Selective cleavage enhanced by acetylating the side chain of lysine

Selective cleavage is of great interest in mass spectrometry studies as it can help sequence identification by promoting simple fragmentation pattern of peptides and proteins. In this work, the collision‐induced dissociation of peptides containing internal lysine and acetylated lysine residues were studied. The experimental and computational results revealed that multiple fragmentation pathways coexisted when the lysine residue was two amino acid residues away from N‐terminal of the peptide. After acetylation of the lysine side‐chain, ions were the most abundant primary fragment products and the Lys_(Ac)–Gly amide bond became the dominant cleavage site via an oxazolone pathway. Acetylating the side‐chain of lysine promoted the selective cleavage of Lys–Xxx amide bond and generated much more information of the peptide backbone sequence. The results re‐evaluate the selective cleavage due to the lysine basic side‐chain and provide information for studying the post‐translational modification of proteins and other bio‐molecules containing Lys residues. Copyright © 2013 John Wiley & Sons, Ltd.     

Energetic switch of the proline effect in collision‐induced dissociation of singly and doubly protonated peptide Ala‐Ala‐Arg‐Pro‐Ala‐Ala

Suppression of the selective cleavage at N‐terminal of proline is observed in the peptide cleavage by proteolytic enzyme trypsin and in the fragment ion mass spectra of peptides containing Arg‐Pro sequence. An insight into the fragmentation mechanism of the influence of arginine residue on the proline effect can help in prediction of mass spectra and in protein structure analysis. In this work, collision‐induced dissociation spectra of singly and doubly charged peptide AARPAA were studied by ESI MS/MS and theoretical calculation methods. The proline effect was evaluated by comparing the experimental ratio of fragments originated from cleavage of different amide bonds. The results revealed that the backbone amide bond cleavage was selected by the energy barrier height of the fragmentation pathway although the strong proton affinity of the Arg side chain affected the stereostructure of the peptide and the dissociation mechanism. The thermodynamic stability of the fragment ions played a secondary role in the abundance ratio of fragments generated via different pathways. Fragmentation studies of protonated peptide AACitPAA supported the energy‐dependent hypothesis. The results provide an explanation to the long‐term arguments between the steric conflict and the proton mobility mechanisms of proline effect.     

Energetics and dynamics of the fragmentation reactions of protonated peptides containing methionine sulfoxide or aspartic acid via energy- and time-resolved surface induced dissociation

The surface-induced dissociation (SID) of six model peptides containing either methionine sulfoxide or aspartic acid (GAILM(O)GAILR, GAILM(O)GAILK, GAILM(O)GAILA, GAILDGAILR, GAILDGAILK, and GAILDGAILA) have been studied using a specially configured Fourier transform ion-cyclotron resonance mass spectrometer (FT-ICR MS). In particular, we have investigated the energetics and dynamics associated with (i) preferential cleavage of the methionine sulfoxide side chain via the loss of CH3SOH (64 Da), and (ii) preferential cleavage of the amide bond C-terminal to aspartic acid. The role of proton mobility in these selective bond cleavage reactions was examined by changing the C-terminal residue of the peptide from arginine (nonmobile proton conditions) to lysine (partially mobile proton conditions) to alanine (mobile proton conditions). Time- and energy-resolved fragmentation efficiency curves (TFECs) reveal that selective cleavages due to the methionine sulfoxide and aspartic acid residues are characterized by slow fragmentation kinetics. RRKM modeling of the experimental data suggests that the slow kinetics is associated with large negative entropy effects and these may be due to the presence of rearrangements prior to fragmentation. It was found that the Arrhenius pre-exponential factor (A) for peptide fragmentations occurring via selective bond cleavages are 1-2 orders of magnitude lower than nonselective peptide fragmentation reactions, while the dissociation threshold (E0) is relatively invariant. This means that selective bond cleavage is kinetically disfavored compared to nonselective amide bond cleavage. It was also found that the energetics and dynamics for the preferential loss of CH3SOH from peptide ions containing methionine sulfoxide are very similar to selective C-terminal amide bond cleavage at the aspartic acid residue. These results suggest that while preferential cleavage can compete with amide bond cleavage energetically, dynamically, these processes are much slower compared to amide bond cleavage, explaining why these selective bond cleavages are not observed if fragmentation is performed under mobile proton conditions. This study further affirms that fragmentation of peptide ions in the gas phase are predominantly governed by entropic effects.     

