Singly charged precursor

Short examination of the fragment ion spectrum from a singly charged precursor is not particularly helpful. Some previously found mass shifts can be verified but no peaks with m/z lower than 800 Th can be assigned. Detailed peak annotation is presented in Fig. 6.16. 

Figure 6.16. Annotated product ion spectra from Fig. 6.14—singly-charged precursor.

The third idea is focused on finding the reverse ion series that might cover some additional amino acids. Masses of b-ions and y-ions emerging from the cleavage of the same bond add to the mass of the whole precursor increased by 1 Da. If the peptide is 10 AA long, the sum of masses of, for example, b8 and y2 ions is equal to the mass of the singly charged precursor +1 Da. This additional dalton is because... 

Fig. 4. Top-down proteomics for spore identification from MALDI TOF/TOF mass spectra of biomarker molecular ions (top) precursor ion spectrum and (bottom) fragment ion spectrum. The inset on top represents the expanded region around the singly charged precursor ion at m/z 6712. The resulting 68 fragment ions are matched against proteins in Swiss-Prot and the protein P0A4F4 (SAS2 BACCR), originating from Bacillus cereus, is identified as the most plausible candidate (adapted with ACS permission from ref. [45]).

Fig. 6. (A) Low-energy CID spectrum (obtained by means of a nano-ESI-QIT instrument) of the doubly charged precursor ion (m/z 859.07) of the peptide with the amino acid sequence lANNVQPILTSrV. (B) Low-energy CID spectrum (obtained by means of a MALDI-QIT/RTOF instrument) of the singly charged precursor ion (m/z 1717.15) of the above-mentioned peptide.

A final category of gas-phase ion-electron reactions as activation methods for tandem mass spectrometry involves interactions between singly charged precursor ions and free electrons (note that ECD, hot ECD, ETD, and EDD require at least doubly charged precursor ions). The first reaction of this type, involving singly protonated peptides, was reported by Zubarev... 

An additional example of the flexibility of the [ ER3 2Cu(QR )2M(QR )2] architecture to direct adjustment of these precursors are the SSPs 7 and 8, which represent the first liquid single-source precursors for the deposition of CuInS2.1 An added benefit of a charge neutral liquid precursor is the potential for greater solubility in nonpolar organic solvents and the possibility of direct... 

Figure 6.14. Example 2 fragmentation spectrum of singly charged peptide, precursor mass of 1098.6.

El predominantly creates singly charged ions from the precursor neutral. If the neutral was a molecule as in most cases, it started as having an even number of electrons, i.e., it was an even-electron (closed-shelT) molecule. The molecular ion formed must then be a radical cation or an odd-electron open-shell) ion as these species are termed, e.g., for methane we obtain ... 

Note As one electron charge is neutralized upon EC, the precursor ion for ECD must at least be a doubly charged positive even-electron ion to yield a singly charged radical ion for subsequent dissociation. 

Example MS on a QIT was used for the identification of beauverolides, cyclic peptides from the fermentation broth of Beauveria bassiana, a fungus of pathogenic activity on insects. [178] All MS (ESI-CID-QIT) experiments started from singly charged [Mh-H]" precursor ions (Fig. 4.48). 

Electron ionization mainly creates singly charged positive ions by ejection of one electron out of the neutral. If the precursor was a molecule, M, it will have an even number of electrons, i.e., an even-electron or closed-shell species. The molecular ion formed upon EI must then be a positive radical ion, M" , odd-electron or open-shell) ion. 

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