Chemical Dynamics Simulations of Energy Transfer in Collisions of

Jun 4, 2008 - Chemical Engineering, Vanderbilt UniVersity, NashVille, Tennessee .... High Performance Computing Center, Texas Tech University. J. Phys...
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J. Phys. Chem. C 2008, 112, 9377–9386

9377

Chemical Dynamics Simulations of Energy Transfer in Collisions of Protonated Peptide-Ions with a Perfluorinated Alkylthiol Self-Assembled Monolayer Surface Li Yang,*,† Oleg A. Mazyar,‡ U. Lourderaj,† Jiangping Wang,§ M. T. Rodgers,§ Emilio Martı´nez-Nu´n˜ez,| Srirangam V. Addepalli,⊥ and William L. Hase*,† Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409, Department of Chemical Engineering, Vanderbilt UniVersity, NashVille, Tennessee 37235-1604, Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202, Departamento de Quı´mica Fı´sica, UniVersidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain, and High Performance Computing Center, Texas Tech UniVersity, Lubbock, Texas 79409 ReceiVed: December 25, 2007; ReVised Manuscript ReceiVed: March 21, 2008

Classical trajectory simulations are performed to study energy transfer in collisions of protonated diglycine, gly2-H+, and dialanine, ala2-H+, ions with a fluorinated octanethiol self-assembled monolayer (F-SAM) surface for collision energies Ei in the range of 5-70 eV and incident angles θi 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, ∆Eint, to the surface, ∆Esurf, and in peptide-ion translation, Ef, are very similar for gly2-H+, and ala2-H+. The average percentage energy transferred to ∆Esurf and Ef increases and decreases, respectively, with an increase in Ei, while the average percentage energy transfer to ∆Eint is nearly independent of Ei. Changing θi from 0 to 45° decreases and increases the percentage of energy transfer to ∆Esurf and Ef, respectively, but has little change in the transfer to ∆Eint. Average percentage energy transfer to the surface is found to approximately depend on Ei according to exp(-b/Ei). Comparisons with previous simulations show that peptide-H+ collisions with the EA F-SAM model transfer approximately a factor of 2 more energy to ∆Eint 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 ∆Eint energy transfer efficiencies for the F-SAM and H-SAM surfaces is a mass effect. The simulations for ala2-H+ colliding with the EA F-SAM surface give P(∆Eint) distributions in good agreement with previous experiments and an average transfer to ∆Eint 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. Ei ) ∆Eint + ∆Esurf + Ef

I. Introduction For over a decade, peptides and proteins have been a principal research interest of mass spectrometry.1–3 The dissociation energies and dynamics, for their fragmentation pathways, have been actively explored by tandem mass spectrometry and the fragmentation products provide a fingerprint of the ion’s structure.4–7 In surface-induced dissociation (SID), ions are energized by their collisions with a surface and this mass spectrometry technique has been successfully applied to study a wide variety of fragmentations ranging from relatively small ions8–13 to high-mass biomolecules.14–18 If electronic excitation is unimportant, the collision’s translational energy Ei is partitioned between the final translational energy Ef, and transferred to the internal vibrational/rotational modes of the ion, ∆Eint, and the vibrations of the surface, ∆Esurf

* Corresponding author. E-mail: [email protected] (L.Y.) and bill.hase@ ttu.edu (W.L.H.). † Department of Chemistry and Biochemistry, Texas Tech University. ‡ Vanderbilt University. § Wayne State University. | Universidad de Santiago de Compostela. ⊥ High Performance Computing Center, Texas Tech University.

(1)

The nature of the surface strongly affects the fraction of the ion’s collision energy that is converted into its internal energy.19,20 Different surfaces are used to fragment ions in SID experiments, and self-assembled monolayer (SAM) surfaces have proven to be particularly useful for the study of low-energy ion/surface collisions.21,22 Both hydrogenated and fluorinated SAM surfaces have been extensively utilized in this regard.12,23,24 Experiments show that fluorinated hydrocarbon surfaces are particularly effective in transferring energy to the ion and inducing its dissociation. For example, the percentage of collision energy transferred to the ion’s internal energy is in the range 12-17% for hydrogenated alkylthiol self-assembled monolayer (H-SAM) surfaces and 18-28% for fluorinated alkylthiol self-assembled monolayer (F-SAM) surfaces.25,26 The hydrogenated and fluorinated surfaces have been described as behaving like a “soft mattress” and a “hard wall”, respectively.19,20 Classical trajectory simulations, based on accurate potentials for the projectile-surface system, have proven to be an important method for determining the distributions of transfer to ∆Eint, ∆Esurf, and Ef, and how they depend on Ei, incident angle θi, the projectile ion’s size and shape, and properties of the surface.25,27–35 Trajectory simulations of Cr+(CO)6 SID,29 utilizing an analytic potential, have given detailed information

10.1021/jp712069b CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008

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regarding the energy transfer distributions. The value for the average fraction of collision energy transferred to Eint is in excellent agreement with experiment.20 The energy transfer that occurs when the protonated glycine (gly) and alanine (ala) ions gly-H+, gly2-H+, gly3-H+, gly5-H+, and ala2-H+ collide with the hydrogenated diamond {111} (H-diamond) and H-SAM surfaces20,25,31 have been simulated by using the molecular mechanical AMBER force field36 for the peptide-ion’s intramolecular potential, and the calculated energy transfer efficiencies are consistent with experimental studies.10 In this article, classical trajectory simulations are reported of the energy-transfer dynamics associated with collisions of gly2H+ and ala2-H+ with the perfluorinated alkylthiol [CF3(CF2)7S] self-assembled monolayer surface (F-SAM) on gold. The simulations were performed with two model potentials for the F-SAM surface, that is, an explicit-atom (EA) model,37 used with success in a recent dynamics study of CO2 scattering from the F-SAM,28 and a united-atom (UA) model,38–44 wherein the CF3 and CF2 units are represented as single pseudoatoms. The energy transfer efficiencies determined from the current study are compared with those of the previous simulations for the H-SAM and H-diamond surfaces to see how energy transfer to ∆Eint, ∆Esurf, and Ef depend on the surfaces. The simulations are performed at collision energies Ei of 5-70 eV and collision angles θi of 0° and 45° to study the role of collision energy and incident angle on the energy transfer dynamics. II. Potential Energy Function The general analytic potential energy function used for the peptide/F-SAM system is given by