The relative charge ratio between C and N atoms in amide bond acts as a key factor to determine peptide fragment efficiency in different charge states

The influence of charge state on the peptide dissociation behavior in tandem mass spectrometry (MS/MS) is worthy of discussion. Comparative studies of singly- and doubly-protonated peptide molecules are performed to explore the effect and mechanism of charge state on peptide fragmentation. In view of the charge-directed cleavage of protonated peptides described in the mobile proton model, radiolytic oxidation was applied to change the charge distribution of peptides but retain the sequence. Experimental studies of collision energy-dependent fragmentation efficiencies coupled with quantum chemical calculations indicated that the cleavage of ARRA and its side-chain oxidation products with oxygen atoms added followed a trend that doubly-protonated peptides fragment more easily than singly-protonated forms, while the oxidation product with the guanidine group deleted showed the opposite trend. By analyzing the charge distribution around the amide bonds, we found that the relative charge ratios between C and N atoms (Q_C/Q_N) in the amide bonds provided a reasonable explanation for peptide fragmentation efficiencies. An increase of the Q_C/Q_N value of the amide bond means that a peptide fragments more easily, and vice versa. The results described in this paper provide an experimental and calculation strategy for predicting peptide fragmentation efficiency.     

The charge ratio between O and N on amide bonds: a new approach to the mobile proton model

The influence of charge distribution on the cleavage of the peptides was investigated by fragmentation efficiency curves and quantum chemical calculations in order to clarify the fragmentation mechanism in this paper. The peptide Arg-Gly-Asp-Cys (RGDC) was oxidized to change the charge distribution, but its main sequence was retained. Under this study, it was illustrated that the fragmentation of the peptide RGDC became easier with each addition of an O atom to the Cys hydrosulfide group and the relative charge ratios between O and N (QO/QN) in the amide bonds had much to do with the cleavage of the peptide RGDC. For each amide bond, the situations coincided with overall conclusion: the increase of the QO/QN values results in a higher fragmentation efficiency and vice versa. The methods which combined fragmentation efficiency curves with the charge distribution of peptides provided a way to refine the mobile proton model for peptide fragmentation and to probe the discrepant fragmentation of peptides in peptide/protein identification.     

Phosphorylated serine and threonine residues promote site‐specific fragmentation of singly charged,arginine‐containing peptide ions

In order to investigate gas‐phase fragmentation reactions of phosphorylated peptide ions, matrix‐assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) tandem mass (MS/MS) spectra were recorded from synthetic phosphopeptides and from phosphopeptides isolated from natural sources. MALDI‐TOF/TOF (TOF: time‐of‐flight) spectra of synthetic arginine‐containing phosphopeptides revealed a significant increase of y ions resulting from bond cleavages on the C‐terminal side of phosphothreonine or phosphoserine. The same effect was found in ESI‐MS/MS spectra recorded from the singly charged but not from the doubly charged ions of these phosphopeptides. ESI‐MS/MS spectra of doubly charged phosphopeptides containing two arginine residues support the following general fragmentation rule: Increased amide bond cleavage on the C‐terminal side of phosphorylated serines or threonines mainly occurs in peptide ions which do not contain mobile protons. In MALDI‐TOF/TOF spectra of phosphopeptides displaying N‐terminal fragment ions, abundant b–H₃PO₄ ions resulting from the enhanced dissociation of the pSer/pThr–X bond were detected (X denotes amino acids). Cleavages at phosphoamino acids were found to be particularly predominant in spectra of phosphopeptides containing pSer/pThr–Pro bonds. A quantitative evaluation of a larger set of MALDI‐TOF/TOF spectra recorded from phosphopeptides indicated that phosphoserine residues in arginine‐containing peptides increase the signal intensities of the respective y ions by almost a factor of 3. A less pronounced cleavage‐enhancing effect was observed in some lysine‐containing phosphopeptides without arginine. The proposed peptide fragmentation pathways involve a nucleophilic attack by phosphate oxygen on the carbon center of the peptide backbone amide, which eventually leads to cleavage of the amide bond. Copyright © 2009 John Wiley & Sons, Ltd.     

