Complexation and synergistic boundary lubrication of porcine gastric mucin and branched poly(ethyleneimine) in neutral aqueous solution

Lubrication of soft polydimethylsiloxane (PDMS) elastomer interfaces was studied in aqueous mixtures ofporcine gastric mucin (PGM) and branched polyethyleneimine (b-PEI) at neutral pH and various ionicstrengths (0.1–1.0 M). While neither PGM nor b-PEI improved lubrication compared to polymer-freebuffer solution, their mixtures produced a synergistic lubricating effect by reducing friction coefficientsby nearly two orders of magnitude, especially at slow sliding speed in the boundary lubrication regime.An array of spectroscopic studies revealed that small cationic b-PEI molecules were able to stronglybind and penetrate into large anionic PGM molecules, producing an overall contraction of the randomlycoiled PGM conformation. The interaction also affected the structure of the folded PGM proteinterminals, decreased the surface potential and increased light absorbance in PGM:b-PEI mixtures. Addingan electrolyte (NaCl) weakened the aggregation between PGM and b-PEI, and degraded the lubricationsynergy, indicating that electrostatic interactions drive PGM:b-PEI complexation.