Towards liquid chromatography time-scale peptide sequencing and characterization of post-translational modifications in the negative-ion mode using electron detachment dissociation tandem mass spectrometry

Electron detachment dissociation (EDD) of peptide poly-anions is gentle towards post-translational modifications (PTMs) and produces predictable and interpretable fragment ion types (a., x ions). However, EDD is considered an inefficient fragmentation technique and has not yet been implemented in large-scale peptide characterization strategies. We successfully increased the EDD fragmentation efficiency (up to 9%), and demonstrate for the first time the utility of EDD-MS/MS in liquid chromatography time-scale experiments. Peptides and phosphopeptides were analyzed in both positive- and negative-ion mode using electron capture/transfer dissociation (ECD/ETD) and EDD in comparison. Using approximately 1 pmol of a BSA tryptic digest, LC-EDD-MS/MS sequenced 14 peptides (27% aa sequence coverage) and LC-ECD-MS/MS sequenced 19 peptides (39% aa sequence coverage). Seven peptides (18% aa sequence coverage) were sequenced by both EDD and ECD. The relative small overlap of identified BSA peptides demonstrates the complementarity of the two dissociation modes. Phosphopeptide mixtures from three trypsin-digested phosphoproteins were subjected to LC-EDD-MS/MS resulting in the identification of five phospho-peptides. Of those, one was not found in a previous study using a similar sample and LC-ETD-MS/MS in the positive-ion mode. In this study, the ECD fragmentation efficiency (15.7% av.) was superior to the EDD fragmentation efficiency (3.6% av.). However, given the increase in amino acid sequence coverage and extended PTM characterization the new regime of EDD in combination with other ion-electron fragmentation techniques in the positive-ion mode is a step towards a more comprehensive strategy of analysis in proteome research.     

The Radical Ion Chemistry of S-Nitrosylated Peptides

The radical ion chemistry of a suite of S-nitrosopeptides has been investigated. Doubly and triply-protonated ions of peptides NYCGLPGEYWLGNDK, NYCGLPGEYWLGNDR, NYCGLPGERWLGNDR, NACGAPGEKWAGNDK, NYCGLPGEKYLGNDK, NYGLPGCEKWYGNDK and NYGLPGEKWYGCNDK were subjected to electron capture dissociation (ECD), and collision-induced dissociation (CID). The peptide sequences were selected such that the effect of the site of S-nitrosylation, the nature and position of the basic amino acid residues, and the nature of the other amino acid side chains, could be interrogated. The ECD mass spectra were dominated by a peak corresponding to loss of ^?NO from the charge-reduced precursor, which can be explained by a modified Utah-Washington mechanism. Some backbone fragmentation in which the nitrosyl modification was preserved was also observed in the ECD of some peptides. Molecular dynamics simulations of peptide ion structure suggest that the ECD behavior was dependent on the surface accessibility of the protonated residue. CID of the S-nitrosylated peptides resulted in homolysis of the S?CN bond to form a long-lived radical with loss of ^?NO. The radical peptide ions were isolated and subjected to ECD and CID. ECD of the radical peptide ions provided an interesting comparison to ECD of the unmodified peptides. The dominant process was electron capture without further dissociation (ECnoD). CID of the radical peptide ions resulted in cysteine, leucine, and asparagine side chain losses, and radical-induced backbone fragmentation at tryptophan, tyrosine, and asparagine residues, in addition to charge-directed backbone fragmentation.     

Identification of Phosphorylated Human Peptides by Accurate Mass Measurement Alone

At sufficiently high mass accuracy, it is possible to distinguish phosphorylated from unmodified peptides by mass measurement alone. We examine the feasibility of that idea, tested against a library of all possible in silico tryptic digest peptides from the human proteome database. The overlaps between in silico tryptic digest phosphopeptides generated from known phosphorylated proteins (1-12 sites) and all possible unmodified human peptides are considered for assumed mass error ranges of ±10, ±50, ±100, ±1,000, and ±10,000 ppb. We find that for mass error ±50 ppb, 95% of all phosphorylated human tryptic peptides can be distinguished from nonmodified peptides by accurate mass alone through the entire nominal mass range. We discuss the prospect of on-line LC MS/MS to identify phosphopeptide precursor ions in MS1 for selected dissociation in MS2 to identify the peptide and site(s) of phosphorylation.     

