). This can be proven if pyrene or other
hydrophobic molecules entrapped in the hydrophobic interior are not released even
after addition of DNA [77]. It has to be noted that micelles are in thermodynamic
equilibrium with unimers and that both species can form electrostatic interactions
with DNA. When the micelles do not undergo a structural change, no rearrangement into a “scrambled eggs” structure takes place between the amphiphilic polycation and DNA, and because DNA is in minority, it adds to the positive shell of the
structure until neutralization (Scheme 11, case 2).
Influence of Salts
After changes in ionic strength (due to the addition of salt), swelling or deswelling
of IPECs occurs immediately, whereas coagulation (i.e., destabilization of colloids
by neutralizing the electric charge of the dispersed nanoparticles, which results in
aggregation of the colloidal particles) is a much slower process and is dependent on
the concentration of the colloidal particles [74]. Two major effects on the formation
of IPECs in the presence of salt were found by Dautzenberg [77]. On the one hand,
the presence of a very small amount of salt during formation dramatically decreased
the level of aggregation, probably due to the less stiff and more coiled structure that
the polymers can adopt. On the other hand, a higher ionic strength resulted in
macroscopic flocculation, explained by the contribution of two factors: particle
swelling because of charge screening of the stabilizing outer shell and particle
aggregation due to colloidal instability. However, the internal structure of most
IPECs is marginally affected by salt [77]. With a further increase in ionic strength,
the point is reached where charges are screened at the level of the polymers, and
polycations and DNA are dissolved as individual polymers.
As already mentioned, counterions seem to be important for the interaction
between polyelectrolytes (uni- or multivalent) and the specific ions involved
(size, chaotropic/kosmotropic) [78, 79].
122
A. Bertin