ИСТИНА

Polybetaines are polyampholytes with cationic and anionic moieties in the same monomer unit. Reactions of polybetaines with positively or negatively charged polymers are poorly described. Recently the results on complexing of poly(ammonium sulfopropylbetaine) with polycations and polyanions were reported by the group of Y. Osada (Polymer 2000, 41, 141; Macromol. Rapid Commun. 2002, 23, 423). However, the using of polybetaines with sulfobetaine functionalities minimizes the pH control over the system. To verify whether pH-sensitive polycarboxybetaines (PCB) also can readily form polyelectrolyte complexes, we synthesized PCB samples with different hydrophobic substituents but the same distance between the charges isolated with 1 methylene group (V.A. Izumrudov et al, J. Phys. Chem. B 2003, 107, 7982). All PCB samples revealed rather poor ability to interact with poly(methacrylic) acid or DNA. In the current work, we present unambiguous evidences that capacity of PCB for the interpolyelectrolyte interactions can be drastically enhanced and efficiently controlled by variation of alkyl spacer between charges in the betaine moieties. Polycarboxybetaines (PCB-n) of pyridiniocarboxylate structure with different number n of methylene groups in the alkyl spacer, n = 1, 2, 4, and 5, were synthesized by alkylation of poly(4-vinylpyridine) with the corresponding ω-bromocarboxylic acid. The affinity of PCB-n to pyrenyl-tagged poly(methacrylic) acid (PMAA*) or DNA was estimated by stability of the polyelectrolyte complexes at different pH and ionic strength that was monitored by fluorescence quenching techniques. PCB-1 formed the least stable complexes with both polyanions due to stable ion pairs in the betaine moieties. The increase in n resulted in pronounced but irregular change of the affinity with clearly defined maximum at n = 2 and minimum at n = 4. The complexes of PCB-2 proved to be very much like the highly stable complexes of polycation poly(N-ethyl-4-vinylpyridinium) bromide with PMAA* or DNA. The maximum affinity implies weak interaction of the charges in betaine moieties of PCB-2 that was supported by potentiometry: among the PCB-n samples, the only PCB-2 was titrated like weak polycarboxylic acid. The minimum is attributable to formation of stable ion pairs in the six-membered betaine rings of PCB-4. At pH > 7.0, the affinity of PCB-n to PMAA* increased in the series: PCB-1 ≈ PCB-4 < PCB-5 < PCB-2, whereas at pH 6.0 the position of PCB-4 in the series was reversed: PCB-1 < PCB-5 < PCB-2 ≈ PCB-4. The DNA-containing complexes revealed the same regularity, but the pronounced growth in stability of the PCB-4 complex occurred in more acidic media with a ΔpH shift of ca. 1 unit. The weaker PCB-4 affinity to DNA may be ascribed to high rigidity of the double helix that hinders the matching of the DNA phosphate groups with pyridinium groups of the polybetaine. The possibility to change dramatically the interpolyelectrolyte interactions by variation of n and pH offers considerable scope for design of polyelectrolyte complexes with appropriate and predictable stability in water-salt media that could be of practical significance, in particular for development of separation techniques in biotechnology. Specifically, the revealed ability of PCB-4 to combine high affinity to DNA at the enzyme-friendly pH and ionic strength with rather strong electrostatic interaction in the unbound betaine moieties seems to be attractive for selective binding and extraction of nucleic acids; the selectivity could be conditioned by poor affinity of the polybetaine with the mutually neutralized charges to the non-targeted substances, e.g. proteins.