Polyelectrolyte complex formation between iota-carrageenan and poly(l-lysine) in dilute aqueous solutions: a spectroscopic and conformational study

The polyelectrolyte complex formation between iota- (ι-)carrageenan and poly(l-lysine) (PLL) has been investigated in HEPES 5 mM, NaCl 150 mM pH 6.5 at 25 °C, in dilute conditions. After building of the ternary phase diagram of the system, the formation of soluble polyelectrolyte complexes in transparent systems has been evidenced by means of spectroscopic techniques. Absorption spectrum of methylene blue (MB+) interacting with ι-carrageenan chains, in presence of increasing amounts of the polypeptide, as well as fluorescence polarization of a FITC-labeled PLL and circular dichroism spectrum of PLL, in presence of increasing amounts of the polysaccharide, have been studied. In all cases, the endpoint occurred for a peptide to polysaccharide weight ratio of about 0.82, indicating an approximate 1/1 charge stoichiometry of the polyelectrolyte complex formed. Furthermore, the polyelectrolyte complex formation process was proved to be conformation-directing as PLL adopts a right-handed α-helical conformation when mixed with ι-carrageenan.