The Role of Charged Groups in the Adsorption of Proteins at Solid Surfaces*

Interaction between electric charges is an important factor in protein adsorption. Similar charge signs on the protein and the sorbent surface opposes adsorption, but whether it prevents ad.: sorption depends on other factors such as dehydration of the protein and the sorbent and structural rearrangements in the protein molecule. Ribonuclease, that does not strongly change its structure upon adsorption, adsorbs on hydrophilic surfaces only if its charge sign is opposite that of the sorbent. On increasing the hydrophobicity of the sorbent adsorption also takes place in the case of the same charge sign. Blood plasma albumin, on the other hand, adsorbs at any interface even if the sorbent is h ydrophilic and has the same charge sign as the protein. In this case the driving force for adsorption stems from dehydration and/or conformational changes in the protein molecule. Although a same charge sign may not prevent adsorption to occur, it may slow down the adsorption process. This has been demonstrated for albumin at negatively charged polystyrene surfaces. The Gibbs free energy of the net electrostatic interaction is relatively insensitive for the charge on the protein and the sorbent. This is due to the role of small ions in the system: ions are eventually transferred from the aqueous solution into the contact region between the protein and the sorbent in order to prevent the development of high electrostatic potentials in this region. The chemical effect of the medium change of these ions is unfavorable and, since it is proportional to the number of trasferred ions, it increases with decreasing charge contrast between the protein and the sorbent surface.