Article pubs.acs.org/JPCB
Intermolecular Potential for Binding of Protonated Peptide Ions with Perfluorinated Hydrocarbon Surfaces Subha Pratihar,† Swapnil C. Kohale,† Saulo A. Vázquez,*,‡ and William L. Hase*,† †
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States Departamento de Química Física and Centro Singular de Investigación en Química Biológica y Materiales Moleculares Campus Vida, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
‡
S Supporting Information *
ABSTRACT: 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.
I. INTRODUCTION In mass spectrometry, soft landing refers to a process in which collisions of mass-selected polyatomic ions with surfaces in vacuum result in intact capture of the ions, with or without retention of the initial charge.1−9 For soft landing to occur, the fraction of the collision energy that is transferred to the surface and to the internal degrees of freedom of the ion must be such that the ion does not have enough recoil translational energy to escape from the surface attraction. Soft landing began to receive much attention in the 1990s, when it was proposed as a convenient method for preparing modified surfaces,2 with potential applications in materials science, microelectronics, biology, and catalysis.4,9 The types of projectile ions that have been selected for soft landing experiments include small and medium-size polyatomic ions,1,2,10−17 atomic and organometallic clusters,18−32 peptides,17,28,33−38 and proteins.4,33,39−42 In many of these experiments, self-assembled monolayers (SAMs)43 have been used as target surfaces.9 SAMs have highly ordered and well-characterized structures, which make them convenient targets for fundamental studies. Also, the SAM chains can be easily functionalized with a wide variety of chemical groups for specific interactions with projectile ions. At hyperthermal collision energies (
