Study of the dissociation of a charge-reduced phosphopeptide formed by electron transfer from an alkali metal target

Electron transfer dissociation mass spectrometry of acidic phosphorylated peptides cationized with trivalent praseodymium

The lanthanide ion praseodymium, Pr(III), was employed to study metallated ion formation and electron transfer dissociation (ETD) of 27 biological and model highly acidic phosphopeptides. All phosphopeptides investigated form metallated ions by electrospray ionization (ESI) that can be studied by ETD to yield abundant sequence information. The ions formed are [M + Pr - H]²⁺ , [M + Pr]³⁺ , and [M + Pr + H]⁴⁺ . All biological phosphopeptides with a chain length of seven or more residues generate [M + Pr]³⁺ . For biological phosphopeptides, [M + Pr]³⁺ undergoes more backbone cleavage by ETD than [M + Pr - H]²⁺ and, in some cases, full sequence coverage occurs. Acidic model phosphorylated hexa-peptides and octa-peptides, composed of alanine residues and one phosphorylated residue, form exclusively [M + Pr - H]²⁺ by ESI. Limited sequence information is obtained by ETD of [M + Pr - H]²⁺ with only metallated product ions being generated. For two biological phosphopeptides, [M + Pr + H]⁴⁺ is observed and may be due to the presence of at least one residue with a highly basic side chain that facilitates the addition of an extra proton. For the model phosphopeptides, more sequence coverage occurs when the phosphorylated residue is in the middle of the sequence than at either the N- or C-terminus. ETD of the metallated precursor ions formed by ESI generates exclusively metallated and nonmetallated c- and z-ions for the biological phosphopeptides, while metallated c-ions, z-ions, and a few y-ions form for the model phosphopeptides. Most of the product ions contain the phosphorylated residue indicating that the metal ion binds predominantly at the deprotonated phosphate group. The results of this study indicate that ETD is a promising tool for sequencing highly acidic phosphorylated peptides by metal adduction with Pr (III) and, by extension, all nonradioactive lanthanide metal ions.

Study of Ion Dynamics by Electron Transfer Dissociation: Alkali Metals as Targets

High energy collision processes for singly charged positive ions using an alkali metal target are confirmed, as a charge inversion mass spectrometry, to occur by electron transfers in successive collisions and the dissociation processes involve the formation of energy-selected neutral species from near-resonant neutralization with alkali metal targets. A doubly charged thermometer molecule was made to collide with alkali metal targets to give singly and doubly charged positive ions. The internal energy resulting from the electron transfer with the alkali metal target was very narrow and centered at a particular energy. This narrow internal energy distribution can be attributed to electron transfer by Landau-Zener potential crossing between the precursor ion and an alkali metal atom, and the coulombic repulsion between singly charged ions in the exit channel. A large cross section of more than 10^-14 cm² was estimated for high-energy electron transfer dissociation (HE-ETD). Doubly protonated phosphorylated peptides obtained by electrospray ionization were collided with Xe and Cs targets to give singly and doubly charged positive ions. Whereas doubly charged fragment ions resulting from CAD were dominant in the case of the Xe target, singly charged fragment ions resulting from ETD were dominant with the Cs target. HE-ETD using the Cs target provided all of the z-type ions by N-C_α bond cleavage without the loss of the phosphate groups. The results demonstrate that HE-ETD with an alkali metal target allowed the position of phosphorylation and the amino acid sequence of peptides with post translational modifications (PTM) to be determined.

Renaissance of cation-radicals in mass spectrometry

This brief overview addresses the topic that was presented in the Thomson Medal Award session at the 19th International Mass Spectrometry Conference in Kyoto, Japan. Mass spectrometry of cation-radicals has enjoyed a remarkable renaissance thanks to the development of new methods for electron attachment to multiply charged peptide ions. The charge-reduced ions that are odd-electron species exhibit interesting reactivity that is useful for peptide and protein sequencing. The paper briefly reviews the fundamental aspects of the formation, energetics, and backbone dissociations of peptide cation-radicals.

