Time-dependent conformational changes of polyelectrolyte complexes in solution

Changes in the conformation of the polyelectrolytes when contacted with oppositely charged polyelectrolytes or when subjected to shifts in solution conditions (pH), have been studied in this work along with reversibility of the changes using fluorescence spectroscopy. While changes due to both of the above are marked, they are measurably different from each other. Thus, the extent of coiling of the complexes formed between the anionic polyelectrolyte, maleicacid anhydride–propene and the cationic polyelectrolytes was much higher than that achieved by the change in pH alone. Also, while the changes due to pH shifts were fast and reversible, that due to complexation between oppositely charged ones involved first a rapid uncoiling followed by slow recoiling to a new structure. Interestingly, shifting the coiled conformation to an even more coiled one resulted in a new reversible state, but shifting to a stretched state by complexation led to a somewhat irreversible structure. Also maximum interaction obtained between the anionic and one cationic polyelectrolyte was markedly higher than that between the former and another cationic polymer. These observations using fluorescence spectroscopy was consistent with that obtained by the potentiometric titration. The study clearly shows the importance of the manner in which the polyelectrolytes are equilibrated to desired solution conditions. These results are interpreted here in terms of deprotonation/protonation of the polyelectroytes upon pH change and complexation with oppositely charged ones resulting in screening of charges as well as stiffening.