Fragmentation of amidinated peptide ions

The collision-induced dissociation characteristics of amidinated and unmodified tryptic peptides are compared using an ion trap mass spectrometer with both electrospray ionization and matrix-assisted laser/desorption ionization (MALDI). Several fragmentation pathways in a number of tryptic peptides of various precursor charge states are found to be enhanced. The additional information conveyed by the observed fragment ions should facilitate protein identifications.     

Chemical Derivatization of Peptide Carboxyl Groups for Highly Efficient Electron Transfer Dissociation

The carboxyl groups of tryptic peptides were derivatized with a tertiary or quaternary amine labeling reagent to generate more highly charged peptide ions that fragment efficiently by electron transfer dissociation (ETD). All peptide carboxyl groups—aspartic and glutamic acid side-chains as well as C-termini—were derivatized with an average reaction efficiency of 99 %. This nearly complete labeling avoids making complex peptide mixtures even more complex because of partially-labeled products, and it allows the use of static modifications during database searching. Alkyl tertiary amines were found to be the optimal labeling reagent among the four types tested. Charge states are substantially higher for derivatized peptides: a modified tryptic digest of bovine serum albumin (BSA) generates ~90% of its precursor ions with z? > ?2, compared with less than 40 % for the unmodified sample. The increased charge density of modified peptide ions yields highly efficient ETD fragmentation, leading to many additional peptide identifications and higher sequence coverage (e.g., 70 % for modified versus only 43 % for unmodified BSA). The utility of this labeling strategy was demonstrated on a tryptic digest of ribosomal proteins isolated from yeast cells. Peptide derivatization of this sample produced an increase in the number of identified proteins, a >50 % increase in the sequence coverage of these proteins, and a doubling of the number of peptide spectral matches. This carboxyl derivatization strategy greatly improves proteome coverage obtained from ETD-MS/MS of tryptic digests, and we anticipate that it will also enhance identification and localization of post-translational modifications.

Electron capture dissociation of singly and multiply phosphorylated peptides

Analysis of phosphotyrosine and phosphoserine containing peptides by nano-electrospray Fourier transform ion cyclotron resonance (FTICR) mass spectrometry established electron capture dissociation (ECD) as a viable method for phosphopeptide sequencing. In general, ECD spectra of synthetic and native phosphopeptides appeared less complex than conventional collision activated dissociation (CAD) mass spectra of these species. ECD of multiply protonated phosphopeptide ions generated mainly c- and z(.)-type peptide fragment ion series. No loss of water, phosphate groups or phosphoric acid from intact phosphopeptide ions nor from the c and z(.) fragment ion products was observed in the ECD spectra. ECD enabled complete or near-complete amino acid sequencing of phosphopeptides for the assignment of up to four phosphorylation sites in peptides in the mass range 1400 to 3500 Da. Nano-scale Fe(III)-affinity chromatography combined with nano-electrospray FTMS/ECD facilitated phosphopeptide analysis and amino acid sequencing from crude proteolytic peptide mixtures.     

Improvement of electron capture efficiency by resonant excitation

A novel pulse sequence improving the efficiency for electron capture dissociation (ECD) of an unmodified Fourier transform ion cyclotron resonance (FTICR) mass spectrometer by more than an order of magnitude is presented. Commercially available FTICR instruments are usually equipped with a filament-based electron source producing an electron beam that has a rather small cross section. An ideal overlap between the rotating ion cloud and the electron beam appears to be a prerequisite for a high ECD efficiency. A reduced interception of the ion cloud and the electron beam is probably due to the contribution of the magnetron motion to the trajectory of the ions, resulting in a precession about the z-axis of the instrument. By increasing the kinetic energy and therefore increasing the cyclotron radii of the precursor ions by resonant excitation, the overlap of the rotating ion cloud with the electron beam is improved. By use of this protocol the efficiency of electron capture is substantially increased and consequently the acquisition time of ECD spectra is reduced significantly. The capability of resonant excitation of the precursor ions during the irradiation with electrons is demonstrated for standard peptides. This approach is particularly valuable for analysis and characterization of O-glycosylated peptides. In addition to amino acid sequence information, the attachment site of the labile glycan moiety is determined, and also radical-site-induced fragmentations of the glycosidic bonds are observed.     

