A Mechanistic Investigation of the Enhanced Cleavage at Histidine in the Gas-Phase Dissociation of Protonated Peptides (original) (raw)
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Proton Mobility in b2 Ion Formation and Fragmentation Reactions of Histidine-Containing Peptides
Journal of the American Society for Mass Spectrometry, 2015
A detailed energy-resolved study of the fragmentation reactions of protonated histidine-containing peptides and their b2 ions has been undertaken. Density functional theory calculations were utilized to predict how the fragmentation reactions occur so that we might discern why the mass spectra demonstrated particular energy dependencies. We compare our results to the current literature and to synthetic b2 ion standards. We show that the position of the His residue does affect the identity of the subsequent b2 ion (diketopiperazine versus oxazolone versus lactam) and that energy-resolved CID can distinguish these isomeric products based on their fragmentation energetics. The histidine side chain facilitates every major transformation except trans-cis isomerization of the first amide bond, a necessary prerequisite to diketopiperazine b2 ion formation. Despite this lack of catalyzation, trans-cis isomerization is predicted to be facile. Concomitantly, the subsequent amide bond cleavage...
Rapid Communications in Mass Spectrometry, 2010
A combination of electrospray ionisation (ESI), multistage and high-resolution mass spectrometry experiments was used to compare the gas-phase chemistry of the amino acids histidine (1), 2-oxohistidine (2), and 2-thioxo-histidine (3). Collision-induced dissociation (CID) of all three different proton-bound heterodimers of these amino acids led to the relative gas-phase proton affinity order of: histidine >2-thioxo-histidine >2-oxo-histidine. Density functional theory (DFT) calculations confirm this order, with the lower proton affinities of the oxidised histidine derivatives arising from their ability to adopt the more stable keto/thioketo tautomeric forms. All protonated amino acids predominately fragment via the combined loss of H 2 O and CO to yield a 1 ions. Protonated 2 and 3 also undergo other small molecule losses including NH 3 and the imine HN¼CHCO 2 H. The observed differences in the fragmentation pathways are rationalised through DFT calculations, which reveal that while modification of histidine via the introduction of the oxygen atom in 2 or the sulfur atom in 3 does not affect the barriers against the loss of H 2 ORCO, barriers against the losses of NH 3 and HN¼CHCO 2 H are lowered relative to protonated histidine.
Dissociation of the peptide bond in protonated peptides
Journal of Mass Spectrometry, 2000
The dissociation of the amide (peptide) bond in protonated peptides, [M Y H] Y , is discussed in terms of the structures and energetics of the resulting N -terminal b n and C -terminal y n sequence ions. The combined data provide strong evidence that dissociation proceeds with no reverse barriers through interconverting proton-bound complexes between the segments emerging upon cleavage of the protonated peptide bond. These complexes contain the C -terminal part as a smaller linear peptide (amino acid if one residue) and the N -terminal part either as an oxazolone or a cyclic peptide (cyclic amide if one residue). Owing to the higher thermodynamic stability but substantially lower gas-phase basicity of cyclic peptides vs isomeric oxazolones, the N -terminus is cleaved as a protonated oxazolone when ionic (b n series) but as a cyclic peptide when neutral (accompanying the C -terminal y n series). It is demonstrated that free energy correlations can be used to derive thermochemical data about sequence ions. In this context, the dependence of the logarithm of the abundance ratio log[y 1 /b 2 ], from protonated GGX (G, glycine; X, varying amino acid) on the gas-phase basicity of X is used to obtain a first experimental estimate of the gas-phase basicity of the simplest b-type oxazolone, viz. 2-aminomethyl-5-oxazolone (b 2 ion with two glycyl residues).
