On the survival of peptide cations after electron capture: Role of internal hydrogen bonding and microsolvation

Abstract

Electron capture by both bare and microsolvated small peptide dications was investigated by colliding these ions with sodium vapor in an accelerator mass spectrometer to provide insight into processes that occur on the microsecond time frame. Survival of the intact peptide monocation after electron capture depends strongly on molecular size. For dipeptides, no intact reduced species were observed; the predominant ions correspond to loss of hydrogen and ammonia. In contrast, the intact reduced species was observed for larger peptides. Calculated structures indicate that the diprotonated dipeptide ions form largely extended structures with low probability of internal ionic hydrogen bonding (i.e., charge solvation) whereas internal ionic H-bonding occurs extensively for larger peptide dications. Solvation of the peptide ions with between one to seven methanol molecules reduces the total extent of H loss even for dipeptides where intact reduced species can survive more than a microsecond after electron capture. The yield of ions corresponding to cleavage of NOC ␣ bonds (c ϩ and z ϩ· ions) does not depend strongly on peptide size but decreases with the extent of microsolvation for the dipeptide dications. H-bonding appears to play an important role for the survival of the intact reduced ions but less so for the formation of c ϩ and z ϩ· ions. Our results indicate that electron capture predominantly occurs at the ammonium groups (at least 70 to 80%), and provides important new insights into the electron capture dissociation process. (J Am Soc

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

“…Loss of water molecules resulting from ECD of proteins has been observed [31], but this is a minor process. Product ions corresponding to the loss of a H atom can be the dominant process observed for ECID of diprotonated dipeptides [32], but loss of a H atom is less significant for larger peptides. In larger peptides and proteins, it has been postulated that a hot hydrogen atom may play a role in the formation of product ions [7].…”

“…The calculated values rapidly decrease with increasing cluster size approaching an asymptotic limit of 3.9 -4.8 and 5.1-5.5 eV for clusters with n ϭ 11-19 at the MP2 and BLYP levels of theory, respectively [69]. For Mg(H 2 O) 32 2ϩ , we find an average internal energy deposition (E avg ) of about 4.5 eV and a maximum value of 4.8 -5.2 eV. These measured values are slightly higher than the recombination energies calculated at the MP2 level, but are comparable to BLYP values for large n [69].…”

Periodic sequence distribution of product ion abundances in electron capture dissociation of amphipathic peptides and proteins

“…ECD FT-ICR MS of six doubly protonated horse myoglobin tryptic fragments ( [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16], [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], [32][33][34][35][36][37][38][39][40][41][42] Figure S2) and with vibrational activation ( Figure 5) confirm preferential and sometimes periodic product ion formation from peptides with mainly ␣-helical or ␤-turn secondary structure in solution (before protein digestion). Predominantly z-ions are observed because of preferential charge retention at the C-terminal Lys or Arg basic residue upon electron capture, as expected for doubly charged tryptic peptides.…”

“…Supplementary Figure S2 demonstrates preferential cleavage at the NOC ␣ bond number 4 (z nϪ3 ) for fragments with a distinct ␣-helical or turn structure at the N-terminus (fragments [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16], [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] -118]). The periodicity is more pronounced in AI-ECD data ( Figure 5), demonstrating a period of 3 amino acids for the specified myoglobin fragments.…”

“…Nevertheless, product ion abundance (PIA) distribution in ECD of peptides is believed to depend on peptide secondary structure in the gas phase [13]. Recent experimental and theoretical progress confirms the important role of hydrogen bonds in the backbone cleavage mechanism [21], allows better prediction of ECD product ion distribution [22][23][24], and establishes correlation with gas-phase peptide and protein structures [12,25,26]. However, the high structural complexity of peptides and proteins limits computational capabilities [21] and complicates analysis of the experimental results.…”