Hydrodynamic radius coincides with the slip plane position in the electrokinetic behavior of lysozyme

Abstract

The zeta potential (ζ) is the effective charge energy of a solvated protein, describing the magnitude of electrostatic interactions in solution. It is commonly used in the assessment of adsorption processes and dispersion stability. Predicting ζ from molecular structure would be useful to the structure-based molecular design of drugs, proteins, and other molecules that hold charge-dependent function while remaining suspended in solution. One challenge in predicting ζ is identifying the location of the slip plane (X ), a distance from the protein surface where ζ is theoretically defined. This study tests the hypothesis that the X can be estimated by the Stokes-Einstein hydrodynamic radius (R ), using globular hen egg white lysozyme as a model system. Although the X and R differ in their theoretical definitions, with the X being the position of the ζ during electrokinetic phenomena (e.g., electrophoresis) and the R being a radius pertaining to the edge of solvation during diffusion, they both represent the point where water and ions no longer adhere to a molecule. This work identifies the limited range of ionic strengths in which the X can be determined using diffusivity measurements and the Stokes-Einstein equation. In addition, a computational protocol is developed for determining the ζ from a protein crystal structure. At low ionic strengths, a hyperdiffusivity regime exists, requiring direct measurement of electrophoretic mobility to determine ζ. This work, therefore, supports a basic tenant of EDL theory that the electric double layer during diffusion and electrophoresis are equivalent in the Stokes-Einstein regime.

Zeta Potential Prediction from Protein Structure in General Aqueous Electrolyte Solutions

“…Therefore, experiments were conducted at sufficiently low protein concentrations to minimize concentration effects during electrophoresis. In previous work, , ZPRED was tested on lysozyme, a nearly spherical protein with a high net charge at neutral pH, in the presence of an indifferent electrolyte, KCl. To assess the extent to which ζ could be accurately computed from structure (ZPRED), we investigated multiple proteins over a range of aqueous solution conditions.…”

“…Previously, it was shown that u e of lysozyme decreases with increasing concentration of KCl, despite having little effect on the position of the slip-plane . It was proposed that other salts may interact with the hydration layer.…”

“…The Stokes–Einstein hydrodynamic radius defines the location of the solvation edge (and thus the X SP ) on a protein. , X SP is a theoretical construct, and the existence of a sharp cutoff between the solvation layer and bulk solution is physically unrealistic. , The solvation layer is a region of solution encompassing a protein, where solvent motion is significantly hindered relative to the bulk, causing an apparent increase in the size of the protein. This region is only a few molecular-sized layers thick (in general about 2 to 4 Å), and ions adsorbed within it can cause “specific-ion effects” that can be modeled with the Gouy–Chapman-Stern EDL. , The X SP defines the thickness of the solvation layer and thus the distance from the protein surface over which surface potential dampening occurs.…”

Complex Formation in the Sulfobetaine-Containing Entirely Ionic Block Copolymer/Ionic Homopolymer System and Their Temperature Responsivity

“…The hydrodynamic radius ( R h ) of the block-/ionic homopolymer system PIC micelles was estimated using a Photal SLS-7000DL (Otsuka Electronics Co., LTD, Osaka, Japan) equipped with a GC-1000 photon correlator and a 15 mW He–Ne laser (wavelength 632.8 nm). R h is an important parameter that pertains to the hydration layer undergoing diffusive motion and the size of a particle . DLS measurements were carried out at various concentrations and temperatures.…”

“…R h is an important parameter that pertains to the hydration layer undergoing diffusive motion and the size of a particle. 36 DLS measurements were carried out at various concentrations and temperatures. The time correlation function of the scattered field was measured at four scattering angles (60°, 75°, 90°, and 105°) with an accumulation time of 10−30 min.…”