Molecular simulations of cytochrome c adsorption on positively charged surfaces: the influence of anion type and concentration

Contradictory results have been reported regarding cytochrome c (Cyt-c) adsorption onto the positively charged SAMs, and the role of small anions in the adsorption is still unclear. In this work, the adsorption of Cyt-c on the amino-terminated SAM (NH2-SAM) and the effect of chloride and phosphate ions on the adsorption were studied using molecular dynamics simulations. The results reveal that Cyt-c could not stably adsorb onto the surface even at a relatively high ionic strength when chloride ions were added, while phosphate ions could promote its adsorption. At a low phosphate concentration, Cyt-c can adsorb on the NH2-SAM mainly with two opposite orientations. One is similar to that characterized in the experiments for Cyt-c adsorbed on the NH2-SAMs, in which the heme group points far away from the surface. The other orientation is similar to that for Cyt-c on the carboxyl-terminated SAMs. In the latter case, phosphate ions formed a distinct counterion layer near the surface and overcompensated the positive charge of the surface. Further analysis shows that chloride ions have no significant tendency to aggregate near the NH2-SAM surface and cannot shield the electrostatic repulsion between Cyt-c and the surface, while the phosphate ions can easily adsorb onto the surface and bind specifically to certain lysine residues of Cyt-c, which mediate its adsorption. At a high phosphate concentration, the phosphate and sodium ions will aggregate to form clusters, which results in random adsorption orientation. This work may provide some guidance for the design of Cyt-c-based bioelectronic devices and controlled enzyme immobilization.