C60-fullerene bound silica for the preconcentration and the fractionation of multiphosphorylated peptides

Phosphorylation of proteins is an important cellular regulatory process. The analysis of protein phosphorylation is challenging due to the high dynamic range and low abundance natures of phosphorylated species. Mass spectrometry (MS) of phosphopeptides obtained from tryptic protein digests is the method-of-choice for characterization of phosphorylated proteins. However, determination of phosphopeptides by MS represents a major challenge, especially in the presence of unmodified peptides. Due to lower ionization efficiency of phosphopeptides, as well as the fact that the stoichiometry of phosphorylation is often present at low relative abundance, efficient enrichment of the phosphorylated peptides prior to MS analysis is therefore of high demand. In addition, successful identification of peptides with different phosphorylation grades still remains challenging.     

Sulfonium Ion Derivatization,Isobaric Stable Isotope Labeling and Data Dependent CID- and ETD-MS/MS for Enhanced Phosphopeptide Quantitation,Identification and Phosphorylation Site Characterization

An amine specific peptide derivatization strategy involving the use of novel isobaric stable isotope encoded ‘fixed charge’ sulfonium ion reagents, coupled with an analysis strategy employing capillary HPLC, ESI-MS, and automated data dependent ion trap CID-MS/MS, -MS³, and/or ETD-MS/MS, has been developed for the improved quantitative analysis of protein phosphorylation, and for identification and characterization of their site(s) of modification. Derivatization of 50 synthetic phosphopeptides with S,S′-dimethylthiobutanoylhydroxysuccinimide ester iodide (DMBNHS), followed by analysis using capillary HPLC-ESI-MS, yielded an average 2.5-fold increase in ionization efficiencies and a significant increase in the presence and/or abundance of higher charge state precursor ions compared to the non-derivatized phosphopeptides. Notably, 44% of the phosphopeptides (22 of 50) in their underivatized states yielded precursor ions whose maximum charge states corresponded to +2, while only 8% (4 of 50) remained at this maximum charge state following DMBNHS derivatization. Quantitative analysis was achieved by measuring the abundances of the diagnostic product ions corresponding to the neutral losses of ‘light’ (S(CH₃)₂) and ‘heavy’ (S(CD₃)₂) dimethylsulfide exclusively formed upon CID-MS/MS of isobaric stable isotope labeled forms of the DMBNHS derivatized phosphopeptides. Under these conditions, the phosphate group stayed intact. Access for a greater number of peptides to provide enhanced phosphopeptide sequence identification and phosphorylation site characterization was achieved via automated data-dependent CID-MS³ or ETD-MS/MS analysis due to the formation of the higher charge state precursor ions. Importantly, improved sequence coverage was observed using ETD-MS/MS following introduction of the sulfonium ion fixed charge, but with no detrimental effects on ETD fragmentation efficiency.     

Effects of the position of internal histidine residues on the collision-induced fragmentation of triply protonated tryptic peptides

The collision-induced dissociation spectra of a series of synthetic, tryptic peptides that differed by the position of an internal histidine residue were studied. Electrospray ionization of these peptides produced both doubly and triply protonated molecular ions. Collision-induced fragmentation of the triply protonated peptide ions had better efficiency than that of the doubly protonated ions, producing a higher abundance of product ions at lower collision energies. The product ion spectra of these triply protonated ions were dominated by a series of doubly charged y-ions and the amount of sequence information was dependent on the position of the histidine residue. In the peptides where the histidine was located towards the C-terminus of the peptide, a more extensive series of sequence specific product ions was observed. As the position of the histidine residue was moved towards the N-terminus of the peptide, systematically less sequence information was observed. The peptides were subsequently modified with diethylpyrocarbonate to manipulate the product ion spectra. Addition of the ethoxyformyl group to the N-terminus and histidine residue shifted the predominant charge state of the modified peptide to the doubly protonated form. These peptide ions fragmented efficiently, producing product ion spectra that contained more sequence information than could be obtained from the corresponding unmodified peptide.     

