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

• Published in: Physical chemistry chemical physics : PCCP 2016-04, Vol.18 (15), p.9979-9989
• Main Authors: Peng, Chunwang, Liu, Jie, Xie, Yun, Zhou, Jian
• Format: Article
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp00170j

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 (NH 2 -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 NH 2 -SAM mainly with two opposite orientations. One is similar to that characterized in the experiments for Cyt- c adsorbed on the NH 2 -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 NH 2 -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. The influence of anion type and concentration on the adsorption of cytochrome c onto the positively charged NH 2 -SAM surface.