V ) Vpeptide + Vsurface + Vpeptide,surface

(2)

where Vpeptide is the protonated peptide intramolecular potential, Vsurface is the potential for the F-SAM surface, and Vpeptide,surface is the peptide/surface intermolecular potential. For the simulations reported here, the AMBER valence force field of Cornell et al.36 was used for the gly2-H+ and ala2-H+ intramolecular potentials. A local energy minimization algorithm of the VENUS computer program was used to determine the conformer structures and the same minimum energy conformers of these peptide-ions were used as in the previous simulations.30,45 The AMBER potential is harmonic for stretch and bend terms, and anharmonicity is introduced via its torsions and nonbonded terms. In previous simulations of gly-H+ and gly2-H+ collisions with the H-terminated diamond {111} surface, for Ei in the range of 30-100 eV, the same energy transfer efficiencies are found using AMBER as using the AM1 and MP2/6-31G* electronic structure theory models in QM + MM direct dynamics simulations.31,45,46 That AMBER, AM1 and MP2/6-31G* give the same energy-transfer efficiencies suggests the collisional energy transfer is direct, impulsive, occurs in a short-time47 and, thus, is only influenced by the peptide-ion’s structure, the forces about the ion’s potential energy minimum, and the ion-surface intermolecular potential.30,46 A. Surface Potential Energy. As indicated above, both explicit-atom (EA) and united-atom (UA) models were considered to represent the F-SAM surface. In the EA model, developed in previous work,37 the F-SAM consists of a 3 by 3 unit cell which contains a total of 108 free chains of the CF3(CF2)7S radical adsorbed on a rigid single layer of 441 constrained Au atoms. The validity of treating the gold atoms as a rigid anchoring slab has been shown elsewhere.34 The S atoms are adsorbed in a shape of a rhombus, to correspond to

experiment,48 with each S atom interacting with the closest three Au atoms via three individual harmonic stretching potentials.49 Both bonded and nonbonded interactions are required to characterize the potential energy of the EA F-SAM surface. The functional form for this potential is:

VF-SAM )

kr kθ (r - re)2 + (θ - θe)2 + 2 2 stretches bends

∑

∑

Vn [1 - cos(nφ - γ)] + 2 dihedrals

∑

∑

Buckingham

[

A exp(-Br) +

]

[

C3 C12 C + + 6 12 r (z - z0)3 Klein (z - z0)

∑

]

(3) All of the parameters for the EA model of the F-SAM surface are given in ref 37. The difference between this EA model and that in ref 28 is that interactions between the CF2 and CF3 groups of the alkyl chains and the gold surface are included, which is represented as the last part of eq 3, where z is the shortest distance between one of the atoms of -CF3 and -CF2- groups and Au surface. The parameters for this potential were derived by Hautmann and Klein.50 This EA model gives a 300 K structure for the F-SAM which is in excellent agreement with experiment;48 for example, the average distance between the terminal C atoms is 5.89 ( 0.01 Å in comparison to the experimental value of 5.7 ( 0.2 Å.48 The united-atom model of the alkylthiolate self-assembled monolayer (UA F-SAM) surface consists of 108 chains of the CF3(CF2)7S radical absorbed on a single layer of 441 constrained Au atoms. The nearest neighbor internuclear distance between the Au atoms is 2.884 Å, consistent with the bulk value of 2.88 Å.49 Sulfur atoms of the CF3(CF2)7S moiety are placed 1.931 Å above the Au{111} surface38 in close-packed rows rotated 30° from the close-packed rows of gold atoms48 maintaining the nearest neighbor chain-chain distance of 5.776 Å. The backbone of CF3(CF2)7S radical has a tilt angle of ∼12° with respect to the Au surface normal. The general expression of the intramolecular potential energy of the UA F-SAM surface is the same as the EA model, but the nonbonded interactions are represented by Lennard-Jones 6-12 potentials instead of Buckingham functions:

V(r) )

A B + 6 12 r r

(4)

Lennard-Jones interactions between united atoms separated by one, two, and three bonds were neglected. Nonbonding interactions for united atoms separated by four or more bonds were included in the potential energy evaluation. A spherical potential truncation at 13.5 Å and no tail corrections were used in the UA F-SAM model. The average distance between the terminal C atoms is 5.95 Å, which is in agreement with AFM results.48 The potential energy parameters for the UA F-SAM surface are summarized in refs 40, 42–45, 48, and 49. Given the impulsiveness of the peptide-H+ + surface collision,30,46 it is expected that a potential for the surface as used here, with harmonic bend and stretch terms and anharmonic torsions and nonbonded terms, is accurate and sufficient for modeling the energy transfer. However, to assess the possible importance of additional anharmonicity for the surface potential, the harmonic stretches were replaced with Morse functions. The βe (Å-1), De (kcal/mol) values for the C-C and C-F bonds are 1.66, 100.0 and 1.76, 130.0, respectively.51 As discussed in

ET in Collisions of Protonated Peptide-Ions

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section IV.A., the energy transfer efficiencies remain statistically the same when these stretch anharmonic terms are added. When the model for the F-SAM surface is used in the chemical dynamics simulations, the correct treatment of the model’s boundaries is crucial because it enables “macroscopic” properties to be calculated from simulations using a relatively small surface. These boundary effects were considered in this study, and two strategies are applied: periodic boundary conditions (PBC)52 and a frozen outermost layer (rigid border).53 B. Peptide/Surface Potential Energy. The interaction term between the peptide-ion and the F-SAM surface in the EA model is expressed by a sum of two-body potentials between the atoms of the peptide and the atoms of F-SAM. The twobody potential is given by

VXY(rij) ) AXY exp(-BXYrij) +

CXY rij5

(5)

where X corresponds to the C and F atoms of the F-SAM and Y corresponds to H, C, O, and N atoms of the peptide. The potentials were derived from high level ab initio potential energy curves54 for the interaction between CF4, as a model for C and F atoms of a fluorinated alkane surface, and CH4, NH3, NH4+, H2CO, and H2O, as models for the atoms of the different functional groups comprising a protonated glycine or alanine peptide-ion. For the UA model of the peptide-ion/F-SAM potential, individual atoms of the peptide-ion interact with united atoms representations of the -CF3 and -CF2- moieties of the F-SAM. These UA moieties were assumed to be represented by an isotropically averaged CF4 molecule. Potential energy curves were calculated between isotropically averaged CF4 and different molecules representing functional groups of the peptide-ion. These curves were then fit with a sum of two body potentials between the peptide-ion’s atoms and the isotropically averaged CF4, representing the UA atoms of the -CF2- and -CF3 moieties. The two body potential function required for these fits is