Proton-Driven Amide Bond-Cleavage Pathways of Gas-Phase Peptide Ions Lacking Mobile Protons
Journal of the American Chemical Society, 2009
The mobile proton model (Dongre, A. R., Jones, J. L., Somogyi, A. and Wysocki, V. H. J. Am. Chem. Soc. 1996, 118 , 8365-8374) of peptide fragmentation states that the ionizing protons play a critical role in the gas-phase fragmentation of protonated peptides upon collision-induced dissociation (CID). The model distinguishes two classes of peptide ions, those with or without easily mobilizable protons. For the former class mild excitation leads to proton transfer reactions which populate amide nitrogen protonation sites. This enables facile amide bond cleavage and thus the formation of b and y sequence ions. In contrast, the latter class of peptide ions contains strongly basic functionalities which sequester the ionizing protons, thereby often hindering formation of sequence ions. Here we describe the proton-driven amide bond cleavages necessary to produce b and y ions from peptide ions lacking easily mobilizable protons. We show that this important class of peptide ions fragments by different means from those with easily mobilizable protons. We present three new amide bond cleavage mechanisms which involve salt-bridge, anhydride, and imine enol intermediates, respectively. All three new mechanisms are less energetically demanding than the classical oxazolone b(n)-y(m) pathway. These mechanisms offer an explanation for the formation of b and y ions from peptide ions with sequestered ionizing protons which are routinely fragmented in large-scale proteomics experiments.
The journal of physical chemistry. B, 2010
The gas-phase structures and fragmentation pathways of the singly protonated peptide arginylglycylaspartic acid (RGD) are investigated by means of collision-induced-dissociation (CID) and detailed molecular mechanics and density functional theory (DFT) calculations. It is demonstrated that despite the ionizing proton being strongly sequestered at the guanidine group, protonated RGD can easily be fragmented on charge directed fragmentation pathways. This is due to facile mobilization of the C-terminal or aspartic acid COOH protons thereby generating salt-bridge (SB) stabilized structures. These SB intermediates can directly fragment to generate b(2) ions or facilely rearrange to form anhydrides from which both b(2) and b(2)+H(2)O fragments can be formed. The salt-bridge stabilized and anhydride transition structures (TSs) necessary to form b(2) and b(2)+H(2)O are much lower in energy than their traditional charge solvated counterparts. These mechanisms provide compelling evidence of ...
Journal of The American Chemical Society, 2008
Protonated peptides containing histidine or arginine residues and a free carboxyl group (His-Ala-Ile, His-Ala-Leu, Ala-His-Leu, Ala-Ala-His-Ala-Leu, His-Ala-Ala-Ala-Leu, and Arg-Ala-Ile) form stable anions upon collisional double electron transfer from Cs atoms at 50 keV kinetic energies. This unusual behavior is explained by hidden rearrangements occurring in peptide radical intermediates formed by transfer of the first electron. The rearrangements occur on a ∼120 ns time scale determined by the radical flight time. Analysis of the conformational space for (His-Ala-Ile + H) + precursor cations identified two major conformer groups, 1a + -1m + and 5a + -5h + , that differed in their H-bonding patterns and were calculated to collectively account for 39% and 60%, respectively, of the gas-phase ions. One-electron reduction in 1a + and 5a + triggers exothermic hydrogen atom migration from the terminal COOH group onto the His imidazole ring, forming imidazoline radical intermediates. The intermediate from 5a is characterized by its charge and spin distribution as a novel cation radical-COOsalt bridge. The intermediate from 1a undergoes spontaneous isomerization by imidazoline N-H migration, re-forming the COOH group and accomplishing exothermic isomerization of the initial (3H)-imidazole radical to a (2H)-imidazole radical. An analogous unimolecular isomerization in simple imidazole and histidine radicals requires activation energies of 150 kJ mol -1 , and its occurrence in 1a and 5a is due to the promoting effect of the proximate COOH group. The rearrangement is substantially reduced in Ala-Leu-His due to an unfavorable spatial orientation of the imidazole and COOH groups and precluded in the absence of a free carboxyl group in His-Ala-Leu amide. In contrast to His-Ala-Ile and Arg-Ala-Ile, protonated Lys-Ala-Ile does not produce stable anions upon double electron transfer. The radical trapping properties of histidine residues are discussed. (4) (a) Cournoyer, J. J.; Pittman, J. L.; Ivleva, V. B.; Fallows, E.; Waskell, L.; Costello, C. E.; O'Connor, P. B.
