Dynamics of Protonated Peptide Ion Collisions with Organic Surfaces

Jul 28, 2016 - For Cr+(CO)6 collisions with the H-SAM the ions recoil off the surface and, after efficient intramolecular vibrational energy redistrib...
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Dynamics of Protonated Peptide Ion Collisions with Organic Surfaces: Consonance of Simulation and Experiment Subha Pratihar,† George L. Barnes,‡ Julia Laskin,§ and William L. Hase*,† †

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States Department of Chemistry and Biochemistry, Siena College, Loudonville, New York 12211, United States § Pacific Northwest National Laboratory, Physical Sciences Division, P.O. Box 999 K8-88, Richland, Washington 99352, United States ‡

ABSTRACT: In this Perspective, mass spectrometry experiments and chemical dynamics simulations are described that have explored the atomistic dynamics of protonated peptide ions, peptide-H+, colliding with organic surfaces. These studies have investigated the energy transfer and fragmentation dynamics for peptide-H+ surface-induced dissociation (SID), peptide-H+ physisorption on the surface, soft landing (SL), and peptide-H+ reaction with the surface, reactive landing (RL). SID provides primary structures of biological ions and information regarding their fragmentation pathways and energetics. Two SID mechanisms are found for peptide-H+ fragmentation. A traditional mechanism in which peptide-H+ is vibrationally excited by its collision with the surface, rebounds off the surface and then dissociates in accord with the statistical, RRKM unimolecular rate theory. The other, shattering, is a nonstatistical mechanism in which peptide-H+ fragments as it collides with the surface, dissociating via many pathways and forming many product ions. Shattering is important for collisions with diamond and perfluorinated self-assembled monolayer (F-SAM) surfaces, increasing in importance with the peptide-H+ collision energy. Chemical dynamics simulations also provide important mechanistic insights on SL and RL of biological ions on surfaces. The simulations indicate that SL occurs via multiple mechanisms consisting of sequences of peptide-H+ physisorption on and penetration in the surface. SL and RL have a broad range of important applications including preparation of protein or peptide microarrays, development of biocompatible substrates and biosensors, and preparation of novel synthetic materials, including nanomaterials. An important RL mechanism is intact deposition of peptide-H+ on the surface.

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on the surface.16 Particularly important RL events are those in which the projectile ion chemisorbs intact on the surface. SL and RL have important applications including purification of compounds from complex mixtures,17−19 preparation of protein or peptide microarrays,20 development of biocompatible substrates and biosensors,21,22 deposition of mass-selected cluster ions,23−25 and preparation of novel synthetic materials,26,27 including nanomaterials.28,29 In RL, molecules are bound covalently to a surface and the use of hyperthermal beams of mass−selected ions provides a promising technique for highly selective surface modification.30 Chemical dynamics simulations are used to obtain an atomistic understanding of collisions and are important for modeling and interpreting experiments of peptide-H+ + surface collisions.31 Energy transfer and fragmentation dynamics for SID,32,33 as well as mechanisms for SL and RL,34,35 are determined from the simulations. Overall, very good agreement is obtained between the results of the simulations and experiments, indicating that the simulations adequately capture the dynamics of ion−surface collisions.31

he study of biological ion collisions with surfaces is an important component of biological mass spectrometry.1 Integral in these studies are hyperthermal (1 nA) of mass-selected ions delivered by this source may be used for nanofabrication of both single or multilayer peptide films by either focusing the ion beam down to submicron size or patterning. High ion currents are also beneficial for facile fabrication of stable biological interfaces using RL. Although initial peptide RL experiments focused on formation of amide bonds to the surface, other interfacial reactions may be explored to enable selective binding of other functional groups to SAMs. For example, SAMs terminated with maleimide functional groups are known to react with biologically active thiols in solution.73 Furthermore, chemical dynamics simulations may guide future RL experiments, in which other types of reactions that are inefficient in solution are used for selective covalent attachment of peptides to surfaces.

digylcine peaks at a collision energy for 40 eV, before dramatically dropping off.44 A faster drop off in efficiency was observed in the simulations compared to experiments, though we note that the chemical details of the NHS-SAM and the COCl-SAM are only qualitatively similar, with the NHS leaving group having significantly larger capacity for accepting excess energy. In any case, the agreement in the peak RL efficiency for RL between simulation and experiment is noteworthy. The qualitative agreement between simulation and experiment for RL is significant. However, as for the SL simulations, it is important to expand the RL simulations so that direct comparisons may be made with experiment. For the simulation QM/MM model it may be important to include multiple reactive sites on the surface. It will be of interest to see if the simulations find dominant RL pathways for the peptide-H+ + surface systems studied experimentally. It is intriguing to consider additional future simulations and experiments which address SID, SL, and RL. There are multiple avenues of study for the simulations. From the experiments and simulations it is found that the peptide-H+ + SAM collision systems have average percentage transfer to ΔEint that is nearly independent of the incident angle θi, the size of the ion, and the incident energy Ei. From detailed analyses of the peptide-H+ trajectories it should be possible to formulate a theoretical model which accurately describes these three interesting dynamical properties, which may be interrelated. Also it is important to determine an atomistic and theoretical understanding of why the widths of the P(ΔEint) distributions for peptide-H+ + surface collisional energy transfer are related from narrowest to broadest according to H-SAM < F-SAM < diamond. Future simulations might include: RL of a lysine containing peptide, RL on the NHS-SAM (or similar SAM) to make more direct comparisons with the experiments of Laskin, and RL of peptide-H+ ions with protonated and deprotonated side chains; and a comparison of SID and SL of folded and β-sheet peptide(Hn)n+ ions, both singly and multiply protonated, on the FSAM. Cleavages at amide bonds and other positions may occur when singly or multiply protonated peptides are excited by high energy surface collisions. The atomistic understanding of mechanisms of such fragmentations will be broadly informative to the scientific community. In addition, the manner in which peptide fragmentation is influenced by the gas phase basicity of amino acid residues, will be another aspect of substantial interest. One possible extension of the chemical dynamics simulations are atomistic studies of SID of noncovalent complexes66−68 and additional simulations of peptide SID,69 with different organic surfaces. Wysocki and co-workers have performed a number of experiments of oligopeptide SID.69 Particularly interesting in these dynamics are time scales for SID, time scales for conformational changes of trapped proteins, and the importance of proton transfer between different proteins of the oligomer.70 By combining QM/MM simulations with RRKM modeling, SID spectra over complex potential surfaces for fragmenting peptide-H+ ions may be determined for direct comparison with the experimental data. It is expected that RRKM theory will be valid to describe the fragmentation kinetics of peptide-H+ ions which have sufficient vibrational energy to fragment, but do not during the time period of the trajectories. IVR should be complete within this time. Arrhenius parameters may be determined for randomly excited peptide-H+ ions, with

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research at Texas Tech University is based upon work supported by the Robert A. Welch Foundation under grant No. D-0005 and the National Science Foundation by multiple grants. Support was also provided by the High Performance Computing Center (HPCC) at Texas Tech, under the direction of Philip W. Smith. The authors also wish to thank the Texas Advanced Computing Center (TACC) for the computational facilities they provided. The research at PNNL is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences Division. PNNL is a multiprogram national laboratory operated for DOE by Battelle.

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DOI: 10.1021/acs.jpclett.6b00978 J. Phys. Chem. Lett. 2016, 7, 3142−3150