probing the gas-phase folding kinetics of peptide ions by IR activated DR-ECD

The effect of infrared (IR) irradiation on the electron capture dissociation (ECD) fragmentation pattern of peptide ions was investigated. IR heating increases the internal energy of the precursor ion, which often amplifies secondary fragmentation, resulting in the formation of w-type ions as well as other secondary fragments. Improved sequence coverage was observed with IR irradiation before ECD, likely due to the increased conformational heterogeneity upon IR heating, rather than faster breakdown of the initially formed product ion complex, as IR heating after ECD did not have similar effect. Although the ECD fragment ion yield of peptide ions does not typically increase with IR heating, in double resonance (DR) ECD experiments, fragment ion yield may be reduced by fast resonant ejection of the charge reduced molecular species, and becomes dependent on the folding state of the precursor ion. In this work, the fragment ion yield was monitored as a function of the delay between IR irradiation and the DR-ECD event to study the gas-phase folding kinetics of the peptide ions. Furthermore, the degree of intracomplex hydrogen transfer of the ECD fragment ion pair was used to probe the folding state of the precursor ion. Both methods gave similar refolding time constants of approximately 1.5 s(-1), revealing that gaseous peptide ions often refold in less than a second, much faster than their protein counterparts. It was also found from the IR-DR-ECD study that the intramolecular H. transfer rate can be an order of magnitude higher than that of the separation of the long-lived c/z product ion complexes, explaining the common observation of c. and z type ions in ECD experiments.     

New aspects in fragmentation of peptide nucleic acids: comparison of positive and negative ions by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

The use of peptide nucleic acids (PNAs) is steadily increasing in biochemistry and diagnostics. So far, PNAs have mostly been investigated using cationic conditions in mass spectrometry. Furthermore, the use of fragmentation techniques developed for peptides and proteins like infrared multiphoton dissociation (IRMPD) and electron capture dissociation (ECD) has barely been examined. However, especially the fragmentation behavior of PNA oligomers in negative ion mode is of high importance, due to the ability to interact with nucleic acids which are almost exclusively analyzed in the negatively charged state. In the current study PNA fragmentations under cationic and anionic conditions were investigated and different fragmentation techniques like collision‐induced dissociation (CID), IRMPD and ECD were applied. Especially when using CID and IRMPD, amide bonds were broken, whereas ECD resulted in the elimination of nucleobases. Differences were also observed between positive and negative ionization, while the sequence coverage for the negative ions was superior to positive ions. The fragmentation behavior using IRMPD led to almost complete sequence coverage. Additionally, in anions the interesting effect of multiple eliminations of HNCO was found. Copyright © 2009 John Wiley & Sons, Ltd.     

Improving fragmentation of poorly fragmenting peptides and phosphopeptides during collision-induced dissociation by malondialdehyde modification of arginine residues

Despite significant technological and methodological advancements in peptide sequencing by mass spectrometry, analyzing peptides that exhibit only poor fragmentation upon collision-induced dissociation (CID) remains a challenge. A major cause for unfavorable fragmentation is insufficient proton 'mobility' due to charge localization at strongly basic sites, in particular, the guanidine group of arginine. We have recently demonstrated that the conversion of the guanidine group of the arginine side chain by malondialdehyde (MDA) is a convenient tool to reduce the basicity of arginine residues and can have beneficial effects for peptide fragmentation. In the present work, we have focused on peptides that typically yield incomplete sequence information in CID-MS/MS experiments. Energy-resolved tandem MS experiments were carried out on angiotensins and arginine-containing phosphopeptides to study in detail the influence of the modification step on the fragmentation process. MDA modification dramatically improved the fragmentation behavior of peptides that exhibited only one or two dominant cleavages in their unmodified form. Neutral loss of phosphoric acid from phosphopeptides carrying phosphoserine and threonine residues was significantly reduced in favor of a higher abundance of fragment ions. Complementary experiments were carried out on three different instrumental platforms (triple-quadrupole, 3D ion trap, quadrupole-linear ion trap hybrid) to ascertain that the observation is a general effect.     

