Spatial distribution of protein molecules adsorbed at a polyelectrolyte multilayer

Abstract

The spatial distribution of protein molecules interacting with a planar polyelectrolyte multilayer was determined using neutron reflectometry. Staphylococcal nuclease (SNase) was used as model protein that was adsorbed to the multilayer at 22 degrees C and 42 degrees C. At each temperature, the protein solution was adjusted to pD -values of 4.9 and 7.5 to vary the net charge of the protein molecules. The multilayer was built up on a silicon wafer by the deposition of poly(ethylene imine) (PEI), poly(styrene sulfonate) (PSS), and poly(allylamine hydrochloride) (PAH) in the order Si-PEI-PSS- (PAH-PSS)(5). Applying the contrast variation technique, two different neutron reflectivity curves were measured at each condition of temperature and pD -value. From the analysis of the curves, protein density profiles normal to the interface were recovered. Remarkably, it has been found that SNase is partially penetrating into the polyelectrolyte multilayer after adsorption at all conditions studied. The measured neutron reflectivities are consistent with a penetration depth of 50 A at pD=4.9 and 25 A at pD=7.5. Since SNase has an isoelectric point of pH=9.5, it carries a net positive charge at both pD -values and interacts with the PSS final layer under electrostatic attraction conditions. However, when increasing the temperature, the amount of adsorbed protein is increasing at both pD -values indicating the dominance of entropic driving forces for the protein adsorption. Interestingly, at pD=4.9 where the protein charge is relatively high, this temperature-induced mass increase of immobilized protein is more pronounced within the polyelectrolyte multilayer, whereas at pD=7.5, closer to the isoelectric point of SNase, raising the temperature has mainly the effect to accumulate protein molecules outside the polyelectrolyte multilayer at the water interface. It is suggested that the penetration of SNase into the polyelectrolyte multilayer is related to a complexation mechanism. The complexation is essentially entropic in nature due to the release of counterions.

Deciphering Membrane Insertion of the Diphtheria Toxin T Domain by Specular Neutron Reflectometry and Solid-State NMR Spectroscopy

“…[19][20][21][22] The judicious use of samples organized in a single phospholipid bilayer and of the contrast-variation technique has led to the determination of new structural details on the interaction of polypeptides with membranes. [23][24][25][26][27][28][29][30] In particular, NR is a unique technique to study the distribution of mass along the axis perpendicular to the membrane plane. It allows one to identify the successive layers of chemicals composing the lipid bilayer (lipid headgroups and acyl chains), as well as the compounds embedded in the membrane and located in its vicinity (polypeptide and water molecules, for instance; see Fig.…”

Multiple Functionalities of Polyelectrolyte Multilayer Films: New Biomedical Applications

“…[95] Recently, the diffusion of Staphylococcal nuclease (SNase) was investigated by neutron reflectometry. [96] It was shown that the SNase is partially penetrating (or diffusing) into a PSS terminating polyelectrolyte multilayer upon adsorption. This penetration is preferred when the positive charge of the protein is high.…”

Understanding protein adsorption phenomena at solid surfaces

“…Consequently, these surfaces have great potential for the design of medical devices or implants which are exposed to blood. In recent years the favorable protein adsorption properties of poly(acrylic acid) (PAA) brushes have been reported [74][75][76]. At low salt concentration these polyelectrolyte layers retain proteins in their native state, i.e.…”