Evaluation of the influence of amino acid composition on the propensity for collision-induced dissociation of model peptides using molecular dynamics simulations

Proton Dynamics in Protein Mass Spectrometry

Native electrospray ionization/ion mobility-mass spectrometry (ESI/IM-MS) allows an accurate determination of low-resolution structural features of proteins. Yet, the presence of proton dynamics, observed already by us for DNA in the gas phase, and its impact on protein structural determinants, have not been investigated so far. Here, we address this issue by a multistep simulation strategy on a pharmacologically relevant peptide, the N-terminal residues of amyloid-β peptide (Aβ(1-16)). Our calculations reproduce the experimental maximum charge state from ESI-MS and are also in fair agreement with collision cross section (CCS) data measured here by ESI/IM-MS. Although the main structural features are preserved, subtle conformational changes do take place in the first ∼0.1 ms of dynamics. In addition, intramolecular proton dynamics processes occur on the picosecond-time scale in the gas phase as emerging from quantum mechanics/molecular mechanics (QM/MM) simulations at the B3LYP level of theory. We conclude that proton transfer phenomena do occur frequently during fly time in ESI-MS experiments (typically on the millisecond time scale). However, the structural changes associated with the process do not significantly affect the structural determinants.

Comparative studies on the discrepant fragmentation mechanisms of the GLy-Asp-Gly-Arg and Arg-Gly-Asp-Gly: evidence for the mobile proton model

The fragmentation mechanisms of singly protonated Gly-Asp-Gly-Arg (GDGRI and Arg-Gly-Asp-Gly (RGDGJ were investigated by mass spectrometry and theoretical methods. Both protonated molecules are fragmented mainly at the Asp-Gly amide bond C-terminal to Asp, as supported by quantum chemical calculations. Charge distributions of C and N atoms (Qc + QN) on the amide bonds were collected when the ionizing proton was fixed at different nitrogen atoms along the backbone for each peptide. Compared with the neutral molecules, the total charges of C and N atoms (Qc + QN] for the singly charged peptides tended to be negative when the proton was located at the backbone nitrogen atoms. A relatively larger value of QC + QN corresponds to a higher trend of fragmentation, which is consistent with the experimental relative abundances data that the predominant ions are y2 for [GDGR + H]+ and b3 for [RGDG + H]+. Also, the anhydride mechanism driven by the C-terminal COOH for [RGDG + H]+ was explored by a quantum-mechanical/molecular-mechanical method. Calculations indicate that the protonated peptide can be cleaved through an unusual charge-directed pathway by forming a salt bridge at the C-termini. The formation of the anhydride linkage is much more feasible since this process needs very little energy and is exother- mic, though the subsequent nucleophilic attack on the Asp carbonyl carbon is more difficult. The combined experimental and theoretical methods substantiate the mobile proton model, which opens a way to analyze quantitatively the discrepant fragmentation of dissociated peptides in peptide/protein identification.

The relative charge ratio between C and N atoms in amide bond acts as a key factor to determine peptide fragment efficiency in different charge states

The influence of charge state on the peptide dissociation behavior in tandem mass spectrometry (MS/MS) is worthy of discussion. Comparative studies of singly- and doubly-protonated peptide molecules are performed to explore the effect and mechanism of charge state on peptide fragmentation. In view of the charge-directed cleavage of protonated peptides described in the mobile proton model, radiolytic oxidation was applied to change the charge distribution of peptides but retain the sequence. Experimental studies of collision energy-dependent fragmentation efficiencies coupled with quantum chemical calculations indicated that the cleavage of ARRA and its side-chain oxidation products with oxygen atoms added followed a trend that doubly-protonated peptides fragment more easily than singly-protonated forms, while the oxidation product with the guanidine group deleted showed the opposite trend. By analyzing the charge distribution around the amide bonds, we found that the relative charge ratios between C and N atoms (Q(C)/Q(N)) in the amide bonds provided a reasonable explanation for peptide fragmentation efficiencies. An increase of the Q(C)/Q(N) value of the amide bond means that a peptide fragments more easily, and vice versa. The results described in this paper provide an experimental and calculation strategy for predicting peptide fragmentation efficiency.