Daughter ion mass spectra from cationized molecules of small oligopeptides in a reflecting time-of-flight mass spectrometer

We report a new fragmentation pathway for the CID of (b 3 Ϫ 1 ϩ Cat) ϩ product ions derived from the model peptide AXAG, where X ϭ␤-alanine, ␥-aminobutyric acid, -amino-n-caproic acid, or 4-aminomethylbenzoic acid. By changing the amino acid to the C-terminal side of the amino acid X, and incorporating 15 N and 13 C labeled residues at the same position, we conclude that the dissociation pathway most likely leads to a metal cationized nitrile. With respect to the various amino acids at position X, the putative nitrile product becomes more prominent, relative to the conventional (a 3 Ϫ 1 ϩ Cat) ϩ species, in the order ␤-alanine Ͻ ␥-aminobutyric acid Ͻ -aminocaproic acid Ͻ 4-aminomethylbenzoic acid. The pathway is not observed for peptides with ␣-amino acids at position X. The product ion is observed most prominently during the CID of Li ϩ and Na ϩ cationized peptides, only to a small extent for Ag ϩ cationized peptides, and not at all from protonated analogues. I n an attempt to enhance the understanding of how cation and sequence influence peptide fragmentation, we recently investigated the dissociation of model N-acetylated tetrapeptides with general sequence AcFGGX that featured C-termini designed to allow transfer of the ϪOH required to generate the (b 3 ϩ 17 ϩ Cat) ϩ product ion, but not necessarily as the most favored pathway [1]. The amino acid placed at position X either required a larger cyclic intermediate than the five-member ring presumably formed with ␣-amino acids (␤-alanine, ␥-aminobutyric acid, and -amino-n-caproic acid to generate 6-, 7-, or 9-member rings, respectively) or prohibited cyclization because of the inclusion of a rigid ring (para-and meta-aminobenzoic acid). For Ag ϩ , Li ϩ and Na ϩ cationized AcFGGX, formation of (b 3 ϩ 17 ϩ Cat) ϩ was suppressed when the amino acids requiring the adoption of larger ring intermediates were used, while amino acids that prohibit cyclization eliminated the reaction pathway completely-an observation in accord with proposed mechanisms for the formation of this important sequence ion [2][3][4][5][6][7][8].During subsequent experiments involving peptides containing similar "alternative" amino acids, we observed an unusual fragmentation pathway when the (b 3 Ϫ 1 ϩ Li) ϩ product ion derived from the synthetic peptide A(␤A)AG was subjected to collision-induced dissociation (CID). The pathway resulted in a neutral loss one mass unit (u) greater than the residue mass of the amino acid that composed the presumed oxazolinone ring [9 -15] of the (b 3 Ϫ 1 ϩ Li) ϩ species, and thus could not be attributed to the formation of the (b 2 Ϫ 1 ϩ Li) ϩ ion. We describe here experiments involving a group of tetrapeptides of general sequence AXAG, where X ϭ ␤-alanine, ␥-aminobutyric acid, -amino-ncaproic acid, and 4-aminomethylbenzoic acid, and analogous peptides with 15 N and 13 C labels in specific positions that were designed to probe the unusual reaction pathway. This specific set of experiments, focused on the fragmentation of the (b n Ϫ 1 ϩ Cat) ϩ ions (as opposed to ...

supporting

confidence: 60%

supporting

confidence: 59%

Novel fragmentation pathway for CID of (b_n−1+Cat)⁺ ions from model, metal cationized peptides

Cooper

Talaty

Stipdonk

2005

“…Upon collision induced dissociation (CID) of sodiated peptides, [b nϪ1 ϩ Na ϩ OH] ϩ products, where n represents the number of amino acid residues in the peptide, are commonly observed [2,[5][6][7][8][9][10]. The [b nϪ1 ϩ Na ϩ OH] ϩ products are equivalent to a truncated version of the respective peptide without the C-terminal residue.…”