VUA,Y ) AY exp(-BYr) + CY /rn + DY /r12 + EY /r6

(6)

where r is the distance between the C atom of the CF4 UA model and the Y atom of the peptide. The parameters for these two-body potentials are given in ref 54. III. Trajectory Simulations The classical trajectory simulations were carried out with the general chemical dynamics package VENUS.55 The center of a beam of peptide-ion projectiles is aimed at the center of the surface, with fixed incident angle θi with respect to the surface normal and fixed initial translational energy Ei. The peptide projectile for each trajectory was randomly placed in the cross section of this beam and randomly rotated about its center of mass so that it has an initial random orientation with respect to the surface. The diameter of the beam was chosen so that it overlapped a unit cell on the surface. The azimuthal angle χ, between the beam and a fixed plane perpendicular to the surface, was sampled randomly between 0 to 2π to simulate collisions with different domains of growth on the F-SAM surface. The distance between the center of the beam and the top of the surface was set to 30 Å. Initial conditions for the vibrational modes of the peptide-ions were chosen via the quasi-classical normal mode method,56–58 which includes zero-point energies. The excess energy, for each normal mode of vibration, was selected from the mode’s 300

TABLE 1: Average Percentage Energy Transfers for the EA F-SAM Modela gly2-H+

ala2-H+

Ei (eV)

∆Eint

∆Esurf

Ef

∆Eint

∆Esurf

Ef

5 10 30 50 70

15 15 11 12 13

40 64 81 83 83

θi ) 0° 45 21 8 5 4

18 14 11 11 13

32 60 82 85 83

50 26 7 4 4

5 30 70

10 15 17

22 55 63

θi ) 45° 68 30 20

11 13 18

15 54 61

74 33 21

a The simulations were performed for the PBC model of the F-SAM surface. Uncertainties (standard deviation of the mean) are (1%.

K harmonic oscillator Boltzmann distribution. The energy of each normal mode was partitioned between kinetic and potential by choosing a random phase for the normal mode. A 300 K rotational energy of RT/2 was added to each principal axis of rotation of the peptide-ions. Initial conditions for a surface were chosen by assigning velocities, sampled from a Maxwell-Boltzmann distribution at 300 K, to the surface atoms. The surface was then equilibrated for 1 ps of molecular dynamics by scaling the atomic velocities so the surface temperature corresponds to that for 300 K classical Boltzmann distributions. The structure and velocity obtained from this equilibration process is then used as the initial structure for an equilibration run at the beginning of each trajectory. Trajectories were halted when the separation between peptide-ion and the surface reached 40 Å. One hundred trajectories were computed for each set of initial conditions with fixed Ei and θi. When the trajectory is terminated, the peptide-ion’s final translational energy, Ef, is determined and the ion’s internal energy change, ∆Eint, is determined by subtracting the initial value of the projectile’s internal energy from its final value. The energy transferred ∆Esurf is then determined from the energy conservation relationship in eq 1. Standard deviations of the mean are calculated for the average percentage energy transfers to ∆Eint, ∆Esurf, and Ef, to provide uncertainties in the average energy transfer efficiencies. The standard deviation of the mean is the standard deviation of the distribution divided by the square-root of the number of events (i.e., trajectories) comprising the distribution.59 IV. F-SAM Explicit-Atom (EA) Model Results The results of simulations with the explicit-atom (EA) and PBC models for the F-SAM surface are summarized in Table 1. The effects of different collision properties on the energy transfer efficiencies are described below. A comparison between the results of the PBC and rigid border models of the F-SAM surface is given at the end of this section. A. Effect of Peptide Structure. Values for the average percentage energy transferred to ∆Eint, ∆Esurf, and Ef for gly2H+ and ala2-H+ collisions with the F-SAM surface are given in Table 1 for θi of 0° and 45° and Ei of 5-70 eV. Overall, the energy transfer efficiencies are similar for gly2-H+ and ala2H+. The most pronounced difference is at Ei ) 5 eV where those for ∆Esurf and Ef are smaller and larger, respectively, for ala2-H+ as compared with gly2-H+. The distributions of energy transfer are similar for gly2-H+ and alal2-H+ and are shown in

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Figure 1. Distributions of energy transfer to ∆Eint, ∆Esurf, and Ef, for ala2-H+ colliding with the EA F-SAM surface at θi of 0° and Ei of 10 and 70 eV. The probability for a bin width equals the number of trajectories in the bin divided by both the total number of trajectories and the bin width. Thus, the area under each histogram is unity.

Figure 1 for ala2-H+ collisions for θi ) 0° and Ei of 10 and 70 eV. The width of the P(∆Eint) distributions is similar to that for gly3-H+ collisions with the H-SAM and much narrower than for collisions of this peptide-ion with the diamond surface.25 These comparisons are discussed in more detail in section V. In previous work, the P(∆Eint), P(∆Esurf), and P(Ef) distributions for gly2-H+ and ala2-H+ were found to be similar for collisions with H-diamond{111}.30 In earlier simulations30 of collisions with diamond at θi ) 45°, energy transfer to ala2-H+ was slightly more efficient than to gly2-H+. Ala2-H+ has two additional CH3-rotor torsional degrees of freedom, and slightly more efficient energy transfer to this ion is consistent with trajectory studies25,30 which show that peptide torsional degrees of freedom are preferentially excited in its collisional activation. However, both previous similations30 and experiments10–18,23,24 show the energy transfer efficiencies do not strongly depend on peptide size. Experi-

mentally, the energy transfer efficiency to ∆Eint is nearly the same for ala2-H+ and the octa-peptide bradykinin.10,24 In the impulsive energy-transfer regime, the size of the peptide has no effect on the energy-transfer efficiency.30,32 B. Effect of Ei. The initial peptide translational energy is expected to affect the energy-transfer dynamics, and this is observed here. The percentage energy transfer to ∆Esurf and Ef increases and decreases, respectively, as the collision energy is increased, while percentage energy transfer to ∆Eint is much less sensitive to the collision energy. These are the trends found in previous simulations of peptide-H+ collisions with the H-diamond surface.30 In experimental studies of peptide-H+ SID, energy transfer to ∆Eint is found to be only weakly sensitive to Ei for collisions with both the H-SAM and the F-SAM surfaces.24 C. Effect of θi. Table 1 shows that changing θi from 0° to 45° has a pronounced effect on the efficiency of energy transfer

