Protein Transport upon Advection at the Air/Water Interface: When Charge Matters

Abstract

The formation of dense protein interfacial layers at a free air–water interface is known to result from both diffusion and advection. Furthermore, protein interactions in concentrated phases are strongly dependent on their overall positive or negative net charge, which is controlled by the solution pH. As a consequence, an interesting question is whether the presence of an advection flow of water toward the interface during protein adsorption produces different kinetics and interfacial structure of the adsorbed layer, depending on the net charge of the involved proteins and, possibly, on the sign of this charge. Here we test a combination of the following parameters using ovalbumin and lysozyme as model proteins: positive or negative net charge and the presence or absence of advection flow. The formation and the organization of the interfacial layers are studied by neutron reflectivity and null-ellipsometry measurements. We show that the combined effect of a positive charge of lysozyme and ovalbumin and the presence of advection flow does induce the formation of interfacial multilayers. Conversely, negatively charged ovalbumin forms monolayers, whether advection flow is present or not. We show that an advection/diffusion model cannot correctly describe the adsorption kinetics of multilayers, even in the hypothesis of a concentration-dependent diffusion coefficient as in colloidal filtration, for instance. Still, it is clear that advection is a necessary condition for making multilayers through a mechanism that remains to be determined, which paves the way for future research.

Lysozyme Influence on Monolayers of Individual and Mixed Lipids

“…DPPC and POPC are typical phospholipids and models for the study. In previous work, the influence of lysozyme on individual lipid films using water subphase was studied [38]. In the present work, the influence of lysozyme on mixed lipid films with oleic acid-DPPC, oleic acid-POPC, cholesterol-POPC, and on the individual lipids using NaCl 0.9% water subphase, a more physiological medium, has been studied.…”

“…This could indicate that at the first stages of compression, the monolayer behavior is more dependent on the protein. Advention flow (convection towards the interface) occurs due to solvent evaporation [38]. In the case of lipids spread on an aqueous subphase, which form a surface layer, and using a closed chamber as in the present work, it can be assumed that solvent evaporation is weak or null.…”

“…In the case of lipids spread on an aqueous subphase, which form a surface layer, and using a closed chamber as in the present work, it can be assumed that solvent evaporation is weak or null. Thus, no protein multilayer formation should occur [38], but this point would need future research using surface physical techniques.…”

“…Interactions of proteins with lipid films and membranes are of great interest for biological systems such as tear films and cellular membranes. Works studying the surface properties of protein solutions and the influence of proteins on lipid monolayers are those of references [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] and those reported in reference [39]. Sah et al [26] observed that reversible hysteresis persists if the protein molecules contain effective positive or negative surface charges.…”

“…Depending on the surface pressure and surface charge of BSA, a monomolecular or bimolecular layer of BSA is formed on the water surface; however, a bimolecular layer is observed when the pH is closer to the BSA isoelectric point of ≈5. Pasquier et al [38] studied protein interfacial layers of ovalbumin and lysozyme at a free air-water interface, using neutron reflectivity and null-ellipsometry. These authors found that the combined effect of a positive charge of protein and the presence of advection flow (convection towards the interface) induce the formation of interfacial multilayers.…”