Salting Up of Proteins at the Air/Water Interface

Abstract

Vibrational sum frequency generation (VSFG) spectroscopy and surface pressure measurements are used to investigate the adsorption of a globular protein, bovine serum albumin (BSA), at the air/water interface with and without the presence of salts. We find at low (2 to 5 ppm) protein concentrations, which is relevant to environmental conditions, both VSFG and surface pressure measurements of BSA behave drastically different than at higher concentrations. Instead of emerging to the surface immediately, as observed at 1000 ppm, protein adsorption kinetics is on the order of tens of minutes at lower concentrations. Most importantly, salts strongly enhance the presence of BSA at the interface. This "salting up" effect differs from the well-known "salting out" effect as it occurs at protein concentrations wellbelow where "salting out" occurs. The dependence on salt concentration suggests this effect relates to a large extent electrostatic interactions and volume exclusion. Additionally, results from other proteins and the pH dependence of the kinetics indicate that salting up depends on the flexibility of proteins. This initial report demonstrates "salting up" as a new type of salt-driven interfacial phenomenon, which is worthy of continued investigation given the importance of salts in biological and environmental aqueous systems.

Salting Up and Salting Down of Bovine Serum Albumin Layers at the Air–Water Interface

“…Very few have measured the effect of ionic strength on the protein behavior in native solution without buffer. In a recent work, Xiong et al 23 compare the adsorption kinetics of BSA in pure water and aqueous salt solution; they find that salts (even a small amount, such as 0.005 M) play a pivotal role in BSA adsorption dynamics at the air/water interface and claim a "salting up" effect in the very dilute regime (2−5 ppm). That is, the salt strongly accelerates the assembly of BSA to the air/ water interface.…”

Catching Speedy Gonzales: Driving forces for Protein Film Formation on Silicone Rubber Tubing During Pumping

“…55 Additionally, the electrostatic screening also decreases the interactions between protein and water which leads to more excluded volume. 56 This in turn may contribute to an increase in packing density. [57][58][59] Additionally, the adsorption rate might also be impacted by hampering attractive interactions.…”

The Ocean’s Elevator: Evolution of the Air–Seawater Interface during a Small-Scale Algal Bloom

“…2,21 The behavior of each subset of algal residues in ocean systems have been reported in the literature, exploring fundamental (one or two component) systems such as protein salt interactions, 22−24 lipid monolayer surfaces (including fatty acids, fatty alcohols, phospholipids, among others), 25,26 and saccharide-enriched interfaces. 3,27 By studying the individual role of each primary subset of algal exudates as demonstrated in the studies above, the fundamental interactions at the ocean's interface and consequent aerosol surfaces can be determined and include interfacial adsorption, 22,27 binding interactions, 28,29 and reaction mechanisms. 30 Biogenic surfactants have also been explored in more complex systems and have been shown to alter surface features, 31 enhance interactions, 24,25,32,33 and influence the overall film properties.…”

“…Diatoms are the most abundant oceanic phytoplankton and exude an abundance of biogenic species over time, including proteins, lipids, and saccharides. , The behavior of each subset of algal residues in ocean systems have been reported in the literature, exploring fundamental (one or two component) systems such as protein salt interactions, − lipid monolayer surfaces (including fatty acids, fatty alcohols, phospholipids, among others), , and saccharide-enriched interfaces. , By studying the individual role of each primary subset of algal exudates as demonstrated in the studies above, the fundamental interactions at the ocean’s interface and consequent aerosol surfaces can be determined and include interfacial adsorption, , binding interactions, , and reaction mechanisms …”