Macromolecular complexes of lysozyme with kappa carrageenan

- Dependence of complex formation on ionic strength is not monotonous.
- Secondary forces take part in complex formation.
- Complexation leads to an increase in the helix structure content of lysozyme.
- Complexation leads to a decrease in the thermal stability of lysozyme.
- Complexation depends on the state of carrageenan (coil versus helix).

We present a structural study of the complexation and binding of lysozyme (Lys) with kappa carrageenan (kCG) by means of turbidity measurements, phase analysis, dynamic and electrophoretic light scattering, differential scanning microcalorimetry (DSMC), confocal laser scanning (CLSM) microscopy, fluorescence and circular dichroism measurements. Complexation is governed by both electrostatic interactions and secondary forces, and exhibits a maximum at the kCG to Lys ratio for which mutual compensation of charges occurs. The effect of the ionic strength (I) on complexation has a nonmonotonous character displaying a maximum in complex formation at I ≈ 0.03. The specific pH value at which complex formation is completely suppressed (pHSet), is only slightly dependent on the I value. Turbidity measurements indicate complexation of Lys with kCG at a pH as high as 11.5 (I = 0.01). Molecules of Lys are placed mainly on the periphery of the complex particles and the localization of kCG has an irregular character without formation of a single center of binding. Complexation in dilute solutions leads to a spectacular increase in the helix content, whereas in semidilute solutions complexation causes a decrease of the temperature of denaturation, suggesting that kCG has a higher affinity for the unfolded state than for the native state of Lys.

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