Complexation of lysozyme with lambda carrageenan: Complex characterization and protein stability

In a previous work, we reported that interaction of lysozyme (Lys) with kappa carrageenan (kappa CG) is governed by both electrostatic interactions and secondary forces, and lead to formation of asymmetric complex particles with an irregular localization of biopolymers without formation of a single center of binding. In this work we characterize complexation of lysozyme with strongly charged lambda carrageenan (lCG) and study protein stability within complex at different composition, pH and ionic strength (I) by a combination of a few main experimental techniques: turbidimetric titration, dynamic and electrophoretic light scattering, differential scanning microcalorimetry (DSMC), fluorescence and circular dichroism measurements. Complexation is governed by only electrostatic interactions, exhibits extremely low (7.10(-4)) q(Onset) values and lead to formation of swarm of large complex aggregates consisting of hundreds of very small separate particles. At pH 7 and I = 0.01 a stoichiometric lCG/Lys composition is equal to 0.309:1 (wt/wt) or 1:119 (mole/mole). It shift monotonically in the direction of higher lCG content with increase I values. The effect of I on stability of the complex formed has nonmonotonic character displaying a maximum at I approximate to 0.08. Complexation leads to a spectacular increase in the helix content and blue shift for Lys in the presence of lCG that probably reflects a more hydrophobic nonpolar environment of surface exposed tryptophan residues. It also significantly decreases thermal stability of Lys within complex in a wide range of pH suggesting that lCG has a higher affinity for the unfolded state than for the native state of Lys.