An analytical potential energy function to model protonated peptide soft-landing experiments. The CH3NH3+/CH4 interactions

Abstract

To model soft-landing of peptide ions on surfaces, it is important to have accurate intermolecular potentials between these ions and surfaces. As part of this goal, ab initio calculations at the MP2/aug-cc-pVTZ level of theory, with basis set superposition error (BSSE) corrections, were performed to determine both the long-range attractive and short-range repulsive potentials for CH₄ interacting with the –NH₃⁺group of CH₃NH₃⁺. Potential energy curves for four different orientations between CH₄ and CH₃NH₃⁺ were determined from the calculations to obtain accurate descriptions of the interactions between the atoms of CH₄ and those of –NH₃⁺. A universal analytic function was not found that could accurately represent both the long-range and short-range potentials for collision energies as high as those obtained in surface-induced-dissociation (SID) experiments. Instead, long-range and short-range analytic potentials were developed separately, by simultaneously fitting the four ab initio potential energy curves with a sum of two-body interactions between the atoms of CH₄ and –NH₃⁺, and then connecting these long-range and short-range two-body potentials with switching functions. Following a previous work [J. Am. Chem. Soc., 2002, 124, 1524], these two-body potentials may be used to describe the interactions of the N and H atoms of the –NH₃⁺group of a protonated peptide ion with the H and C atoms of alkane-type surfaces such as alkyl thiol self-assembled monolayers and H-terminated diamond. Accurate short-range and long-range potentials are imperative to model protonated peptide ion soft-landing experiments. The former controls the collision energy transfer, whereas the latter describes the binding of the ion to the surface. A comparison of the ab initio potential energy curves for CH₃NH₃⁺/CH₄ with those for NH₄⁺/CH₄ shows that they give nearly identical two-body interactions between the atoms of –NH₃⁺ and those of CH₄, showing that the smaller NH₄⁺/CH₄ system may be used to obtain the two-body potentials. A comparison of the four ab initio potential energy curves reported here for CH₃NH₃⁺/CH₄, with those given by the AMBER and CHARMM molecular mechanical potentials, show that these latter potentials “roughly” approximate the long-range attractions, but are grossly in error for the short-range repulsions. The work reported here illustrates that high-level ab initio calculations of intermolecular potentials between small model molecules may be used to develop accurate analytical intermolecular potentials between peptide ions and surfaces.