Lysozyme adsorption at a silica surface using simulation and experiment: effects of pH on protein layer structure

Abstract

a Hen Egg White Lysozyme (HEWL) is a widely used exemplar to study protein adsorption on surfaces and interfaces. Here we use fully atomistic Molecular Dynamics (MD) simulations, Multi-Parametric Surface Plasmon Resonance (MP-SPR), contact angle and zeta potential measurements to study HEWL adsorption at a silica surface. The simulations provide a detailed description of the adsorption mechanism and indicate that at pH7 the main adsorption driving force is electrostatics, supplemented by weaker hydrophobic forces. Moreover, they reveal the preferred orientation of the adsorbed protein and show that its structure is only slightly altered at the interface with the surface. This provides the basis for interpreting the experimental results, which indicate the surface adsorbs a close-packed monolayer at about pH10 where the surface has a large negative zeta potential and the HEWL is positively charged. At higher pH, the adsorption amount of the protein layer is greatly reduced due to the loss of charge on the protein. At lower pH, the smaller zeta potential of the surface leads to lower HEWL adsorption. These interpretations are complemented by the contact angle measurements that show how the hydrophobicity of the surface is greatest when the surface coverage is highest. The simulations provide details of the hydrophobic residues exposed to solution by the adsorbed HEWL, completing the picture of the protein layer structure.

Lysozyme encapsulated gold nanoclusters: effects of cluster synthesis on natural protein characteristics

“…The associated zeta potentials were calculated using Henry's equation, [3], and displayed in Figure 3. The zeta potential curve for Lysozyme in a solution with ionic strength I = 1 x 10 -2 M compares well with previously published results 33 .The isoelectric point of Lysozyme in this case is approximately 10.0, in agreement with previous results 26 . In the case of lysozyme AuNC nucleation dramatically changes the zeta potential.…”

Electrostatically Driven Protein Adsorption: Charge Patches versus Charge Regulation

“…51 The orientation predicted by our model for lysozyme at its isoelectric point (pH 11.2) on a negative surface is very close to the most stable orientation predicted by these MD simulations. [49][50][51][52] Moreover, MD studies have identified the same aminoacids as our study (Lys1, Arg5, Arg125 and Arg128) as the aminoacids that reside closest to the surface and contribute the most to the interaction with the silica substrate.…”

Steering protein adsorption at charged surfaces: electric fields and ionic screening

“…Well-designed simulations can show not only how proteins adsorb to different surfaces, but also provide understanding of what the driving forces are and how they can be controlled through surface chemistry, and how bio-activity can be preserved by retaining the protein secondary and tertiary structures. There is growing consensus that the orientation of the protein at the surface is paramount, and that this depends on the surface chemistry [11][12][13][14][15] . Of particular note is the idea that electric fields above charged surfaces will play a crucial role in steering the protein during adsorption, and so determine protein orientation once adsorbed [14][15][16] .…”

“…There is growing consensus that the orientation of the protein at the surface is paramount, and that this depends on the surface chemistry [11][12][13][14][15] . Of particular note is the idea that electric fields above charged surfaces will play a crucial role in steering the protein during adsorption, and so determine protein orientation once adsorbed [14][15][16] . Provided that any unfolding induced by interactions with the surface are of limited extent, this provides the means to engineer surface functionalization through controlled protein adsorption 12 .…”