Modeling the Interaction between Integrin-Binding Peptide (RGD) and Rutile Surface: The Effect of Cation Mediation on Asp Adsorption

The binding of a negatively charged residue, aspartic acid (Asp) in tripeptide arginine-glycine-aspartic acid, onto a negatively charged hydroxylated rutile (110) surface in aqueous solution, containing divalent (Mg ²⁺, Ca ²⁺, or Sr ²⁺) or monovalent (Na ⁺, K ⁺, or Rb ⁺) cations, was studied by molecular dynamics (MD) simulations. The results indicate that ionic radii and charges will significantly affect the hydration, adsorption geometry, and distance of cations from the rutile surface, thereby regulating the Asp/rutile binding mode. The adsorption strength of monovalent cations on the rutile surface in the order Na ⁺ > K ⁺ > Rb ⁺ shows a reverselyotropic trend, while the divalent cations on the same surface exhibit a regularlyotropic behavior with decreasing crystallographic radii (the adsorption strength of divalent cations: Sr ²⁺ > Ca ²⁺ > Mg ²⁺). The Asp side chain in NaCl, KCl, and RbCl solutions remains stably H-bonded to the surface hydroxyls and the inner-sphere adsorbed compensating monovalent cations act as a bridge between the COO ^- group and the rutile, helping to trapthe negatively charged Asp side chain on the negatively charged surface. In contrast, the mediating divalent cations actively participate in linking the COO ^- group to the rutile surface; thus the Asp side chain can remain stably on the rutile (110) surface, even if it is not involved in any hydrogen bonds with the surface hydroxyls. Inner- and outer-sphere geometries are all possible mediation modes for divalent cations in bridging the peptide to the rutile surface.