Investigation of the interaction between mucins and β-lactoglobulin under tribological stress

Abstract

The interaction characteristics between mucins and beta-lactoglobulin (BLG) under tribological stress were investigated by comparing the lubricity of mixed solutions of mucin–BLG with that of neat protein solutions at compliant hydrophobic interfaces. Surface adsorption properties of the proteins as characterized by bicinchoninic acid (BCA) assay revealed that both bovine submaxillary mucin (BSM) and porcine gastric mucin (PGM) showed distinctly higher adsorbed masses compared to BLG onto polydimethylsiloxane (PDMS) or polystyrene (PS) surfaces. The adsorbed masses of the mixed protein solutions, namely BLG–BSM and BLG–PGM, reduced significantly, and BLG appeared to dominate the surface adsorption event, presumably due to the reduced concentration of mucins and the Vroman effect. While pin-on-disk tribometry and mini-traction machine (MTM) were employed to provide the tribological contacts with varying contact pressure, speed range, and slide/roll ratio, the dominant lubrication mechanism of the protein solutions was boundary lubrication. BLG–BSM mixture showed the highest level of degradation in the lubricity of BSM at pH 5, although BLG–saliva interaction is known to degrade the lubricity most rapidly at more acidic pH, such as at pH 3.5. More importantly, pH dependent lubricating properties of BLG–BSM mixed solutions appeared to be determined by competitive adsorption of the two proteins onto the substrates, which suggests that they do not form as strong aggregates as BLG–saliva, especially under tribological stress.