Spatial structure and composition of polysaccharide-protein complexes from small angle neutron scattering

Area of Science:

• Biophysics
• Materials Science
• Polymer Science

Background:

• Lysozyme is a globular protein, and pectin is a natural anionic polysaccharide.
• The charge density of pectin can be modulated by its degree of methylation (DM) and pH.
• Understanding protein-polysaccharide complex formation is crucial for various applications.

Purpose of the Study:

• To investigate the structural characteristics of lysozyme-pectin complexes.
• To determine how charge density influences complex formation and inner composition.
• To elucidate the driving forces behind complex assembly.

Main Methods:

• Small Angle Neutron Scattering (SANS) with an original analysis method.
• Systematic variation of pectin charge density through pH and degree of methylation (DM).
• Control of experimental conditions including ionic strength and lysozyme-to-pectin ratio.

Main Results:

• Protein clustering observed within complexes, indicated by a correlation peak at 0.2 A(-1).
• Complexes form spherical globules (10-50 nm radius) containing thousands of proteins.
• Globule size is determined by pectin's linear charge density and molecular weight; protein stacking increases with pectin charge.
• At very low pectin charge, a gel-like structure forms instead of globules.

Conclusions:

• Complex formation is primarily driven by electrostatic interactions, not hydrophobic interactions.
• Pectin's charge density, controlled by DM and pH, dictates the size and structure of lysozyme-pectin globules.
• Molecular weight significantly influences globule size, with longer chains forming larger structures.