The ornithine effect in peptide cation dissociation

Facile cleavage C‐terminal to ornithine residues in gas phase peptides has been observed and termed the ornithine effect. Peptides containing internal or C‐terminal ornithine residues, which are formed from deguanidination of arginine in solution, were fragmented to produce either a y‐ion or water loss, respectively, and the complementary b‐ion. The fragmentation patterns of several peptides containing arginine were compared to those of the ornithine analogues. Conversion of arginine to ornithine results in a decrease of the gas phase proton affinity of the residue, thereby increasing the mobility of the ionizing proton. This alteration allows the nucleophilic amine to facilitate a neighboring group reaction to induce a cleavage of the adjacent amide bond. The selective cleavage at the ornithine residue is proposed to result from the highly favorable generation of a six‐membered lactam ring. The ornithine effect was compared with the well‐known proline and aspartic acid effects in peptide fragmentation using angiotensin II, DRVYIHPF and the ornithine analogue, DOVYIHPF. Under conditions favorable to either the aspartic acid (i.e. singly protonated peptide) or proline effect (i.e. doubly protonated peptide), the ornithine effect was consistently observed to be the more favorable fragmentation pathway. The highly selective nature of the ornithine effect opens up the possibility for conversion of arginine to ornithine residues to induce selective cleavages in polypeptide ions. Such an approach may complement strategies that seek to generate non‐selective cleavages of the related peptides. Copyright © 2013 John Wiley & Sons, Ltd.     

Impact of proline and aspartic acid residues on the dissociation of intermolecularly crosslinked peptides

The dissociation of intermolecularly crosslinked peptides was evaluated for a series of peptides with proline or aspartic acid residues positioned adjacent to the crosslinking sites (lysine residues). The peptides were crosslinked with either disuccinimidyl suberate (DSS) or disuccinimidyl L-tartrate (DST), and the influence of proline and aspartic acid residues on the fragmentation patterns were investigated for precursor ions with and without a mobile proton. Collisionally activated dissociation (CAD) spectra of aspartic acid-containing crosslinked peptide ions, doubly-charged with both protons sequestered, were dominated by cleavage C-terminal to the Asp residue, similar to that of unmodified peptides. The proline-containing crosslinked peptides exhibited a high degree of internal ion formation, with the resulting product ions having an N-terminal proline residue. Upon dissociation of the doubly-charged crosslinked peptides, twenty to fifty percent of the fragment ion abundance was accounted for by multiple cleavage products. Crosslinked peptides possessing a mobile proton yielded almost a full series of b- and y-type fragment ions, with only proline-directed fragments still observed at high abundances. Interestingly, the crosslinked peptides exhibited a tendency to dissociate at the amide bond C-terminal to the crosslinked lysine residue, relative to the N-terminal side. One could envision updating computer algorithms to include these crosslinker specific product ions--particularly for precursor ions with localized protons--that provide complementary and confirmatory information, to offer more confident identification of both the crosslinked peptides and the location of the crosslink, as well as affording predictive guidelines for interpretation of the product-ion spectra of crosslinked peptides.     

Mobile and localized protons: a framework for understanding peptide dissociation

Protein identification and peptide sequencing by tandem mass spectrometry requires knowledge of how peptides fragment in the gas phase, specifically which bonds are broken and where the charge(s) resides in the products. For many peptides, cleavage at the amide bonds dominate, producing a series of ions that are designated b and y. For other peptides, enhanced cleavage occurs at just one or two amino acid residues. Surface-induced dissociation, along with gas-phase collision-induced dissociation performed under a variety of conditions, has been used to refine the general 'mobile proton' model and to determine how and why enhanced cleavages occur at aspartic acid residues and protonated histidine residues. Enhanced cleavage at acidic residues occurs when the charge is unavailable to the peptide backbone or the acidic side-chain. The acidic H of the side-chain then serves to initiate cleavage at the amide bond immediately C-terminal to Asp (or Glu), producing an anhydride. In contrast, enhanced cleavage occurs at His when the His side-chain is protonated, turning His into a weak acid that can initiate backbone cleavage by transferring a proton to the backbone. This allows the nucleophilic nitrogen of the His side-chain to attack and form a cyclic structure that is different from the 'typical' backbone cleavage structures.     

Investigation of c ions formed by N‐terminally charged peptides upon collision‐induced dissociation

Peptide fragments such as b and y sequence ions generated upon low‐energy collision‐induced dissociation have been routinely used for tandem mass spectrometry (MS/MS)‐based peptide/protein identification. The underlying formation mechanisms have been studied extensively and described within the literature. As a result, the ‘mobile proton model’ and ‘pathways in competition model’ have been built to interpret a majority of peptide fragmentation behavior. However, unusual peptide fragments which involve unfamiliar fragmentation pathways or various rearrangement reactions occasionally appear in MS/MS spectra, resulting in confused MS/MS interpretations. In this work, a series of unfamiliar c ions are detected in MS/MS spectra of the model peptides having an N‐terminal Arg or deuterohemin group upon low‐energy collision‐induced dissociation process. Both the protonated Arg and deuterohemin group play an important role in retention of a positive charge at the N‐terminus that is remote from the cleavage sites. According to previous reports and our studies involving amino acid substitutions and hydrogen–deuterium exchange, we propose a McLafferty‐type rearrangement via charge‐remote fragmentation as the potential mechanism to explain the formation of c ions from precursor peptide ions or unconventional b ions. Density functional theory calculations are also employed in order to elucidate the proposed fragmentation mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.     

