N-Terminal Derivatization and Fragmentation of Neutral Peptides via Ion−Molecule Reactions with Acylium Ions:  Toward Gas-Phase Edman Degradation?

Abstract

The gas-phase ion-molecule reactions of neutral alanylglycine have been examined with various mass-selected acylium ions RCO(+) (R= CH(3), CD(3), C(6)H(5), C(6)F(5) and (CH(3))( 2)N), as well as the transacylation reagent O-benzoylbenzophenone in a Fourier transform ion cyclotron resonance mass spectrometer. Reactions of the gaseous dipeptide with acylium ions trapped in the ICR cell result in the formation of energized [M + RCO](+) adduct ions that fragment to yield N-terminal b-type and C-terminal y-type product ions, including a modified b(1) ion which is typically not observed in the fragmentation of protonated peptides. Judicious choice of the acylium ion employed allows some control over the product ion types that are observed (i.e., b versus y ions). The product ion distributions from these ion--molecule reactions are similar to those obtained by collision-activated dissociation in a triple quadrupole mass spectrometer of the authentic N-acylated alanylglycine derivatives. These data indicate that derivatization of the peptide in the gas-phase occurs at the N-terminal amine. Ab initio molecular orbital calculations, performed to estimate the thermochemistry of the steps associated with adduct formation as well as product ion formation, indicate that (i) the initially formed adduct is energized and hence likely to rapidly undergo fragmentation, and (ii) the likelihood for the formation of modified b(1) ions in preference to y(1) ions is dependent on the R substituent of the acylium ion. The reaction of the tetrapeptide valine--alanine--alanine--phenylalanine with the benzoyl cation was also found to yield a number of product ions, including a modified b(1) ion. This result suggests that the new experimental approach described here may provide a tool to address one of the major limitations associated with traditional mass spectrometric peptide sequencing approaches, that is, determination of the identity and order of the two N-terminal amino acids. Analogies are made between the reactions observed here and the derivatization and N-terminal cleavage reactions employed in the condensed-phase Edman degradation method.

Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization via VUV-Generating Microplasmas

“…After absorption of this pulse, molecular species are ejected from the opposite side of the foil with relatively little energy transferred to the desorbed species [11,12]. Since LIAD is known to generate mostly neutral and intact molecules, this technique is well suited for softly producing pulses of low-polarity thermally-labile gas-phase molecules [11,13,14]. Initial studies describing the LIAD process suggested an acoustic wave mechanism for volatilization [15].…”

Probing isomeric differences of phosphorylated carbohydrates through the use of ion/molecule reactions and FT-ICR MS

“…Other recent examples include those by Kenttämaa and coworkers who utilized ion/molecule reactions to differentiate isomeric species such as diols [20,21] and steroids [22]. In other studies, ion/molecule reactions have been utilized to develop a potential gas phase sequencing method for peptides [23], to determine enantiomeric excess [24 -26], and to explore non-covalent interactions [27]. In addition, McLuckey and coworkers have employed ion/ion proton transfer reactions to enrich specific charge states of protein ions as part of their "ion parking" methodology [28].…”

Meerwein reaction of phosphonium ions with epoxides and thioepoxides in the gas phase

“…systematically investigated the intrinsic reactivity of mass-selected gaseous acylium ions and have demonstrated synthetic and analytical applications of these reactions [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Gaseous acylium ions undergo the following reactions: polar [4 ϩ 2 ϩ ] cycloaddition with dienes [11,12], ketalization with diols and analogs [13,14], and transacetalization with five-and six-membered cyclic acetals [15][16][17][18][19][20] or with seven-membered acetals.…”

“…Halogen (X) acylium ions, X™CO ϩ (F Ͼ Cl Ͼ Br), promote carbonylation of benzene [24,25] and of five-membered heterocyclics via selective functionalization of their inert C™H aromatic bonds. Gaseous acylium ions also react with neutral peptides by N-terminal derivatization followed by dissociation that produces structurally-diagnostic, modified b 1 sequence ions [26] ϩ product ions in proportions that are characteristic of the amine [27,28]. Acylium ions also react with isomeric ␣-, ␤-and ␥-hydroxyketones via structurally diagnostic cyclization reactions [29] and with ␣,␤-unsaturated carbonyl compounds by single and double [4 ϩ 2 ϩ ] Diels-Alder cycloaddition [30].…”