Chemical Dynamics Simulations of Energy Transfer in Collisions of Protonated Peptide−Ions with a Perfluorinated Alkylthiol Self-Assembled Monolayer Surface

Abstract

Classical trajectory simulations are performed to study energy transfer in collisions of protonated diglycine, gly 2 -H + , and dialanine, ala 2 -H + , ions with a fluorinated octanethiol self-assembled monolayer (F-SAM) surface for collision energies E i in the range of 5-70 eV and incident angles θ i of 0 and 45°with respect to the surface normal. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface. The simulations show the distribution of energy transfer to the peptide-ion's internal degrees of freedom, ∆E int , to the surface, ∆E surf , and in peptide-ion translation, E f , are very similar for gly 2 -H + , and ala 2 -H + . The average percentage energy transferred to ∆E surf and E f increases and decreases, respectively, with an increase in E i , while the average percentage energy transfer to ∆E int is nearly independent of E i . Changing θ i from 0 to 45°decreases and increases the percentage of energy transfer to ∆E surf and E f , respectively, but has little change in the transfer to ∆E int . Average percentage energy transfer to the surface is found to approximately depend on E i according to exp(-b/E i ). Comparisons with previous simulations show that peptide-H + collisions with the EA F-SAM model transfer approximately a factor of 2 more energy to ∆E int than do collisions with the hydrogenated SAM, that is, H-SAM. Replacing the mass of the F atoms by that of a H atom in the simulations, without changing the potential, shows that the different ∆E int energy transfer efficiencies for the F-SAM and H-SAM surfaces is a mass effect. The simulations for ala 2 -H + colliding with the EA F-SAM surface give P(∆E int ) distributions in good agreement with previous experiments and an average transfer to ∆E int of 15% as compared with the experimental value of 21%. The UA F-SAM model gives energy transfer efficiencies in qualitative agreement with those of the EA model, but there are important quantitative differences.

Importance of shattering fragmentation in the surface-induced dissociation of protonated octaglycine

“…They found the same result in collisions of the same ion, C 2 H 6 O ϩ• , with perfluoro (F-SAM), hydrogenated (H-SAM), and -COOH terminated (HOOC-SAM) alkylthiol selfassembled monolayer surfaces for collisions with i in the same range [41,42]. In simulation of gly 2 -H ϩ collisions with a perfluorinated octanethiol F-SAM surface [38], the energy-transfer to ⌬E int is similar for collisions with i of 0°and 45°. For i ϭ 0°, the percentage transfer to ⌬E int changes from 11% to 13% as E i is increased from 30 eV to 70 eV.…”

“…For the other incident angle studied, 45°, these percentages are 26%, 12%, and 62%. The strong dependence of the energy-transfer to ⌬E int , on the incident angle, has not been observed previously in either experimental [40 -42] or computational [38] studies of protonated peptide ion collisions with hydrocarbon molecule and selfassembled monolayer (SAM) surfaces. For a smooth and rigid surface, there are no mechanisms for interconverting the normal and parallel components of the incident velocity and, as a result, the energy-transfer dynamics depends only on the normal component with the parallel component conserved [47].…”

“…Such a pronounced effect is not seen in a simulation of gly 2 -H ϩ collisions with a perfluorinated octanethiol self-assembled monolayer (F-SAM) surface, for E i in the range of 30 -70 eV [38]. For this system, energy-transfer to ⌬E int increases by only a factor 1.3 in changing i from 45°to 0°.…”

“…Such an effect is not seen experimentally in collisions with a surface consisting of large organic molecules or self-assembled monolayers, [40 -42] and computationally in collisions with a self assembled monolayer (SAM) [38]. Herman and coworkers [40] studied collisions of ethanol molecular ions, C 2 H 6 O ϩ• , with stainless-steel surfaces covered by a multilayer of hydrogen-containing substances, usually identified as (pump oil) hydrocarbons, and found that the probability of energy-transfer to the ion's internal degrees of freedom, ⌬E int , is independent of i in the range of 40°to 80°.…”

Energy Transfer and Thermal Accommodation in Ozone Scattering from a Perfluorinated Self-Assembled Monolayer

“…The model shows a linear dependence of ⟨ΔE⟩ versus E i for high incident energies, which explains the fairly constant values of ⟨ΔE int ⟩/E i as a function of the incident energy found previously for a number of systems. 8,32,48,49 Figure 3 shows the average values of energy transferred to vibration, rotation and translation of the projectile obtained in our O 3 + F-SAM simulations (symbols) and the fits of eq 11 to the simulation data (solid lines). The parameters obtained in the fits are collected in Table 2.…”