Selective cleavage of protonated penetratin and its substitutes under low‐energy collision‐induced dissociation condition

An understanding of the dissociation of penetratin is important for improving its metabolic stability and cargo‐delivery efficiency. In this study, we describe the selective cleavage of the K15–K16 amide bond of penetratin under the low‐energy collision‐induced dissociation condition in mass spectrometry. A variety of penetratin substitutes have been studied in which key basic amino acids have been substituted within the sequence. The calculated structure indicates that an α‐helix structure prevents the fragmentation of the central peptide domain and the side chain of lysine is involved in the proton translocation process. Copyright © 2010 John Wiley & Sons, Ltd.     

N-Protonated Isomers as Gateways to Peptide Ion Fragmentation

According to the popular “mobile proton model” for peptide ion fragmentation in tandem mass spectrometry, peptide bond cleavage is typically preceded by intramolecular proton transfer from basic sites to an amide nitrogen in the backbone. If the intrinsic barrier to dissociation is the same for all backbone sites, the fragmentation propensity at each amide bond should reflect the stability of the corresponding N-protonated isomer. This hypothesis was tested by using ab initio and force-field computations on several polyalanines and Leu-enkephalin. The results agree acceptably with experimental reports, supporting the hypothesis. It was found that backbone N-protonation is most favorable near the C-terminus. The preference for C-terminal N-protonation, which is stronger for longer polyalanines, may be understood in terms of the well known “helix macrodipole” in the corresponding helical conformations. The opposite stability trend is found for peptides constrained to be linear, which is initially surprising but turns out to be consistent with the reversed direction of the macrodipole in the linear conformation.     

Occurrence of C-Terminal Residue Exclusion in Peptide Fragmentation by ESI and MALDI Tandem Mass Spectrometry

By screening a data set of 392 synthetic peptides MS/MS spectra, we found that a known C-terminal rearrangement was unexpectedly frequently occurring from monoprotonated molecular ions in both ESI and MALDI tandem mass spectrometry upon low and high energy collision activated dissociations with QqTOF and TOF/TOF mass analyzer configuration, respectively. Any residue localized at the C-terminal carboxylic acid end, even a basic one, was lost, provided that a basic amino acid such arginine and to a lesser extent histidine and lysine was present in the sequence leading to a fragment ion, usually depicted as (b_n-1 + H₂O) ion, corresponding to a shortened non-scrambled peptide chain. Far from being an epiphenomenon, such a residue exclusion from the peptide chain C-terminal extremity gave a fragment ion that was the base peak of the MS/MS spectrum in certain cases. Within the frame of the mobile proton model, the ionizing proton being sequestered onto the basic amino acid side chain, it is known that the charge directed fragmentation mechanism involved the C-terminal carboxylic acid function forming an anhydride intermediate structure. The same mechanism was also demonstrated from cationized peptides. To confirm such assessment, we have prepared some of the peptides that displayed such C-terminal residue exclusion as a C-terminal backbone amide. As expected in this peptide amide series, the production of truncated chains was completely suppressed. Besides, multiply charged molecular ions of all peptides recorded in ESI mass spectrometry did not undergo such fragmentation validating that any mobile ionizing proton will prevent such a competitive C-terminal backbone rearrangement. Among all well-known nondirect sequence fragment ions issued from non specific loss of neutral molecules (mainly H₂O and NH₃) and multiple backbone amide ruptures (b-type internal ions), the described C-terminal residue exclusion is highly identifiable giving raise to a single fragment ion in the high mass range of the MS/MS spectra. The mass difference between this signal and the protonated molecular ion corresponds to the mass of the C-terminal residue. It allowed a straightforward identification of the amino acid positioned at this extremity. It must be emphasized that a neutral residue loss can be misattributed to the formation of a y_m-1 ion, i.e., to the loss of the N-terminal residue following the a₁-y_m–1 fragmentation channel. Extreme caution must be adopted when reading the direct sequence ion on the positive ion MS/MS spectra of singly charged peptides not to mix up the attribution of the N- and C-terminal amino acids. Although such peculiar fragmentation behavior is of obvious interest for de novo peptide sequencing, it can also be exploited in proteomics, especially for studies involving digestion protocols carried out with proteolytic enzymes other than trypsin (Lys-N, Glu-C, and Asp-N) that produce arginine-containing peptides.     

