Protein adsorption at solid-liquid interfaces is an important issue in many fields such as: bio- and hemocompatible materials, diagnostic kits or enzymatic activity in mineral soils. The theoretical description of the protein adsorption phenomena is very complicated due to combination of many types of interactions involved as: electrostatic, dispersive, hydrophobic/hydrophilic, specific ion effects. Just considering only the electrostatic interactions with solid surface we need to take into account that the surface charge of proteins is pH dependent and non-uniformly distributed. The concept of non-uniform charge distribution is usually used to explain anomalous adsorption of proteins at the charged surfaces carrying the surface charge of the same sign as the mean charge of protein. For example human serum albumin (HAS), which above pH 5 has net negative charge and can be deposited at mica surface [1], whereas casein can be deposited on silica surface in the broad pH value regardless that it has isoelectric point around pH 5.
We used molecular modeling calculation to determine the distribution of the surface charge at the selected proteins (α-chymotrypsin, α- and β-casein, HSA). The modeling was conducted in Accelrys Discovery Studio 3.5 package using CHARMm force field. The enzyme’s pI was calculated with ‘Calculate Protein Ionization and Residue pK’ protocol and the protein net charge and its electric dipole moment was estimated for pH conditions at, below, and above pI point. We compared the dependencies of the net charge of proteins on pH obtained from the calculation with ones measured experimentally by the microelectrophoresis. Then, we evaluated the electrostatic interactions between proteins having nonuniform distribution of surface charge with uniformly charged, planar interface. The interaction force exerted on the particle were calculated by solving numerically three dimensional nonlinear Poisson-Boltzmann equation. The interaction energy was calculated by integration of the force distance relationship. These theoretical predictions were used for the interpretation of experimental data [1-4] for the adsorption and desorption kinetics of proteins at mica surface.
References:
[1] M. Dabkowska, Z. Adamczyk, Langmuir. 2012 28, 15663−15673.
[2] M. Wasilewska, Z. Adamczyk, Langmuir. 2011 27, 686-96.
[3] V.Z. Spassov; L. Yan, Protein Science 2008, 17, 1955-1970.
[4] E.N. Vasina, P. Dejardin, Langmuir, 2004, 20, 8699-8706
ncwarszy@cyf-kr.edu.pl
