Interpolyelectrolyte reactions in solutions of polycarboxybetaines, 2: influence of alkyl spacer in the betaine moieties on complexing with polyanions

Series of polycarboxybetaines (PCB-n) of pyridiniocarboxylate structure with the same degree of polymerization but differing in the number, n, of methylene groups in the alkyl spacer between charges in the betaine moieties, n = 1, 2, 3, 4, 5, and 8, were synthesized. The utility of PCB-n as positively charged components of polyelectrolyte complexes was elucidated by potentiometry, turbidimetry, and fluorescence spectroscopy. Affinity of PCB-n to the pyrenyl-tagged poly(methacrylic) acid (PMAA) or DNA was judged from the stability of the corresponding polyelectrolyte complexes in water-salt solutions at different pH values as monitored by fluorescence quenching techniques. At pH = 9.0, PCB-1 formed the least stable complexes due to strong interaction of charged groups positioned in close proximity in the betaine moieties. The increase in n resulted in the irregular change of the affinity. Thus, as expected, PCB-2 formed noticeably more stable complexes than PCB-1. However, PCB-3 and, in particular, PCB-4 revealed weaker affinity to PMAA or DNA that is attributed to formation of stable ion pairs between charges in the betaine rings. At neutral and slightly acidic pH values binding of all PCB-n except PCB-1 was drastically enhanced due to protonation of PCB-n carboxylic groups that occurred with a DeltapH shift of 2-3 units to higher values as compared with the protonation of free PCB-n. The ability of added polyanion to compete with the betaine carboxylic groups in binding with the pyridinium groups was supported by potentiometric titration of PCB-n mixtures with sodium poly(styrenesulfonate): for n > or = 2, the binding of the polyanion-competitor also shifted protonation of carboxylic groups to higher values with DeltapH of more than 2 units. Practical ramifications of the revealed role of the alkyl spacer in polyelectrolyte complexation as well as the pH-induced stabilization of the complexes that occurs under enzyme-friendly conditions might extend to areas of biotechnology, specifically in bioseparation and gene delivery.