ET in Collisions of Protonated Peptide-Ions to ∆Esurf and Ef, with the percentage energy transfer to ∆Esurf and Ef decreasing and increasing, respectively, as θi is changed from a normal to 45° collision. Energy transfer to ∆Eint is much less dependent on θi, except for the lowest collision energy of 5 eV where the percent energy transfer decreases in going from 0° to 45°. This finding, that energy transfer to ∆Eint is nearly independent of incident angle, is also observed for collisions with H-SAM surfaces in experiments by Hermann and co-workers.60,61 In contrast, simulations of peptide-H+ collisions with the crystalline diamond {111} surface show that energy transfer to ∆Eint decreases as θi is increased from 0° to 45°.30,62 D. Importance of the Hautmann-Klein SAM/Au Potential and Additional Anharmonicity in the SAM Potential. Included in the F-SAM potential energy function used here, eq 3, are interactions between the CF2 and the CF3 groups of the alkyl chains and the Au surface as represented by the analytic function of Hautmann and Klein.50 Though these interactions were not included in the previous study of CO2/F-SAM scattering,28 they were viewed as possibly quite important for the current simulation given the 5-70 eV collision energy of the peptide-H+ ions as compared with the 0.1-1.0 eV for the CO2 collisions.28 To assess the possible importance of the Hautmann-Klein potential for describing peptide-H+/F-SAM energy transfer, the simulations were repeated with this potential term removed from the F-SAM potential in eq 3. It was found that the HautmannKlein potential has a negligible effect on the percentage transfer to ∆Eint and only small effects on the transfers to ∆Esurf and Ef. For the gly2-H+/F-SAM simulations with Ei ) 5-70 eV and θi of 0° and 45°, the average magnitude of the difference in the percentage of energy transfer to ∆Eint, with and without the Hautmann-Klein potential, is only 1% and within the statistical uncertainty of the simulations. The corresponding differences in the percentage energy transfer, to ∆Esurf and Ef, are each 3% and somewhat larger than that for ∆Eint but still small. Animations of the gly2-H+/F-SAM collisions, with and without the Hautmann-Klein term, suggested that removing it from the F-SAM potential should significantly affect the energy transfer. For collisions with Ei of 30 eV and higher, some of trajectories resulted in several of the alkyl chains “passing through” the Au surface when the repulsive interactions between CF2 and CF3 and the surface of the Hautmann-Klein potential were not present. However, as shown above, this did not have a major effect on the calculated energy transfer percentages, consistent with the impulsiveness of the collisions and energy transfer.30,32 The potential energy function used for the F-SAM surface has harmonic stretch and bend terms. To investigate the possibility that additional anharmonicity for the F-SAM potential would affect the energy transfer efficiencies, simulations were performed for ala2-H+ collisions with the EA F-SAM model and the C-C and C-F harmonic stretches of the F-SAM replaced with Morse oscillators. These simulations were performed for θi ) 0° and Ei ) 5, 30, and 70 eV. The resulting respective average energy transfer percentages to ∆Eint, ∆Esurf, and Ef for these three Ei are 5 eV -17, 33, 50; 30 eV -10, 82, 8; 70 eV - 13, 83, 4. A comparison of these percentages with those in Table 1, for the F-SAM model with harmonic stretches, shows they differ at most by only 1% and, thus, replacing the harmonic C-C and C-F stretch potentials with Morse oscillators has no statistically meaningful effect on the energy transfer efficiencies.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9381 TABLE 2: Comparison of Energy Transfer Efficiencies for gly2-H+/F-SAM Energy Transfer with the PBC and Rigid Border EA F-SAM Modelsa ∆Eint

∆Esurf

Ei (eV)

PBC

rigid

5 10 30 50 70

15 15 11 12 13

13 12 12 14 15

5 30 70

10 15 17

9 15 14

PBC

Ef

rigid

PBC

rigid

θi ) 0° 40 64 81 83 83

48 66 79 79 80

45 21 8 5 4

39 22 9 7 5

θi ) 45° 22 55 63

28 58 65

68 30 20

63 27 21

a The numbers are percentages of energy transfer. Uncertainties (standard deviation of the mean) are (1%.

E. Effects of the Rigid Border F-SAM Model and F-SAM Surface Size. The above simulations were performed using twodimensional periodic boundary conditions (PBC) for the F-SAM surface (section II.A.). Another possible model for the F-SAM surface is to have a rigid exterior border of alkyl chains surrounding the group of chains interacting with the colliding peptide-H+ ion, to ensure these interior chains do not become unphysically disordered. This model, with 46 rigid, exterior alkyl chains and 75 interior chains, was investigated for gly2-H+ collisions at θi ) 0° and 45° and Ei of 5-70 eV. The EA potential given by eqs 3 and 5 was used for these simulations. The results are given in Table 2, where they are compared with the PBC results presented and discussed above. The ∆Eint energy transfer percentages are statistically the same for the PBC and rigid border F-SAM models. Except for the Ei ) 5 eV collisions, the PBC and rigid border models also give similar transfer percentages for both ∆Esurf and Ef. At this low Ei, the collisions may be less impulsive and the specific features of the F-SAM model may become important for the energy transfer. However, this does not affect the energy transfer to the internal modes of the peptide-ion. A calculation was performed, using the above rigid exterior border model, to determine the effect on the energy transfer efficiencies of decreasing the size of the F-SAM surface. The number of alkyl chains was decreased from the above 121 to 57 for this additional gly2-H+ + F-SAM calculation at Ei ) 50 eV and θi ) 0°. Within statistical uncertainties, the energy transfer efficiencies were found to be insensitive to this decrease in the size of the F-SAM model. The energy transfer percentages to ∆Eint, ∆Esurf, and Ef are 12, 81, and 7% as compared with the values of 14, 79, and 7% in Table 2 for the larger F-SAM model. V. F-SAM United-Atoms (UA) Model Results To compare with the above results, based on the EA model for the F-SAM, calculations were performed for gly2-H+ collisions with the united-atom (UA) model of the F-SAM. In this model the -CF3 and -CF2- moieties are treated as united atoms. Both PBC and rigid border models (section II.A.) were used for the UA F-SAM surface. The PBC model has 108 free chains in its primary cell and the rigid border model has 75 mobile chains surrounded by 46 rigid chains. These are the same number of chains as used for the EA F-SAM model (section IV). To simulate collisions of projectiles with a surface, a model for the surface with many atoms is often required. Simulations

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TABLE 3: Average Percentage Energy Transfers for gly2-H+ Collisions with the UA F-SAM Modela ∆Eint

∆Esurf

Ei (eV)

PBC

rigid

5 10 30 50 70

26 28 26 26 26

27 28 27 27 27

5 30 70

22 26 30

22 25 29

a

PBC

TABLE 4: Effects of Surface and Incident Energy and Angle on Energy Transfera H-SAMb

