Interactions of Charged Polypeptide Complexes by Atomic Force Microscopy and Soft Colloidal Probe Force Spectroscopy: Fluidization, H-Bonding and Electrostatic Interactions

High Resolution Image Download MS PowerPoint Slide Polyelectrolyte complexes (PECs) are central to biological structure formation and advanced materials, yet the interplay between their physical phase state and adhesive properties remains poorly understood. We employed a dual-scale approach to investigate poly( l -lysine) and poly( l -glutamic acid) complexes as a model system for peptidic PECs. Soft colloidal probe (SCP) spectroscopy and reflection interference contrast microscopy (RICM) quantified mesoscopic adhesion, while atomic force microscopy (AFM) probed nanoscale chain interactions. We demonstrate that adhesion is fundamentally dictated by the complex’s phase state. In pure water, cooperative hydrogen bonds lock the complex into a solid precipitate, resulting in restricted macroscopic contact and brittle nanoscale chain rupture. Disrupting these hydrogen bonds with urea fluidizes the complex into a liquid coacervate. This phase transition maximizes macroscopic interfacial contact area and reveals the underlying isotropic ionic interactions, transforming nanoscale rupture into a highly ductile, viscous extension. Furthermore, high ionic strength enhances adhesion via hydrophobic collapse. These insights establish that macroscopic adhesion is profoundly dictated by interfacial fluidity and the ability to maximize contact, offering vital design principles for dynamic biomaterials and underwater adhesives.