Specific ion binding to macromolecules: Effects of hydrophobicity and ion pairing

Using molecular dynamics simulations in an explicit aqueous solvent, we examine the binding of fluoride versus iodide to a spherical macromolecule with both hydrophobic and positively charged patches. Rationalizing our observations, we divide the ion association interaction into two mechanisms: (1) poorly solvated iodide ions are attracted to hydrophobic surface patches, while (2) the strongly solvated fluoride and to a minor extent also iodide bind via cation-anion interactions. Quantitatively, the binding affinities vary significantly with the accessibility of the charged groups as well as the surface potential; therefore, we expect the ion-macromolecule association to be modulated by the local surface characteristics of the (bio-)macromolecule. The observed cation-anion pairing preference is in excellent agreement with experimental data.