Structure, morphology and reversible hysteresis nature of human serum albumin (HSA) monolayer on water surface

Compression-decompression surface pressure (pi)-specific molecular area (A) isotherm cycle of human serum albumin (HSA) monolayer is performed on water surface at four different subphase pH conditions, i.e., below and above the isoelectric point (pI approximate to 4.7) of HSA molecule. For all pH conditions, the decompression curve nearly follows the compression curve, however, at pH approximate to 5.0, hysteresis is observed at higher surface pressure. Out-of-plane structures and in-plane morphologies obtained from the X-ray reflectivity and AFM studies show that only the film thickness variation takes place with the change in surface pressure, which is also evidenced from the BAM images. With increase in surface pressure, the oblate-shaped HSA molecules start tilting making an angle with the water surface and as the monolayer is decompressed the molecules regain their initial untilted monomolecular configuration. Depending upon the subphase pH and local surface charge of the specific protein molecule, electrostatic repulsive interaction dominates over the van der Waals attraction and as a result decompression curve follows the compression curve as the molecules repel each other, however, closer to the isoelectric point as strength of the interactions reverses, a hysteresis is obtained at higher surface pressure and accordingly monolayer behaviour modifies on the water surface. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study investigated the compression-decompression behavior of human serum albumin monolayer on a water surface under different subphase pH conditions. It was found that hysteresis occurred between the compression and decompression curves, leading to modification of monolayer behavior on the water surface. The interactions between molecules were influenced by the subphase pH and local surface charge, resulting in electrostatic repulsion dominating over van der Waals attraction in certain conditions.