Effect of protein topology on hierarchical complexation of epsilon-polylysine and protein: A multiscale structural analysis

epsilon-polylysine (EPL) is an antimicrobial peptide. Despite the utilization of EPL as a natural preservative worldwide, it will be deactivated owing to phase separation when it encounters some macroions. Interestingly, previous studies demonstrated the approach to preserve its antimicrobial efficacy by forming another delicate structure, the "soluble" complexes, but their interaction mechanism and structure remain unclear. This work elucidated the hierarchically structural complexation of EPL with three representative proteins beta-casein, beta-lactoglobulin and bovine serum albumin. It was found that these hierarchical structures separated into compact mesophase dispersions and porous precipitations, and range from nanometers to microns length scales. The driving forces for the association between EPL and protein have been mainly considered as electrostatic and subsequently hydrophobic attractions. Our results illustrated how the differences in topology (spatial structure, charge anisotropy, and hydrophobicity) among various proteins determined the specifically hierarchical structures of EPL-protein complexation. Unfolded protein with the concentrated negative charge and hydrophobicity patches inclined to form aggregation readily, whereas the folded protein with a relative disperses geometry could create the structural forces to form partial steady-state complexes. This work provides a general structural evolution of EPL-protein complexation and paved the way for its future application in food science and biotechnology.