A Computational Comparison of Soft Landing of α-Helical vs Globular Peptides

Exploring the Effects of Methylation on the CID of Protonated Lysine: A Combined Experimental and Computational Approach

We report the results of experiments, simulations, and DFT calculations that focus on describing the reaction dynamics observed within the collision-induced dissociation of l-lysine-H⁺ and its side-chain methylated analogues, N_ε-methyl-l-lysine-H⁺ (Me₁-lysine-H⁺), N_ε,N_ε-dimethyl-l-lysine-H⁺ (Me₂-lysine-H⁺), and N_ε,N_ε,N_ε-trimethyl-l-lysine-H⁺ (Me₃-lysine-H⁺). The major pathways observed in the experimental measurements were m/z 130 and 84, with the former dominant at low collision energies and the latter at intermediate to high collision energies. The m/z 130 peak corresponds to loss of N(CH₃)_nH_3-n, while m/z 84 has the additional loss of H₂CO₂ likely in the form of H₂O + CO. Within the time frame of the direct dynamics simulations, m/z 130 and 101 were the most populous peaks, with the latter identified as an intermediate to m/z 84. The simulations allowed for the determination of several reaction pathways that result in these products. A graph theory analysis enabled the elucidation of the significant structures that compose each peak. Methylation results in the preferential loss of the side-chain amide group and a reduction of cyclic structures within the m/z 84 peak population in simulations.

Modeling the Effects of O-Sulfonation on the CID of Serine

We present the results of direct dynamics simulations and DFT calculations aimed at elucidating the effect of O-sulfonation on the collision-induced dissociation for serine. Toward this end, direct dynamics simulations of both serine and sulfoserine were performed at multiple collision energies and theoretical mass spectra obtained. Comparisons to experimental results are favorable for both systems. Peaks related to the sulfo group are identified and the reaction dynamics explored. In particular, three significant peaks (m/z 106, 88, and 81) seen in the theoretical mass spectrum directly related to the sulfo group are analyzed as well as major peaks shared by both systems. Our analysis shows that the m/z 106 peaks result from intramolecular rearrangements, intermolecular proton transfer among complexes composed of initial fragmentation products, and at high energy side-chain fragmentation. The m/z 88 peak was found to contain multiple constitutional isomers, including a previously unconsidered, low energy structure. It was also observed that the RM1 semiempirical method was not able to obtain all of the major peaks seen in experimens for sulfoserine. In contrast, PM6 did obtain all major experimental peaks.

Role of Chemical Dynamics Simulations in Mass Spectrometry Studies of Collision-Induced Dissociation and Collisions of Biological Ions with Organic Surfaces

In this article, a perspective is given of chemical dynamics simulations of collisions of biological ions with surfaces and of collision-induced dissociation (CID) of ions. The simulations provide an atomic-level understanding of the collisions and, overall, are in quite good agreement with experiment. An integral component of ion/surface collisions is energy transfer to the internal degrees of freedom of both the ion and the surface. The simulations reveal how this energy transfer depends on the collision energy, incident angle, biological ion, and surface. With energy transfer to the ion's vibration fragmentation may occur, i.e. surface-induced dissociation (SID), and the simulations discovered a new fragmentation mechanism, called shattering, for which the ion fragments as it collides with the surface. The simulations also provide insight into the atomistic dynamics of soft-landing and reactive-landing of ions on surfaces. The CID simulations compared activation by multiple "soft" collisions, resulting in random excitation, versus high energy single collisions and nonrandom excitation. These two activation methods may result in different fragment ions. Simulations provide fragmentation products in agreement with experiments and, hence, can provide additional information regarding the reaction mechanisms taking place in experiment. Such studies paved the way on using simulations as an independent and predictive tool in increasing fundamental understanding of CID and related processes.