Intermolecular Potential for Binding of Protonated Peptide Ions with Perfluorinated Hydrocarbon Surfaces

An analytic potential energy function was developed to model both short-range and long-range interactions between protonated peptide ions and perfluorinated hydrocarbon chains. The potential function is defined as a sum of two-body potentials of the Buckingham form. The parameters of the two-body potentials were obtained by fits to intermolecular potential energy curves (IPECs) calculated for CF₄, which represents the F and C atoms of a perfluoroalkane chain, interacting with small molecules chosen as representatives of the main functional groups and atoms present in protonated peptide ions: specifically, CH₄, NH₃, NH₄⁺, and HCOOH. The IPECs were calculated at the MP2/aug-cc-pVTZ level of theory, with basis set superposition error (BSSE) corrections. Good fits were obtained for an energy range extending up to about 400 kcal/mol. It is shown that the pair potentials derived from the NH₃/CF₄ and HCOOH/CF₄ fits reproduce acceptably well the intermolecular interactions in HCONH₂/CF₄, which indicates that the parameters obtained for the amine and carbonyl atoms may be transferable to the corresponding atoms of the amide group. The derived potential energy function may be used in chemical dynamics simulations of collisions of peptide-H⁺ ions with perfluorinated hydrocarbon surfaces.