Electron capture dissociation and infrared multiphoton dissociation of oligodeoxynucleotide dications

We report electron capture dissociation (ECD) and infrared multiphoton dissociation (IRMPD) of doubly protonated and protonated/alkali metal ionized oligodeoxynucleotides. Mass spectra following ECD of the homodeoxynucleotides polydC, polydG, and polydA contain w or d "sequence" ions. For polydC and polydA, the observed fragments are even-electron ions, whereas radical w/d ions are observed for polydG. Base loss is seen for polydG and polydA but is a minor fragmentation pathway in ECD of polydC. We also observe fragment ions corresponding to w/d plus water in the spectra of polydC and d(GCATGC). Although the structure of these ions is not clear, they are suggested to proceed through a pentavalent phosphorane intermediate. The major fragment in ECD of d(GCATGC) is a d ion. Radical a- or z-type fragment ions are observed in most cases. IRMPD primarily results in base loss, but backbone fragmentation is also observed. IRMPD provides more sequence information than ECD, but the spectra are more complex due to extensive base and water losses. It is proposed that the smaller degree of sequence coverage in ECD, with fragmentation mostly occurring close to the ends of the molecules, is a consequence of a mechanism in which the electron is captured at a P=O bond, resulting in a negatively charged phosphate group. Consequently, at least two protons (or alkali metal cations) must be present to observe a w or d fragment ion, a requirement that is less likely for small fragments.     

Characterization of Tyrosine Nitration and Cysteine Nitrosylation Modifications by Metastable Atom-Activation Dissociation Mass Spectrometry

The fragmentation behavior of nitrated and S-nitrosylated peptides were studied using collision induced dissociation (CID) and metastable atom-activated dissociation mass spectrometry (MAD-MS). Various charge states, such as 1+, 2+, 3+, 2–, of modified and unmodified peptides were exposed to a beam of high kinetic energy helium (He) metastable atoms resulting in extensive backbone fragmentation with significant retention of the post-translation modifications (PTMs). Whereas the high electron affinity of the nitrotyrosine moiety quenches radical chemistry and fragmentation in electron capture dissociation (ECD) and electron transfer dissociation (ETD), MAD does produce numerous backbone cleavages in the vicinity of the modification. Fragment ions of nitrosylated cysteine modifications typically exhibit more abundant neutral losses than nitrated tyrosine modifications because of the extremely labile nature of the nitrosylated cysteine residues. However, compared with CID, MAD produced between 66% and 86% more fragment ions, which preserved the labile –NO modification. MAD was also able to differentiate I/L residues in the modified peptides. MAD is able to induce radical ion chemistry even in the presence of strong radical traps and therefore offers unique advantages to ECD, ETD, and CID for determination of PTMs such as nitrated and S-nitrosylated peptides.     

Charge location directs electron capture dissociation of peptide dications

The effect of peptide dication charge location on electron capture dissociation (ECD) fragmentation pattern is investigated. ECD fragmentation patterns are compared for peptides with amide and free acid C-terminal groups. ECD of free acid compared with C-terminally amidated peptides with basic residues near the N-terminus demonstrates increased formation of a-type ions. Similarly, ECD of free acid compared with C-terminally amidated peptides with basic residues near the C-terminus exhibits increased formation of y-type ions. Alteration of the peptide sequence to inhibit the formation of charged side chains (i.e., amino acid substitution and acetylation) provides further evidence for charge location effect on ECD. We propose that formation of zwitterionic peptide structures increases the likelihood of amide nitrogen protonation (versus basic side chains), which is responsible for the increase in a- and y-type ion formation.     

Identification of single and double sites of phosphorylation by ECD FT-ICR/MS in peptides related to the phosphorylation site domain of the myristoylated alanine-rich c kinase protein

A series of phosphorylated test peptides was studied by electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry (ECD FT-ICR MS). The extensive ECD-induced fragmentation made identification of phosphorylation sites for these peptides straightforward. The site(s) of initial phosphorylation of a synthetic peptide with a sequence identical to that of the phosphorylation site domain (PSD) of the myristoylated alanine-rich C kinase (MARCKS) protein was then determined. Despite success in analyzing fragmentation of the smaller test peptides, a unique site on the PSD for the first step of phosphorylation could not be identified because the phosphorylation reaction produced a heterogeneous mixture of products. Some molecules were phosphorylated on the serine closest to the N-terminus, and others on one of the two serines closest to the C-terminus of the peptide. Although no definitive evidence for phosphorylation on either of the remaining two serines in the PSD was found, modification there could not be ruled out by the ECD fragmentation data.     