Ef

rigid

PBC

rigid

θi ) 0° 64 65 68 70 71

64 64 68 69 70

10 7 6 4 3

9 8 5 4 3

θi ) 45° 41 46 49

41 46 48

37 28 21

37 29 23

Uncertainties (standard deviation of the mean) are (1%.

for such a large model may require a substantial amount of computer time and, thus, it is of interest to study UA surface models, which reduce the number of atoms treated explicitly. Table 3 gives the average percentage energy transfers to ∆Eint, ∆Esurf, and Ef for gly2-H+ collision with the UA F-SAM surface, at collision energies Ei of 5-70 eV and incident angles θi of 0° and 45°. For all of the initial conditions, the energy transfer percentages are statistically the same for the PBC and rigid border F-SAM models, similar to the above EA F-SAM results. Compared with the results of the F-SAM EA model in Table 1, the UA model leads to similar trends to those obtained by the more CPU-time demanding EA model. The transfer of energy to ∆Eint is unaffected by Ei, while the transfers to ∆Esurf and Ef increase and decrease, respectively, when Ei is increased. However, there are significant differences in the actual energy transfer percentages for the F-SAM UA and EA models. The values for the average percentage energy transfer to the internal degrees of freedom of gly2-H+, ∆Eint, is larger for the UA F-SAM. Also, the transfers of energy to ∆Esurf and Ef are much less sensitive to Ei for the UA F-SAM. For the collisions at θi of 0°, the percentage energy transfer to ∆Esurf for the EA F-SAM increases from 40% to 83% as Ei is increased from 5 to 70 eV, but for UA F-SAM, this percentage only changes from 64% to 71% for the same range of collision energy. The difference in the percentage energy remaining in gly2-H+ translation, that is, Ef, for the UA and EA models is particularly striking. This percentage varies significantly from 45% to 4% for the EA F-SAM but rather insignificantly from 10% to 2% for UA F-SAM, with θi ) 0°, and Ei increased from 5 to 70 eV. It is noteworthy that for the largest Ei of 70 eV, the UA and EA models give nearly the same energy transfer percentage to Ef. However, the percentage transfers to ∆Eint and ∆Esurf are larger and smaller, respectively, for the UA F-SAM. The above comparisons between the F-SAM UA and the EA models are for θi ) 0°. As shown in Tables 1 and 3, the same findings are obtained at θi ) 45°. Both the F-SAM and the peptide-H+/F-SAM potentials, that is, Vsurface and Vpeptide,surface in eq 2, affect the collision energy transfer efficiencies. The significant differences in the energy transfer efficiencies for the EA and UA F-SAM models suggest there are important differences in their Vsurface and/or Vpeptide,surface potentials. However, it is difficult to identify whether there are important differences for both potential terms or for only one. At low Ei, energy transfer to the surface is much more efficient for the UA model, suggesting the UA Vsurface potential is “softer” than that for the EA model. In addition, the Vpeptide,surface potential is longer range and more repulsive for the UA model which should affect the energy transfer efficiencies.34,54 For the future,

F-SAMc

Diamondd

peptide Ei (eV) ∆Eint, ∆Esurf, Ef ∆Eint, ∆Esurf, Ef ∆Eint, ∆Esurf, Ef gly-H+ gly2-H+

gly-H+ gly2-H+

ala2-H+

gly3-H+

gly5-H+

θi ) 0° 70 5 10 30 50 70 100

17, 47, 36 15, 40, 45 15, 64, 21 11, 81, 8, 12, 83, 5 13, 83, 4

24, 27, 49 20, 40, 40, 17, 48, 35

θi ) 45° 70 5 10 30 70 110 5 10 30 70 110 10 30 70 110 30

10, 22, 68 15, 55, 30 17, 63, 20 11, 15, 74 13, 54, 33 18, 61, 21 7, 63, 30e

12, 38, 50 16, 0, 84 17, 4, 79 16, 12, 72 14, 25, 61 13, 34, 53 19, 1, 80 19, 5, 76 18, 12, 70 21, 21, 58 15, 31, 54 9, 2, 91f 18, 9, 73 17, 21, 62 14, 29, 57 23, 5, 72

a The numbers are average percentage energy transfer. Uncertainties (standard deviation of the mean) are (1%. b Results from ref 25. c This work. d Results from refs 25, 31, 45, 46. e Results for folded gly3-H+. For extended gly3-H+ the percentages are 8, 60, 38. f Results for folded gly3-H+. For extended gly3-H+, at Ei ) 30 eV, the percentages are 20, 8, 80.

it will be of interest to investigate in detail differences in the Vpeptide,surface potentials for the EA and UA F-SAM models. VI. Peptide-H+/F-SAM Energy Transfer Dynamics A. Comparison with Previous Simulations. In previous work, classical chemical dynamics simulations, like those reported here, have been used to study energy transfer efficiencies for collisions of peptide-H+ ions with EA models for the H-SAM and H-diamond surfaces.25,31,45,48 The ions considered are gly-H+, gly2-H+, ala2-H+, gly3-H+, and gly5-H+ at collision energies ranging from 5-110 eV and incident angles of θi of 0° and 45°. The energy transfer percentages determined from these studies are compared in Table 4 with the values obtained here for the EA F-SAM model. Comparing the current F-SAM results for ala2-H+ and gly2H+ with those obtained previously for gly3-H+ colliding with the H-SAM shows that the percentage energy transfer to ∆Eint is approximately a factor of 2 larger for the F-SAM collisions. This increased peptide-H+ internal energy comes from the expense of ∆Esurf, with the percentage energy in Ef nearly the same for the H-SAM and F-SAM surfaces. The less efficient energy transfer to the peptide-ion for the collisions with the H-SAM as compared with the F-SAM is considered below in section VII as part of the discussion of mass effects. The trends in the energy transfer efficiencies versus Ei are similar for the F-SAM and diamond surfaces. The percentage transfer to ∆Eint is only weakly dependent on Ei, while the percentage transfers to ∆Esurf and Ef increase and decrease, respectively, with increased Ei. However, the values for the energy transfer efficiencies differ for the two surfaces. The