Section: T Andem Mass Spectrometry (Ms/ms) Of Cation-mentioning

confidence: 99%

Generation and manipulation of sodium cationized peptides in the gas phase

Newton

McLuckey

2004

Sodiated peptides are often generated by electrospray ionization (ESI) of solutions containing peptides and a sodium salt. Fragmentation of singly sodiated, singly charged peptide ions commonly provides specific sequence information. However, these ions may be difficult to form by directly electrospraying a mixture. In the application of a recently described technique for forming metal containing peptide ions in the gas phase, singly sodiated, singly charged ions are formed via cation-switching ion/ion reactions of multiply protonated peptides. Proton transfer ion/ion reactions can also be used to form [M ϩ Na] ϩ through the reduction of charge states of multiply charged, singly sodiated ions. The specificity and flexibility of the techniques employed provide a highly controlled means of generating sodiated peptide and protein ions. Thus, the methodologies presented here have potential for forming ions not readily observed via ESI or MALDI. Furthermore, the use of ion/ion reactions to form sodiated peptides facilitates direct comparisons of the fragmentation behavior of [M ϩ Na] ϩ peptides formed in the absence of solvent with that of [M ϩ Na] ϩ peptides generated by directly electrospraying a sodium salt/peptide mixture. Thus, in addition to descriptions of the formation of [M ϩ Na] ϩ peptides in the gas phase using ion/ion reactions, results from CID of reaction products are presented herein. T andem mass spectrometry (MS/MS) of cationized peptides and proteins is a commonly used approach for obtaining primary sequence information. The fragmentation behavior of protonated peptides has been extensively examined. However, complementary fragmentation and/or specific sequence information may be obtained by performing collisioninduced dissociation experiments on metal cationized peptides. Several groups have reported the effects of using alkali metals or transition metals instead of protons as cationizing reagents on peptide fragmentation [1][2][3][4]. A potentially analytically useful observation from such studies is the possibility for C-terminal sequencing of peptides when sodium is the cationizing reagent [5][6][7].Sodiated peptides are often generated by electrospray ionization (ESI) of solutions containing peptides and a sodium salt. Upon collision induced dissociation (CID) of sodiated peptides, [b nϪ1 ϩ Na ϩ OH] ϩ products, where n represents the number of amino acid residues in the peptide, are commonly observed [2,[5][6][7][8][9][10]. The [b nϪ1 ϩ Na ϩ OH] ϩ products are equivalent to a truncated version of the respective peptide without the C-terminal residue. Subsequent MS/MS of first generation [b nϪ1 ϩ Na ϩ OH] ϩ products may yield [b m ϩ Na ϩ OH] ϩ ions, where m is one less than (n Ϫ 1) [5,6]. Thus, MS n of sodiated peptide ions can facilitate determination of peptide ion sequences through sequential cleavages corresponding to loss of C-terminal residues. Competition among fragmentation channels other than the one leading to formation of [b nϪ1 ϩ Na ϩ OH] ϩ constrains the ability to obtain complete s...

“…Because the number and location of sulfur-containing residues are often desired information in the determination of unknown peptides, any specific metal binding to these sulfur ligands is potentially analytically useful with the metal ions serving as flags or indicators of the sulfurcontaining residues. Furthermore, the fragmentation pathways of metallated peptides are often quite different from those of protonated peptides [2,5,6,[24][25][26], and some studies (e.g., [2]) were carried out partially with the goal of peptide sequencing in mind.As adduct reagents for proteins, transition-metal ions have received some attention lately because their binding chemistry tends to be different from that of alkali and alkaline metal ions. Tang et al [24] studied the product ion spectra of a number of proton-, alkali metal-, and silver-containing peptides generated via cesium-ion bombardment on solid peptide samples deposited on silver.…”

mentioning

confidence: 99%

mentioning

confidence: 99%

Complexes of silver(I) with peptides and proteins as produced in electrospray mass spectrometry

Siu

Guevremont

et al. 1997

Silver(I) forms aqueous phase complexes with both sulfur and nonsulfur containing peptides and proteins. These complexes were introduced into the gas phase via electrospray, and their structures probed by means of tandem mass spectrometry. Experiments with di-, tri-, and oligopeptides show that the abundance of silver(I)-containing ions increases relative to that of proton-containing ions as peptide length increases. This increase is much more dramatic for methionine-containing peptides. Collision-induced dissociation of silver-peptide complexes yields a multitude of product ions that are silver containing. However, even for methioninecontaining peptides, very few of these product ions contain the methionine residue. The solution-phase structure and the gas-phase structure of the silver/peptide complex are not identical. The methionine sulfur acts as the silver anchoring point in solution. Desolvation in the gas phase leads to a rearrangement of the silver/peptide complex such that the silver ion becomes chelated to the nitrogen and oxygen atom on the peptide backbone in addition to the methionine sulfur. This rearrangement decreases the importance of the silver/sulfur bond to the extent that it is frequently broken upon collision activation and leads to the formation of silver/peptide product ions that are nonsulfur bearing. (J Am Soc Mass Spectrom 1997, 8, 781-792) Crown copyright 1997 M ass spectrometric investigations of metal-ion binding to peptides and proteins in the gas phase are frequently prompted by the desire to compare the same binding in solution . Aside from its sensitivity and selectivity, mass spectrometry, because it measures the properties of an analyte in vacuum, offers a view of the intrinsic metal binding properties of proteins, unencumbered with solvation. A number of groups have devoted much time and effort on complexes of alkali [1-10], alkaline-earth [8,[11][12][13][14], and transition metals [15][16][17][18][19][20][21]. While most of these investigations centered on cationic metal complexes [1-6, 8, 9, 16-21], some involved anionic complexes as well [7,[10][11][12][13][14][15]. In earlier studies, fast atom bombardment (FAB) was the sample-introduction technique of choice due to its ease of producing a significant population of metal ions within the source. In later studies, the choice was between FAB and electrospray, the latter of the two, of course, offers the added convenience and [9,10,14,[16][17][18][19].The silver(I) ion is considered a "soft" cation according to Pearson's classification, and as such it prefers binding to soft ligands, ligands that are relatively polarizable [22]. Of all the functional groups of a protein, the softest groups are the sulfur ligand on the side-chain of cysteine and that of methionine [23]. Because the number and location of sulfur-containing residues are often desired information in the determination of unknown peptides, any specific metal binding to these sulfur ligands is potentially analytically useful with the metal ions serving as flags o...