Gas-phase structures and proton affinities of N-terminal proline containing b 2 + ions from protonated model peptides (original) (raw)
Related papers
Journal of The American Society for Mass Spectrometry, 2011
Charge-directed fragmentation has been shown to be the prevalent dissociation step for protonated peptides under the low-energy activation (eV) regime. Thus, the determination of the ion structure and, in particular, the characterization of the protonation site(s) of peptides and their fragments is a key approach to substantiate and refine peptide fragmentation mechanisms. Here we report on the characterization of the protonation site of oxazolone b 2 ions formed in collision-induced dissociation (CID) of the doubly protonated tryptic model-peptide YIGSR. In support of earlier work, here we provide complementary IR spectra in the 2800–3800 cm–1 range acquired on a table-top laser system. Combining this tunable laser with a high power CO2 laser to improve spectroscopic sensitivity, well resolved bands are observed, with an excellent correspondence to the IR absorption bands of the ring-protonated oxazolone isomer as predicted by quantum chemical calculations. In particular, it is shown that a band at 3445 cm–1, corresponding to the asymmetric N–H stretch of the (nonprotonated) N-terminal NH2 group, is a distinct vibrational signature of the ring-protonated oxazolone structure.
International Journal of Mass Spectrometry and Ion Processes, 1997
Three-stage tandem mass spectrometry (MS 3 ) and neutral fragment reionization (N f R) are utilized to investigate the structures of the N-terminal ionic (b i ) and neutral backbone fragments, respectively, produced from break-up of the amide bond in protonated peptides that have been collisionally activated. The b-type ion from [M + H] + of C 6 H 5 CO-GF, which is produced by loss of the C-terminal phenylalanine, has the structure of protonated 2-phenyl-5-oxazolone. Conversely, the neutral fragment accompanying the y-type ion (protonated phenylalanine) is 2-phenyl-5-oxazolone. The b 2 ions arising from [M + H] + of underivatized tripeptides are also found to be protonated oxazolones. On the other hand, the neutral fragments released from the N-terminus of the tripeptides upon formation of y 1 are shown to be diketopiperazines and not oxazolones. The combined MS 3 and N f R data help propose dissociation mechanisms that account for the observed structures of ionic and neutral backbone fragments. ᭧ 1997 Elsevier Science B.V.
Dissociation of the peptide bond in protonated peptides
Journal of Mass Spectrometry, 2000
The dissociation of the amide (peptide) bond in protonated peptides, [M Y H] Y , is discussed in terms of the structures and energetics of the resulting N -terminal b n and C -terminal y n sequence ions. The combined data provide strong evidence that dissociation proceeds with no reverse barriers through interconverting proton-bound complexes between the segments emerging upon cleavage of the protonated peptide bond. These complexes contain the C -terminal part as a smaller linear peptide (amino acid if one residue) and the N -terminal part either as an oxazolone or a cyclic peptide (cyclic amide if one residue). Owing to the higher thermodynamic stability but substantially lower gas-phase basicity of cyclic peptides vs isomeric oxazolones, the N -terminus is cleaved as a protonated oxazolone when ionic (b n series) but as a cyclic peptide when neutral (accompanying the C -terminal y n series). It is demonstrated that free energy correlations can be used to derive thermochemical data about sequence ions. In this context, the dependence of the logarithm of the abundance ratio log[y 1 /b 2 ], from protonated GGX (G, glycine; X, varying amino acid) on the gas-phase basicity of X is used to obtain a first experimental estimate of the gas-phase basicity of the simplest b-type oxazolone, viz. 2-aminomethyl-5-oxazolone (b 2 ion with two glycyl residues).
Fragmentation of doubly-protonated Pro-His-Xaa tripeptides: Formation of b 2 2 + ions
Journal of The American Society for Mass Spectrometry, 2009
When ionized by electrospray from acidic solutions, the tripeptides Pro-His-Xaa (Xaa=Gly, Ala, Leu) form abundant doubly-protonated ions, [M+2H]2+. Collision-induced dissociation (CID) of these doubly-protonated species results, in part, in formation of b 2 2+ ions, which fragment further by loss of CO to form a 2 2+ ions; the latter fragment by loss of CO to form the Pro and His iminium [immonium is commonly used in peptide MS work] ions. Although larger doubly-charged b ions are known, this represents the first detailed study of b 2 2+ ions in CID of small doubly protonated peptides. The most abundant CID products of the studied doubly-protonated peptides arise mainly in charge separation involving two primary fragmentation channels, formation of the b 2 /y 1 pair and formation of the a 1 /y 2 pair. Combined molecular dynamics and density functional theory calculations are used to gain insight into the structures and fragmentation pathways of doubly-protonated Pro-His-Gly including the energetics of potential protonation sites, backbone cleavages, post-cleavage charge-separation reactions and the isomeric structures of b 2 2+ ions. Three possible structures are considered for the b 2 2+ ions: the oxazolone, diketopiperazine, and fused ring isomers. The last is formed by cleavage of the His-Gly amide bond on a pathway that is initiated by nucleophilic attack of one of the His side-chain imidazole nitrogens. Our calculations indicate the b 2 2+ ion population is dominated by the oxazolone and/or fused ring isomers.