International Journal of Mass Spectrometry and Ion Processes, 1997
Three-stage tandem mass spectrometry (MS 3 ) and neutral fragment reionization (N f R) are utilized to investigate the structures of the N-terminal ionic (b i ) and neutral backbone fragments, respectively, produced from break-up of the amide bond in protonated peptides that have been collisionally activated. The b-type ion from [M + H] + of C 6 H 5 CO-GF, which is produced by loss of the C-terminal phenylalanine, has the structure of protonated 2-phenyl-5-oxazolone. Conversely, the neutral fragment accompanying the y-type ion (protonated phenylalanine) is 2-phenyl-5-oxazolone. The b 2 ions arising from [M + H] + of underivatized tripeptides are also found to be protonated oxazolones. On the other hand, the neutral fragments released from the N-terminus of the tripeptides upon formation of y 1 are shown to be diketopiperazines and not oxazolones. The combined MS 3 and N f R data help propose dissociation mechanisms that account for the observed structures of ionic and neutral backbone fragments. ᭧ 1997 Elsevier Science B.V.
The Journal of Physical Chemistry B, 2012
Infrared multiphoton dissociation (IRMPD) spectroscopy, using a free-electron laser, and ion mobility measurements, using both drift-cell and traveling-wave instruments, were used to investigate the structure of gas-phase peptide (AAHAL + 2H)(2+) ions produced by electrospray ionization. The experimental data from the IRMPD spectra and collisional cross section (Ω) measurements were consistent with the respective infrared spectra and Ω calculated for the lowest-energy peptide ion conformer obtained by extensive molecular dynamics searches and combined density functional theory and ab initio geometry optimizations and energy calculations. Traveling-wave ion mobility measurements were employed to obtain the Ω of charge-reduced peptide cation-radicals, (AAHAL + 2H)(+●), and the c(3), c(4), z(3), and z(4) fragments from electron-transfer dissociation (ETD) of (AAHAL + 2H)(2+). The experimental Ω for the ETD charge-reduced and fragment ions were consistent with the values calculated for fully optimized ion structures and indicated that the ions retained specific hydrogen bonding motifs from the precursor ion. In particular, the Ω for the doubly protonated ions and charge-reduced cation-radicals were nearly identical, indicating negligible unfolding and small secondary structure changes upon electron transfer. The experimental Ω for the (AAHAL + 2H)(+●) cation-radicals were compatible with both zwitterionic and histidine radical structures formed by electron attachment to different sites in the precursor ion, but did not allow their distinction. The best agreement with the experimental Ω was found for ion structures fully optimized with M06-2X/6-31+G(d,p) and using both projection approximation and trajectory methods to calculate the theoretical Ω values.
Rapid Communications in Mass Spectrometry, 2012
Motivated by the need for chemical strategies designed to tune peptide fragmentation to selective cleavage reactions, benzyl ring substituent influence on the relative formation of carbocation elimination (CCE) products from peptides with benzylamine-derivatized lysyl residues has been examined using collision-induced dissociation (CID) tandem mass spectrometry. Unsubstituted benzylamine-derivatized peptides yield a mixture of products derived from amide backbone cleavage and CCE. The latter involves side-chain cleavage of the derivatized lysyl residue to form a benzylic carbocation [C 7 H 7 ] + and an intact peptide product ion [(MH n) n+-(C 7 H 7) + ] (n-1)+. The CCE pathway is contingent upon protonation of the secondary «-amino group (N «) of the derivatized lysyl residue. Using the Hammett methodology to evaluate the electronic contributions of benzyl ring substituents on chemical reactivity, a direct correlation was observed between changes in the CCE product ion intensity ratios (relative to backbone fragmentation) and the Hammett substituent constants, s, of the corresponding substituents. There was no correlation between the substituent-influenced gas-phase proton affinity of N « and the relative ratios of CCE product ions. However, a strong correlation was observed between the p orbital interaction energies (ΔE int) of the eliminated benzylic carbocation and the logarithm of the relative ratios, indicating the predominant factor in the CCE pathway is the substituent effect on the level of hyperconjugation and resonance stability of the eliminated benzylic carbocation. This work effectively demonstrates the applicability of s (and ΔE int) as substituent selection parameters for the design of benzylbased peptide-reactive reagents which tune CCE product formation as desired for specific applications.