Like-charged protein/nanoparticle pairing governs interfacial assembly at the air-water interface

Hypothesis: Mixed protein-nanoparticle (NP) layers at fluid interfaces underpin applications ranging from Pickering emulsion biocatalysis to stimuli-responsive drug delivery and food-grade materials. However, the role of protein structure in shaping NP interfacial behaviour remains poorly understood. This is most striking for like-charged protein-NP pairs, in which bulk interactions are governed by non-electrostatic protein-specific contributions. Experiments: Here, the interaction of negatively charged silica NPs with three like-charged model proteins differing in molecular architecture and conformational flexibility, namely bovine serum albumin (BSA), β-lactoglobulin (BLG), and β-casein (BCN), is probed by combining bulk-sensitive (circular dichroism, fluorescence spectroscopy) with interface-specific techniques (dynamic surface tension, grazing-incidence small-angle X-ray scattering, sum-frequency generation vibrational spectroscopy, and compression isotherms). Findings: A reciprocal coupling emerges whereby protein structure and dynamics guide NP adsorption, packing, and mechanical response at the air-water interface, while NPs in turn reshape protein conformation, stability, and interfacial behaviour. The three proteins exemplify distinct coupling regimes: strong protein-NP binding promotes co-adsorption of structurally redefined systems (BSA); weak, transient coupling leaves both partners largely unperturbed and suppresses both protein and NP interfacial assembly (BLG); conformational adaptability enables efficient NP organization through dynamic interaction rather than stable complexation (BCN). Binding strength, molecular architecture, and conformational flexibility thus emerge as orthogonal design parameters for tuning the structure, density, and mechanics of mixed interfacial materials.