Fragmentation of doubly-protonated Pro-His-Xaa tripeptides: Formation of b22+ ions
Journal of the American Society for Mass Spectrometry, 2009
When ionized by electrospray from acidic solutions, the tripeptides Pro-His-Xaa (Xaa ϭ Gly, Ala, Leu) form abundant doubly-protonated ions, [M ϩ 2H] 2ϩ. Collision-induced dissociation (CID) of these doubly-protonated species results, in part, in formation of b 2 2ϩ ions, which fragment further by loss of CO to form a 2 2ϩ ions; the latter fragment by loss of CO to form the Pro and His iminium [immonium is commonly used in peptide MS work] ions. Although larger doubly-charged b ions are known, this represents the first detailed study of b 2 2ϩ ions in CID of small doubly protonated peptides. The most abundant CID products of the studied doubly-protonated peptides arise mainly in charge separation involving two primary fragmentation channels, formation of the b 2 /y 1 pair and formation of the a 1 /y 2 pair. Combined molecular dynamics and density functional theory calculations are used to gain insight into the structures and fragmentation pathways of doubly-protonated Pro-His-Gly including the energetics of potential protonation sites, backbone cleavages, post-cleavage charge-separation reactions and the isomeric structures of b 2 2ϩ ions. Three possible structures are considered for the b 2 2ϩ ions: the oxazolone, diketopiperazine, and fused ring isomers. The last is formed by cleavage of the His-Gly amide bond on a pathway that is initiated by nucleophilic attack of one of the His side-chain imidazole nitrogens. Our calculations indicate the b 2 2ϩ ion population is dominated by the oxazolone and/or fused ring isomers.
Journal of The American Society for Mass Spectrometry, 2013
In this study, C-terminal protonated dipeptide eliminations were reported for both b 5 and b 4 ions of side chain hydroxyl group (-OH) containing pentapeptides. The study utilized the model C-terminal amidated pentapeptides having sequences of XGGFL and AXVYI, where X represents serine (S), threonine (T), glutamic acid (E), aspartic acid (D), or tyrosine (Y) residue. Upon low-energy collision-induced dissociation (CID) of XGGFL (where X=S, T, E, D, and Y) model peptide series, the ions at m/z 279 and 223 were observed as common fragments in all b 5 and b 4 ion (except b 4 ion of YGGFL) mass spectra, respectively. By contrast, peptides, namely S Me GGFL-NH 2 and E OMe GGFL-NH 2 , did not show either the ion at m/z 279 or the ion at m/z 223. It is shown that the side chain hydroxyl group is required for the possible mechanism to take place that furnishes the protonated dipeptide loss from b 5 and b 4 ions. In addition, the ions at m/z 295 and 281 were detected as common fragments in all b 5 and b 4 ion (except b 4 ion of AYVYI) mass spectra, respectively, for AXVYI model peptide series. The MS 4 experiments exhibited that the fragment ions at m/z 279, 223, 295, and 281 entirely reflect the same fragmentation behavior of [M+H] + ion generated from commercial dipeptides FL-OH, GF-OH, YI-OH, and VY-OH. These novel eliminations reported here for b 5 and b 4 ions can be useful in assigning the correct and reliable peptide sequences for high-throughput proteomic studies.
Proton Mobility in b2 Ion Formation and Fragmentation Reactions of Histidine-Containing Peptides
Journal of the American Society for Mass Spectrometry, 2015
A detailed energy-resolved study of the fragmentation reactions of protonated histidine-containing peptides and their b2 ions has been undertaken. Density functional theory calculations were utilized to predict how the fragmentation reactions occur so that we might discern why the mass spectra demonstrated particular energy dependencies. We compare our results to the current literature and to synthetic b2 ion standards. We show that the position of the His residue does affect the identity of the subsequent b2 ion (diketopiperazine versus oxazolone versus lactam) and that energy-resolved CID can distinguish these isomeric products based on their fragmentation energetics. The histidine side chain facilitates every major transformation except trans-cis isomerization of the first amide bond, a necessary prerequisite to diketopiperazine b2 ion formation. Despite this lack of catalyzation, trans-cis isomerization is predicted to be facile. Concomitantly, the subsequent amide bond cleavage...
The structure and fragmentation of B n (n≥3) ions in peptide spectra
Journal of the American Society for Mass Spectrometry, 1996
The unimolecular and low energy collision-induced fragmentation reactions of the MI-I+ ions of N-acetyl-tri-alanine, N-acetyl-tri-alanine methyl ester, N-acetyl-tetra-alanine, tetra-alanine, penta-alanine, hexa-glycine, and Leu-enkephalin have been studied with a particular emphasis on the formation and fragmentation of B, (n = 3,4,5) ions. In addition, the metastable ion fragmentation reactions of protonated tetra-glycine, penta-glycine, and Leu-enkephalin amide have been studied. B, ions are prominent stable species in all spectra. The B, ions fragment, in part, by elimination of CO to form A ,, ions; this reaction occurs on the metastable ion time scale with a substantial release of kinetic energy U1,2 = 0.3-0.5 eV) that indicates that a stable configuration of the B,, ion fragments by way of a reacting configuration that is higher in energy than the fragmentation products, An + CO. Ab initio calculations strongly suggest that the stable configuration of the B, and B, ions is a protonated oxazolone formed by interaction of the developing charge with the next-nearest carbonyl group as HX is lost from the protonated species H-(Yyy),,-X * H+. The higher B,, ions also fragment, in part, to form the next-lower B ion, presumably in its stable protonated oxazolone form. This reaction is rationalized in terms of the three-dimensional structure of the B,, ions and it is proposed that the neutral eliminated is an cY-lactam. (J Am Sot Muss Spectrom 2996, 7, 233-242) Address reprint requests to