3 Polyampholyte/DNA Complexes
Ampholytes are amphoteric molecules that contain both acidic and basic groups
and exist mostly as zwitterions in a certain pH range. The pH at which the average
charge is zero is known as the molecule’s isoelectric point.
3.1 Polyzwitterions
A zwitterion is a neutral molecule with a positive charge introduced via a cationic
functional group such as quaternary ammonium or phosphonium, which bears no
hydrogen atom, and with a negative electrical charge introduced via a functional
group such as carboxylate, which may not be adjacent to the cationic site. The
overall neutrality of the molecule arises via a kind of intramolecular acid–base
reaction between, for instance, an ammonium and a carboxylate group. Good
examples of zwitterions are phosphorylcholine and betaines.
3.1.1 Polycarboxybetaines
The complexation of DNA with poly(pyridinio carboxylate)s with various lengths
of additional alkyl chain at the α carbon and a carboxylate always in β position
(Fig. 23a), also called polycarboxybetaines (PCB), was studied by Izumrudov et al.
[225]. Given that these PCB possess quaternary ammoniums that are charged at
any pH, these polymers indeed proved to be soluble in the pH range 2–11. For the
pH range where DNA remains in the native state (4.0–10.0) these PCB were not
able to bind DNA strongly enough to squeeze out EtBr (~15% reduction in
fluorescence at N:P ratio of 2), which was explained by the authors by the amino
group and the carboxylate group in β position forming a rather stable ionic pair.
The influence of the length of additional alkyl chain at the α carbon was negligible.
More interesting are poly(pyridinio carboxylate)s with various lengths of alkyl
chain (m) between charges (PCB m , Fig. 23b) [226]. Even with increasing the length
of the spacer between the quaternary ammonium and the carboxylate group
(m ¼ 1–8), a stable ion pair was formed in neutral and weakly acidic media,
reflected by the hindered protonation of the carboxylate group (as shown by
potentiometric titration), and could lead to a potential inhibition of their electrostatic interactions with DNA. Despite the similar potentiometric behavior of the
PCB m series, the length of the spacer in these polybetaines had an influence on their
complexation with DNA. At N:P ratio of 5, PCB 5 showed a good propensity to
exclude EtBr at pH 9 (reduction of 75% of fluorescence), followed by PCB 2
(reduction of 15% in fluorescence), while PCB 4 and PCB 8 showed similar
behaviors (less than 5% reduction in fluorescence). The polyplexes followed the
same trend in the dissociation of the complex in presence of salt (NaCl). The
authors suggested that the propensity of PCB m (for m ¼ 3 and 4) to form betaine
176
A. Bertin
Ampholytes are amphoteric molecules that contain both acidic and basic groups
and exist mostly as zwitterions in a certain pH range. The pH at which the average
charge is zero is known as the molecule’s isoelectric point.
3.1 Polyzwitterions
A zwitterion is a neutral molecule with a positive charge introduced via a cationic
functional group such as quaternary ammonium or phosphonium, which bears no
hydrogen atom, and with a negative electrical charge introduced via a functional
group such as carboxylate, which may not be adjacent to the cationic site. The
overall neutrality of the molecule arises via a kind of intramolecular acid–base
reaction between, for instance, an ammonium and a carboxylate group. Good
examples of zwitterions are phosphorylcholine and betaines.
3.1.1 Polycarboxybetaines
The complexation of DNA with poly(pyridinio carboxylate)s with various lengths
of additional alkyl chain at the α carbon and a carboxylate always in β position
(Fig. 23a), also called polycarboxybetaines (PCB), was studied by Izumrudov et al.
[225]. Given that these PCB possess quaternary ammoniums that are charged at
any pH, these polymers indeed proved to be soluble in the pH range 2–11. For the
pH range where DNA remains in the native state (4.0–10.0) these PCB were not
able to bind DNA strongly enough to squeeze out EtBr (~15% reduction in
fluorescence at N:P ratio of 2), which was explained by the authors by the amino
group and the carboxylate group in β position forming a rather stable ionic pair.
The influence of the length of additional alkyl chain at the α carbon was negligible.
More interesting are poly(pyridinio carboxylate)s with various lengths of alkyl
chain (m) between charges (PCB m , Fig. 23b) [226]. Even with increasing the length
of the spacer between the quaternary ammonium and the carboxylate group
(m ¼ 1–8), a stable ion pair was formed in neutral and weakly acidic media,
reflected by the hindered protonation of the carboxylate group (as shown by
potentiometric titration), and could lead to a potential inhibition of their electrostatic interactions with DNA. Despite the similar potentiometric behavior of the
PCB m series, the length of the spacer in these polybetaines had an influence on their
complexation with DNA. At N:P ratio of 5, PCB 5 showed a good propensity to
exclude EtBr at pH 9 (reduction of 75% of fluorescence), followed by PCB 2
(reduction of 15% in fluorescence), while PCB 4 and PCB 8 showed similar
behaviors (less than 5% reduction in fluorescence). The polyplexes followed the
same trend in the dissociation of the complex in presence of salt (NaCl). The
authors suggested that the propensity of PCB m (for m ¼ 3 and 4) to form betaine
176
A. Bertin