Effects of external electric fields on lysozyme adsorption by molecular dynamics simulations

Lysozyme adsorption on carboxyl-terminated self-assembled monolayers under external electric fields has been studied by all-atom molecular dynamics simulations. Lysozyme adsorption on negatively charged surfaces could generally be enhanced by positive electric fields and retarded by negative ones. Under positive electric fields, electrostatic interactions between protein and surface are strengthened; however, the interaction energy descends with field strength increases probably due to the co-adsorption of counterions onto the surface to neutralize surface charge. Comparison of orientation distributions of lysozyme adsorption on the surface in the presence and in the absence of electric fields reveals that an applied electric field could narrow the distribution and therefore helps to immobilize protein on surface with uniform orientation. Orientation angle analysis shows that lysozyme is adsorbed on the surface with "bottom end-on", "side-on", "back-on" or "top end-on" orientation under different field strengths, suggesting the possibility of controlling the preferred orientation of lysozyme on surface by applying electric fields. Conformation analysis of protein implies that the structure deformation of adsorbed lysozyme does not increase monotonically with the rising field strength. Under some field strengths, there is no additional structure deformation caused by the electric fields compared with that in the absence of electric fields; while under some other field strengths, there are larger conformational change occurrences. We propose that due to the rearrangement of positions of the local atomic charges of protein to couple its dipole with an external electric field, large position alterations of atoms might be avoided and thus conformational changes be restricted. This work may provide guidance for controlling protein adsorption behaviors via external electric fields for applications of protein immobilization and anti-fouling surfaces.