Surface pH induced hysteresis behavior of lipid (DMPA)-protein (BSA) complex monolayer

Compression-decompression cycles of the surface pressure (π) - specific molecular area (A) isotherms of lipid (DMPA)-protein (BSA) monolayers show pH dependent reversible/irreversible hysteresis. Below the isoelectric point of BSA (pH ≈ 4.0), i.e., when the protein molecules have effective positive charge and is weakly hydrophobic show reversible hysteresis whereas for pH above the isoelectric point of BSA (pH ≈ 7.0) when molecules have effective negative charge and relatively higher hydrophobicity show irreversible hysteresis indicating a permanent transformation in its morphology which was also evident from the Brewster angle microscopy (BAM). From FTIR spectra it is clear that the BSA protein in protein-lipid complex did not undergo any major conformational changes during the compression-decompression cycle for both the pH conditions. Competitive electrostatic and hydrophobic-hydrophobic interactions are probably responsible for such hysteresis behaviors.