ET in Collisions of Protonated Peptide-Ions transfer to ∆Eint is larger for diamond, with the transfer to ∆Esurf larger and to Ef smaller for the F-SAM. A particularly interesting difference in the energy transfer to the two surfaces is that, for the F-SAM, transfer to ∆Eint is at most only weakly dependent on θi, while for the diamond surface, this energy transfer depends on θi, decreasing as θi is increased from perpendicular 0° collisions to 45°. B. Comparison with Experiment. A number of experimental studies have been performed to study the efficiency of transfer of translational energy to the projectile ion’s internal degrees of freedom in their collisions with organic surfaces.10–14,19,20,24,26,60,61,63–74 This energy transfer is not measured directly but deduced indirectly by either the deconvolution method,75 the use of a thermometer ion,20 an RRKM analysis of the projectile’s fragmentation,10 or measuring the translational energy distributions of the scattered projectile and its dissociation fragments.65 These analyses find that the transfer of internal energy to the ion is, at most, only weakly dependent on its size and that collisions with fluorinated alkane liquid, polymer, and SAM surfaces transfer approximately twice as much energy to the ion’s internal modes as do collisions with hydrogenated surfaces of these types. As shown in Table 4, the results of the trajectory simulations are consistent with these findings. Particularly relevant are comparisons of the simulation results with experiments by Laskin and Futrell.10,24 For ala2-H+ collisions with a F-SAM surface, for θi ) 0° and Ei in the range of 3-23 eV, the experiments10 find that the average energy transfer to the ion’s internal modes is 21%. The simulations’ ∆Eint energy transfer efficiency for this collision energy range is 14-18% for the EA F-SAM (Table 1) and 26-28% for the UA F-SAM (Table 3). As discussed above, an EA model is expected to be more accurate than a UA model, and the EA F-SAM model used here slightly underestimates the percentage T f V energy transfer to ∆Eint as compared with the experimental result. The width of the P(∆Eint) distributions determined from the simulations are in quite good agreement with the experimental results.10 For ala2-H+ colliding with the F-SAM, the full-width at half-maximum of the experimental P(∆Eint) is 30 kcal/mol for Ei ) 9 eV and θi ) 0° (see Figure 9 in ref 10). As shown in Figure 1, the simulation at Ei ) 10 eV gives a very similar result. At Ei ) 22.5 eV, the full-width at halfmaximum of the experimental P(∆Eint) is larger and 50 kcal/ mol.10 Though the experiments were performed using a F-SAM prepared from dodecanethiol10 and an octanethiolate F-SAM was used for the simulations, this difference is not expected to affect the experiment/simulation comparison since the melting point for both the C12 and C8 F-SAM is expected to be higher than the 300 K F-SAM temperature for the experiments and simulations. The H-SAM prepared from octanethiol has a melting point of 327 ( 5 K,76 and with its stronger intermolecular attractive potential, the melting point for the octanethiol F-SAM should be higher. Nevertheless, in future studies it will be of interest to compare peptide-H+ SID using H-SAM and F-SAM surfaces with alkylthiols of different chain lengths. Experiments24 of peptide-ion collisions with different hydrocarbon surfaces show that the width of P(∆Eint) increases in the order H-SAM < F-SAM < diamond. Previous simulations25 give a full-width at half-maximum of ∼40 kcal/mol and ∼125 kcal/mol for gly3-H+ colliding with the H-SAM and diamond surfaces, respectively, at Ei ) 30 eV and θi ) 45°. The P(∆Eint) described above for the F-SAM indicates that, at this collision energy, P(∆Eint) for the F-SAM is somewhat broader than that for the H-SAM but considerably less broad than that for diamond. Thus, the simulation results agree with

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9383 TABLE 5: Surface Mass Effect for Collisional Energy Transfera gly2-H+ ∆Eint ∆Esurf Ef

gly3-H+/H-SAMd

F-SAMb

F-SAM/Hc

folded

extended

15 55 30

8 65 27

7 63 30

8 54 38

a The collision energy is 30 eV and incident angle is 45°. The numbers are average percentage energy transfer. Uncertainties (standard deviation of the mean) are (1%. b Results for the EA F-SAM model in Table 1. c Results for the EA F-SAM model, but with the mass of the F atom replaced by that of the H atom. d Folded and extended gly3-H+ collisions with a n-hexyl thiolate SAM.25

the trend deduced from the experiments. In the future, it would be of interest to perform additional simulations to obtain a more precise comparison between the simulation and the experimental P(∆Eint) distributions. Laskin and Futrell studied energy transfer in collisions of des-Arg1-bradykinin, an octa-peptide, with H-SAM, F-SAM, and H-diamond surfaces.24 For θi ) 0°, the average amount of internal energy deposited in the ion was studied for Ei in the range of 28-105 eV, for collisions with the H-SAM surface and 12-58 eV for collisions with the F-SAM and H-diamond surfaces. The average T f V energy transfer percentages found for these Ei ranges are 10.1%, 20.5%, and 19.2% for the H-SAM, F-SAM, and H-diamond surfaces, respectively. The trends in these energy transfer efficiencies are the same as found from the current simulations (see Table 4). These efficiencies for the H-SAM and diamond are also similar, respectively, to the previous simulation results for gly3-H+/H-SAM at θi ) 45° and gly2-H+/diamond at θi ) 0°.25 However, for collision with the F-SAM at θi ) 0°, and within the experimental range of Ei, the EA simulation T f V energy transfer percentages for gly2H+ and ala2-H+ are approximately a factor of 2 smaller than the experimental results of 20.5% for des-Arg1- bradykinin. Figure 1 shows that for both the Ei of 10 and that of 70 eV simulations, there are ala2-H+ collisions with the F-SAM surface for which the scattered ions have quite low translational energies. The same result, not shown but indicated by Table 1, is found for the gly2-H+ projectile. These low translational energy scattered ions may not be able to overcome the ion-surface attractive energy and may become trapped on the surface, which is consistent with soft-landing experiments utilizing F-SAM surfaces.18,77,78 The peptide-ion/surface intermolecular potential used for this study, section II.B., was developed to accurately represent the repulsive interaction, critical for the initial energy transfer but at best to only approximate the peptide-ion/surface attractive interaction, critical for binding of the ion to the surface. Work is in progress79 to develop peptide-H+/F-SAM potentials which incorporate accurate representations of both the repulsive and the attractive interactions. C. Mass Effects. It has been suggested that the more efficient energy transfer to ∆Eint for the F-SAM surface, as compared with the H-SAM surface, is a mass effect.13,20,63,65 To investigate this suggestion, a simulation was performed for gly2-H+ colliding with the EA F-SAM surface model but with the mass of the F-atom replaced by that of the H-atom, that is, the F-SAM/H model. Ei and θi for this simulation are 30 eV and 45°, respectively. The resulting energy transfer efficiencies are listed in Table 5 where they are compared with those for the EA F-SAM model without the mass change (Table 1) and those reported previously for both folded and extended gly3-H+ colliding with a H-SAM surface. The percentage energy transfer

9384 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Yang et al.