Dipole-guided electron capture causes abnormal dissociations of phosphorylated pentapeptides

Electron transfer and capture mass spectra of a series of doubly charged ions that were phosphorylated pentapeptides of a tryptic type (pS,A,A,A,R) showed conspicuous differences in dissociations of charge-reduced ions. Electron transfer from both gaseous cesium atoms at 100 keV kinetic energies and fluoranthene anion radicals in an ion trap resulted in the loss of a hydrogen atom, ammonia, and backbone cleavages forming complete series of sequence z ions. Elimination of phosphoric acid was negligible. In contrast, capture of low-energy electrons by doubly charged ions in a Penning ion trap induced loss of a hydrogen atom followed by elimination of phosphoric acid as the dominant dissociation channel. Backbone dissociations of charge-reduced ions also occurred but were accompanied by extensive fragmentation of the primary products. z-Ions that were terminated with a deaminated phosphoserine radical competitively eliminated phosphoric acid and H(2)PO(4) radicals. A mechanism is proposed for this novel dissociation on the basis of a computational analysis of reaction pathways and transition states. Electronic structure theory calculations in combination with extensive molecular dynamics mapping of the potential energy surface provided structures for the precursor phosphopeptide dications. Electron attachment produces a multitude of low lying electronic states in charge-reduced ions that determine their reactivity in backbone dissociations and H- atom loss. The predominant loss of H atoms in ECD is explained by a distortion of the Rydberg orbital space by the strong dipolar field of the peptide dication framework. The dipolar field steers the incoming electron to preferentially attach to the positively charged arginine side chain to form guanidinium radicals and trigger their dissociations.

High-energy electron transfer dissociation (HE-ETD) using alkali metal targets for sequence analysis of post-translational peptides

Post-translational modifications (PTMs) of proteins are important in the activation, localization, and regulation of protein function in vivo. The usefulness of electron capture dissociation (ECD) and electron-transfer dissociation (ETD) in tandem mass spectrometry (MS/MS) using low-energy (LE) trap type mass spectrometer is associated with no loss of a labile PTM group regarding peptide and protein sequencing. The experimental results of high-energy (HE) collision induced dissociation (CID) using the Xe and Cs targets and LE-ETD were compared for doubly-phosphorylated peptides TGFLT(p)EY(p)VATR (1). Although HE-CID using the Xe target did not provide information on the amino acid sequence, HE-CID using the Cs target provided all the z-type ions without loss of the phosphate groups as a result of HE-ETD process, while LE-ETD using fluoranthene anion gave only z-type ions from z(5) to z(11). The difference in the results of HE-CID between the Xe and Cs targets demonstrated that HE-ETD process with the Cs target took place much more dominantly than collisional activation. The difference between HE-ETD using Cs targets and LE-ETD using the anion demonstrated that mass discrimination was much weaker in the high-energy process. HE-ETD was also applied to three other phosphopeptides YGGMHRQEX(p)VDC (2: X = S, 3: X = T, 4: X = Y). The HE-CID spectra of the doubly-protonated phosphopeptides (= [M + 2H](2+)) of 2, 3, and 4 using the Cs target showed a very similar feature that the c-type ions from c(7) to c(11) and the z-type ions from z(7) to z(11) were formed via N-C alpha bond cleavage without a loss of the phosphate group.

Transition metals as electron traps. I. Structures, energetics, electron capture, and electron-transfer-induced dissociations of ternary copper-peptide complexes in the gas phase

Electron-induced dissociations of gas-phase ternary copper-2,2'-bipyridine complexes of Gly-Gly-Gly and Gly-Gly-Leu were studied on a time scale ranging from 130 ns to several milliseconds using a combination of charge-reversal ((+)CR(-)) and electron-capture-induced dissociation (ECID) measured on a beam instrument and electron capture dissociation (ECD) measured in a Penning trap. Charge-reduced intermediates were observed on the short time scale in the (+)CR(-) and ECID experiments but not in ECD. Ion dissociations following electron transfer or capture mostly occurred by competitive bpy or peptide ligand loss, whereas peptide backbone fragmentations were suppressed in the presence of the ligated metal ion. Extensive electron structure theory calculations using density functional theory and large basis sets provided optimized structures and energies for the precursor ions, charge-reduced intermediates, and dissociation products. The Cu complexes underwent substantial structure changes upon electron capture. Cu was calculated to be pentacoordinated in the most stable singly charged complexes of the [Cu(peptide-H)bpy](+*) type where it carried a approximately +1 atomic charge. Cu coordination in charge-reduced [Cu(peptide-H)bpy] intermediates depended on the spin state. The themodynamically more stable singlet states had tricoordinated Cu, whereas triplet states had a tetracoordinated Cu. Cu was tricoordinated in stable [Cu(peptide-H)bpy](-*) products of electron transfer. [Cu(peptide)bpy](2+*) complexes contained the peptide ligand in a zwitterionic form while Cu was tetracoordinated. Upon electron capture, Cu was tri- or tetracoordinated in the [Cu(peptide)bpy](+) charge-reduced analogs and the peptide ligands underwent prototropic isomerization to canonical forms. The role of excited singlet and triplet electronic states is assessed.