Protein-neutral polysaccharide nano- and micro-biopolymer complexes fabricated by lactoferrin and oat β-glucan: Structural characteristics and molecular interaction mechanisms

Biopolymer complexes fabricated by proteins and neutral polysaccharides may have some specific or innovative functionalities. However, only little is known about the structural characteristics and molecular interaction mechanisms of proteins-neutral polysaccharides biopolymer complexes. Understanding these informations is of major interest for the design of new proteins-polysaccharides biopolymer complexes with specific and/or innovative functionalities. Thus, the aim of the present study was to investigate the structural characteristics and molecular interaction mechanisms of lactoferrin (LF) and oat β-glucan (OG) with and without heat treatment at different OG concentrations (0.2, 0.5 and 0.9%, w/v). Isothermal titration calorimetry (ITC) results showed that LF and OG could interact with each other. The binding behavior between LF and OG at 25 °C was a spontaneous process, and electrostatic interactions, hydrogen bonding and van der Waals forces contributed to the LF–OG self-assembling behavior. OG concentration influenced the thermodynamic characterization of interactions between LF and OG. LF was susceptible to aggregation and thermal denaturation in the presence of OG. The increased turbidity and particle size of LF–OG complexes suggested the formation of large complexes in aqueous solution. SEM results showed that LF–OG self-assembles exhibited physically cross-linked networks at low OG concentration, while formed some spherical complexes at high OG concentration; LF–OG thermally modified complexes exhibited the honeycomb-like structures with different particle sizes in a concentration-dependent manner. Fluorescence spectroscopy results indicated that OG can change the structure of LF, leading to the exposure of Trp residues of LF molecules toward a more polar microenvironment. Raman difference spectra and circular dichroism revealed that the addition of OG could alter the secondary structure of LF, and the most noticeable changes were in the regions connected β-structures. The apparent viscosity of LF–OG complexes were higher than that of LF or OG alone, indicating that there was synergism between LF and OG. Overall, the self-assembling complexes and thermal complexes of LF and OG can be formed at 25 °C and 90 °C, respectively. These formed LF–OG nanocomplexes and microcomplexes (both the self-assembling and thermal complexes) with unique structures can be widely used in food, pharmaceutical and cosmetic industries, which will be suitable for encapsulation and transportation of bioactive compounds, or as fat substitutes.