Investigation of the presence of <Emphasis Type="Italic">b</Emphasis> ions in electron capture dissociation mass spectra

Previously, we have indicated (Cooper, H.J., et al. Int. J. Mass Spectrom., 2003, 228, 723-728) that electron capture dissociation (ECD) of the doubly protonated peptides, Leu(4)-Sar-Leu(3)-Lys-OH, Leu(4)-Ala-Leu(3)-Lys-OH, Gly(4)-Sar-Gly(3)-Lys-NH(2), and Gly(3)-Pro-Sar-Gly(3)-Lys-NH(2), results in abundant b ions, which derive from fragmentation of backbone amide bonds, a nonstandard fragmentation channel in ECD. The instrumental conditions were such that the possibility that collision-induced dissociation processes were contributing to the observed spectra was eliminated. In a separate study (Fung, Y.M.E., et al. Eur. J. Mass Spectrom., 2004, 10, 449-457. ECD of peptides Arg-(Gly)(n)-Xxx-(Gly)(n)-Arg, where Xxx is the amino acid of interest, did not result in b ions. The variation in ECD observed for strikingly similar peptides suggests that the nature of the charge carrier (Arg or Lys) is instrumental in governing the fragmentation channels. Here, we describe the ECD behavior of a suite of model peptides designed such that the nature and position of the charge carrier could be probed. The results suggest that the presence of b ions in ECD spectra is a consequence of both charge carrier and peptide structure. Possible mechanisms for the formation of b ions following electron capture are discussed.     

Fragmentation of oligoribonucleotides from gas-phase ion-electron reactions

We have recently demonstrated that both electron capture dissociation (ECD) and electron detachment dissociation (EDD) can provide complementary sequence-specific cleavage of DNA compared with collision activated dissociation (CAD) and infrared multiphoton dissociation (IRMPD). However, EDD is preferred because of more extensive fragmentation at higher sensitivity (due to its negative ion mode operation). Here, we extend the radical ion chemistry of these two gas-phase ion-electron reaction techniques to the characterization of RNA. Compared with DNA, rather limited information is currently available on the gas-phase fragmentation of RNA. We found that the ECD fragmentation patterns of the oligoribonucleotides A6, C6, and CGGGGC are nucleobase dependent, suggesting that cleavage proceeds following electron capture at the nucleobases. Only limited backbone cleavage was observed in ECD. EDD, on the other hand, provided complete sequence coverage for the RNAs A6, C6, G6, U6, CGGGGC, and GCAUAC. The EDD fragmentation patterns were different from those observed with CAD and IRMPD in that the dominant product ions correspond to d- and w-type ions rather than c- and y-type ions. The minimum differences between oligoribonucleotides suggest that EDD proceeds following direct electron detachment from the phosphate backbone.     

IRMPD and ECD fragmentation of intermolecular cross-linked peptides

Despite the increasing number of studies using mass spectrometry for three dimensional analyses of proteins (MS3D), the identification of cross-linked peptides remains a bottleneck of the method. One of the main reasons for this is the lack of knowledge about the fragmentation of these species. Intermolecular cross-linked peptides are considered the most informative species present in MS3D experiment, since different peptides are connected by a cross-linker, the peptides chain can be either from a single protein, providing information about protein folding, or from two different proteins in a complex, providing information about binding partners, complex topology and interaction sites. These species tend to be large and highly charged in ESI, making comprehensive fragmentation by CID MS/MS problematic. On the other hand, these highly charged peptides are very suitable for dissociation using both infrared multiphoton dissociation (IRMPD) and electron capture dissociation (ECD). Herein, we report the fragmentation study of intermolecular cross-linked peptides using IRMPD and ECD. Using synthetic peptides and different commercial cross-linkers, a series of intermolecular cross-linked peptides were generate, and subsequently fragmented by IRMPD and ECD in a FT-ICR-MS instrument. Due to the high mass accuracy and resolution of the FT-ICR, the fragment ions could be attributed with high confidence. The peptides sequence coverage and fragmentation features obtained from IRMPD and ECD were compared for all charge states.