TABLE 6: Peptide Mass Effect for Collisional Energy Transfera Ei (eV) 5 10 30 50 70

∆Eint (15)b

14 13 (15) 8 (11) 8 (12) 7 (13)

∆Esurf

Ef

44 (40) 58 (64) 82 (81) 84 (83) 86 (83)

42 (45) 29 (21) 10 (8) 8 (5) 7 (4)

Energy transfer percentage for gly2-H+ colliding with the EA F-SAM surface, but with the masses of the C, N, and O atoms of gly2-H+ changed to 30.0 amu. This increases the mass of gly2-H+ from 119 to 279 amu. The collision angle, θi, is 0°. b The energy transfer percentages in parentheses are those for the standard gly2-H+ model, without the mass changes, and are taken from Table 1. Uncertainties (standard deviation of the mean) are (1%. a

to ∆Eint for gly2-H+ colliding with the F-SAM/H model surface is identical to that for the gly3-H+ + H-SAM collisions and a factor of 2 smaller than that for the gly2-H+ + F-SAM collisions, strongly suggesting the different energy transfer efficiencies to ∆Eint for the H-SAM and F-SAM surfaces is purely a mass effect. The energy remaining in projectile translation, Ef, is similar for gly2-H+ collisions with the F-SAM and F-SAM/H surfaces, and the different ∆Eint efficiencies for the two surfaces is concomitant with different transfers to ∆Esurf. The energy transfer efficiencies for the gly2-H+ + F-SAM/H collisions are nearly identical to those for folded gly3-H+ colliding with the H-SAM. The results listed in Table 4 show that the energy transfer efficiencies are not strongly dependent on the mass and size of the peptide-H+ ions. For the same surface and same Ei and θi, the efficiencies are similar for gly2-H+ and ala2-H+, as well as for the glyn-H+ ions. To differentiate between possible mass and size effects for energy transfer, the effect of only varying the mass was studied by a model, for gly2-H+ colliding with the EA F-SAM surface, in which the masses of the C, N, and O atoms were increased to 30 amu. This artificially increased the mass of gly2-H+ from 119 to 279 amu and decreased its velocity by a factor of 1.53 for the same Ei. These simulations were performed for θi ) 0° and Ei ranging from 5 to 70 eV, and the results are listed in Table 6. With Ei fixed, it is seen that this artificial increase in the mass of gly2-H+ does not have a significant effect on the transfer to ∆Esurf. The interesting trend is that, for the more massive gly2-H+ model, energy transfer to ∆Eint and ∆Ef decrease and increase, respectively, as compared with normal gly2-H+ when Ei is increased. Comparing the results for the different collision energies shows that this trend does not arise from the different velocities for the two gly2-H+ models at the same Ei, as a result of their different masses. D. Model for the Energy Transfer Efficiencies. The results reported here and in previous simulations,25,30,80 of projectile ion + surface collisions, show that the percentage energy transfer to the projectile’s internal degrees of freedom (∆Eint) does not strongly depend on the collision energy Ei. An approximate model for the energy transfer efficiencies, as found from the simulations, is that the percentage transfer to ∆Eint is independent of Ei, while the percentage transfers to ∆Esurf and ∆Ef increase and decrease, respectively, with increase in Ei. The adiabaticity parameter81 ξ provides a model for energy transfer to the surface. For T f V energy transfer, this parameter is given by

ξ ) tc ⁄ tV

(7)

where tc, inversely proportional to the collision velocity, represents the duration of the collision, and tν is an effective

Figure 2. Plots of eq 9 versus simulation results for gly2-H+ and ala2H+ colliding with the EA F-SAM surface at θi ) 0°. ξ ) b/υi for the upper graph and b/Ei for the lower graph. The Po and b fitting parameters are given in the text.

vibrational period for the surface modes receiving the energy transfer. Thus, ξ has the relationship

ξ ) aν ⁄ V

(8)

where ν is an effective vibrational frequency for the surface, υ is the collision velocity, and a is the proportionality constant. Energy transfer becomes more efficient with a decrease in ξ, and the average probability of energy transfer to the surface is approximately related to ξ according to81

Psurf(Ei) ) 〈∆Esurf〉/Ei ) Po exp(-ξ)

(9)

where Po is the limiting, small ξ, probability of energy transfer. Equation 9 was used to analyze the probability of energy transfer to the surface for gly2-H+ and ala2-H+ colliding with the EA F-SAM surface model. Expressing ξ as b/υi, following eq 8, the Psurf (Ei) for gly2-H+ and ala2-H+ were fit to eq 9 with Po and b as adjustable parameters, and the resulting fits are shown in Figure 2 for θi ) 0°. For gly2-H+, the fitting parameters are Po ) 1.10 ( 0.08 and b ) 1.99 ( 0.35 eV and for ala2-H+ they are Po ) 1.21 ( 0.12 and b ) 2.54 ( 0.49 eV (the uncertainty is the standard error). These fits are at best only approximate and the limiting probability, Po, is greater than unity, which is not physically realistic. A much better and physically realistic representation of the Psurf (Ei) probabilities is obtained by expressing ξ as b/Ei. These fits are also shown in Figure 2, and their parameters are Po ) 0.90 ( 0.02 and b ) 3.80 ( 0.30 eV for gly2-H+ and Po ) 0.93 ( 0.02 and b ) 4.94 ( 0.47 eV for ala2-H+. The similarity in these parameters for gly2-H+ and ala2-H+ is in accord with their similar energy transfer efficiencies as discussed above. The high energy limiting energy transfer to the surface is ∼90% for both gly2-H+ and ala2-H+. That the ξ ) b/Ei model gives a better representation

ET in Collisions of Protonated Peptide-Ions of the simulation results, than does ξ ) b/υi may arise from the approximate nature of eq 9 and/or the dependence of the surface’s effective vibrational frequency ν, eq 8, on the collision energy. In addition, the ξ ) b/υi model results from a 3-atom collinear representation of the energy transfer82 and the actual energy transfer dynamics for a polyatomic projectile’s collision with a surface may be substantially different. Quite good fits to Psurf (Ei) values for other simulations were also made using eq 9 and ξ ) b/Ei. For the model in section IV.C., with the masses of the C, N, and O atoms of gly2-H+ increased to 30 amu, the fitting parameters are Po ) 0.91 ( 0.07 and b ) 3.88 ( 0.34 eV for θi ) 0° and are nearly the same as those given above for the normal gly2-H+ model. For gly2-H+ and ala2-H+ colliding with the EA F-SAM surface at θi ) 45° (Table 1), the fitting parameters are Po ) 0.68 ( 0.01 and b ) 5.66 ( 0.31 eV for gly2-H+ and Po ) 0.68 ( 0.01 and b ) 7.51 ( 0.32 eV for ala2-H+. The limiting energy transfer efficiency Po is significantly smaller at θi of 45° as compared to 0°. VII. Summary Chemical dynamics trajectory simulations were performed to explore energy transfer in collisions of the gly2-H+ and ala2H+ peptide-ions with a perfluorinated F-SAM surface, at incident energies of 5-70 eV and an incident angle at 0 and 45°. Both explicit-atom (EA) and the united-atom (UA) models were used to represent the F-SAM. The energy transfer efficiencies are very similar for gly2-H+ and ala2-H+. Most of the incident energy goes to the surface’s vibrational modes, with the transfer to the surface ∆Esurf, increasing from 30-40 to 80% as the initial energy Ei is increased from 5 to 70 eV for θi ) 0°. Concomitantly, the percent energy remaining in peptide-ions translation Ef, decreases. The relative partitioning of the collision energy between the projectile modes, the surface vibrations, and the projectile final translation depends on the relationships between the time scales for projectile vibration, surface vibration, and projectile translation.83–85 At low Ei, the higher frequency surface modes are less active and receive approximately 50% of the collision energy. However, as Ei is increased, the time scales for projectile translation and surface vibration become in accord and more energy is transferred to surface vibration at the expense of the scattered projectile’s translation. In comparison to these transfers to ∆Esurf and Ef, the percentage energy transfer to the peptide-ion’s internal vibrational/rotational modes, ∆Eint, is only weakly dependent on Ei. Similarly, the transfer to ∆Esurf and Ef depends on the incident angle θi, but the transfer to ∆Eint does not. Experimental studies10 of ala2-H+ + F-SAM energy transfer, for Ei of 3 - 23 eV and θi ) 0°, find an average energy transfer to ∆Eint of 21%. For these collision conditions, the trajectory simulations for the EA F-SAM model give a similar, but somewhat smaller, 14-18% transfer to ∆Eint. In addition, the widths of the experimental and simulation P(∆Eint) energy transfer distributions are in good agreement. The experimental and simulation trends in the average energy transfers and energy transfer distributions, for peptide-H+ ion collisions with the H-SAM, F-SAM, and diamond surfaces, are quite similar. The results obtained by both the EA and the UA models for the F-SAM agree with the experimental observation that the percentage energy transfer to the peptide’s internal degrees of freedom is insensitive to the collision energy, but the EA model underestimates by a factor 1.3 and the UA model overestimates by the same factor this percentage energy transfer as compared

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9385 with the experimental finding.10 The EA model for the F-SAM is expected to be more accurate,86 and in future work, it is important to determine why the percentage energy transfers to ∆Eint, for the EA and UA models, differ with experiment by the same factor. There are significant differences between the EA and the UA models in their percentage energy transfers to ∆Esurf and Ef. For the EA model, these percentage energy transfers strongly depend on Ei, while they are nearly independent of Ei for the UA model. The differences between the energy transfers of the EA and UA F-SAM models may arise from inaccuracies in the peptide-H+/surface intermolecular potential and/or the surface potential for the UA model. For each, the -CF3 and -CF2- moieties are treated as united-atoms. More work needs to be done to determine the possible shortcoming(s) of the UA F-SAM model. It may be fundamentally flawed, since treating the -CF3 and -CF2- moieties as a single atom may not be physically realistic. The simulations reported here and experiments24 indicate the transfer to the peptide-ion’s ∆Eint is approximately a factor of 2 smaller for collisions with the H-SAM surface as compared with the F-SAM. Replacing the F atoms of the F-SAM surface by H atoms in the simulation, and not modifying the potentials for the peptide-H+ and F-SAM, yields a F-SAM/H model which gives a transfer efficiency to ∆Eint nearly identical to that for the H-SAM surface. This suggests the difference in the energy transfer efficiencies for the F-SAM and H-SAM surfaces, and possibly other fluorinated and hydrogenated surfaces,13,18,20,63,65,84,85,87,88 is a mass effect. The efficiencies of transfer to ∆Eint, ∆Esurf, and Ef for gly2H+ and ala2-H+ colliding with the F-SAM surface are wellrepresented by a model which energy transfer to ∆Eint is independent of Ei, and the average transfer to ∆Esurf varies with Ei according to 〈∆Esurf〉/Ei ) Po exp(-b/Ei). In the future, it will be of interest to apply this model to study energy transfer for other projectile-surface systems. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. CHE-0615321 and the Robert A. Welch Foundation under Grant No. D-0005. Support was also provided by the High-Performance Computing Center (HPCC) at Texas Tech University, under the direction of Philip W. Smith. The authors also wish to thank Julia Laskin for providing the specific collision energies at which the ala2-H+ + F-SAM experiments were performed. References and Notes (1) Biemann, K.; Scoble, H. A. Science 1987, 237, 992. (2) Speir, J. P.; Amster, I. J. J. Am. Chem. Soc. 1995, 6, 1069. (3) Vachet, R. W.; Glish, G. L. Anal. Chem. 1998, 70, 340. (4) Klassen, J. S.; Kebarle, P. J. Am. Chem. Soc. 1997, 119, 6552. (5) Price, W. D.; Schnier, P. D.; Jockusch, R. A.; Strittmatter, E. F.; Williams, E. R. J. Am. Chem. Soc. 1996, 1189, 10640. (6) Michael, C. M.; Richard, E. C.; Busch, K. L.; Schey, K. L.; Bartlett, M. G. J. Mass Spectrom. 1998, 33, 75. (7) Marzluff, E. M.; Campbell, S.; Rodgers, M. T.; Beauchamp, J. L. J. Am. Chem. Soc. 1994, 116, 7787. (8) Wysocki, V. H.; Jones, J. L.; Ding, J. M. J. Am. Chem. Soc. 1991, 113, 8969. (9) Somogyi, A.; Kane, T. E.; Ding, J. M.; Wysocki, V. H. J. Am. Chem. Soc. 1993, 115, 5275. (10) Laskin, J.; Denisov, E.; Futrell, J. H. J. Am. Chem. Soc. 2000, 122, 9703. (11) Laskin, J.; Denisov, E.; Futrell, J. H. J. Phys. Chem. B 2001, 105, 1895. (12) Schultz, D. G.; Lim, H.; Gabris, S.; Hanley, L. J. Mass Spectrom. 1999, 34, 217. (13) Winger, B. E.; Julian, R. K., Jr.; Cooks, R. G.; Chidsey, C. E. D. J. Am. Chem. Soc. 1991, 113, 8967.

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