A novel salt bridge mechanism highlights the need for nonmobile proton conditions to promote disulfide bond cleavage in protonated peptides under low-energy collisional activation

The gas-phase fragmentation mechanisms of small models for peptides containing intermolecular disulfide links have been studied using a combination of tandem mass spectrometry experiments, isotopic labeling, structural labeling, accurate mass measurements of product ions, and theoretical calculations (at the MP2/6-311 + G(2d,p)//B3LYP/3-21G(d) level of theory). Cystine and its C-terminal derivatives were observed to fragment via a range of pathways, including loss of neutral molecules, amide bond cleavage, and S-S and C-S bond cleavages. Various mechanisms were considered to rationalize S-S and C-S bond cleavage processes, including charge directed neighboring group processes and nonmobile proton salt bridge mechanism. Three low-energy fragmentation pathways were identified from theoretical calculations on cystine N-methyl amide: (1) S-S bond cleavage dominated by a neighboring group process involving the C-terminal amide N to form either a protonated cysteine derivative or protonated sulfenyl amide product ion (44.3 kcal mol(-1)); (2) C-S bond cleavage via a salt bridge mechanism, involving abstraction of the alpha-hydrogen by the N-terminal amino group to form a protonated thiocysteine derivative (35.0 kcal mol(-1)); and (3) C-S bond cleavage via a Grob-like fragmentation process in which the nucleophilic N-terminal amino group forms a protonated dithiazolidine (57.9 kcal mol(-1)). Interestingly, C-S bond cleavage by neighboring group processes have high activation barriers (63.1 kcal mol(-1)) and are thus not expected to be accessible during low-energy CID experiments. In comparison to the energetics of simple amide bond cleavage, these S-S and C-S bond cleavage reactions are higher in energy, which helps rationalize why bond cleavage processes involving the disulfide bond are rarely observed for low-energy CID of peptides with mobile proton(s) containing intermolecular disulfide bonds. On the other hand, the absence of a mobile proton appears to "switch on" disulfide bond cleavage reactions, which can be rationalized by the salt bridge mechanism. This potentially has important ramifications in explaining the prevalence of disulfide bond cleavage in singly protonated peptides under MALDI conditions.     

Sequencing Lys-N Proteolytic Peptides by ESI and MALDI Tandem Mass Spectrometry

In this study, we explored the MS/MS behavior of various synthetic peptides that possess a lysine residue at the N-terminal position. These peptides were designed to mimic peptides produced upon proteolysis by the Lys-N enzyme, a metalloendopeptidase issued from a Japanese fungus Grifola frondosa that was recently investigated in proteomic studies as an alternative to trypsin digestion, as a specific cleavage at the amide X-Lys chain is obtained that provides N-terminal lysine peptide fragments. In contrast to tryptic peptides exhibiting a lysine or arginine residue solely at the C-terminal position, and are thus devoid of such basic amino acids within the sequence, these Lys-N proteolytic peptides can contain the highly basic arginine residue anywhere within the peptide chain. The fragmentation patterns of such sequences with the ESI-QqTOF and MALDI-TOF/TOF mass spectrometers commonly used in proteomic bottom-up experiments were investigated.     

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.     

Peptidomic analysis of human acquired enamel pellicle

Human acquired enamel pellicle is the result of a selective interaction of salivary proteins and peptides with the tooth surface. In the present work, the characterization of the peptides as well as the type of interactions established with the enamel surface was performed. Peptides from in vivo bovine enamel implants in the human oral cavity were sequentially extracted using guanidine and trifluoroacetic acid solutions and the fractions obtained were analysed by LC-MS and LC-MS/MS. Based on the LC-MS data, six phosphorylated peptides were identified in an intact form, strongly adsorbed to the enamel surface. Data from the LC-MS/MS analyses allowed us to identified 30 fragment peptides non-covalently bonded to enamel [basic proline-rich proteins, histatins (1 and 3) and acidic proline-rich protein classes]. The tandem mass spectrometry experiments showed the existence of a pattern of amide bond cleavage for the different identified peptide classes suggesting a selective proteolytic activity. For histatins, a predominance of cleavage at Arg, Lys and His residues was observed, while for basic proline-rich proteins, cleavage at Arg and Pro residues prevailed. In the case of acidic proline-rich proteins, a clearly predominance of cleavage of the Gln-Gly amide bond was evident.     

Electron capture induced dissociation of doubly protonated pentapeptides: dependence on molecular structure and charge separation

We have studied electron capture induced dissociation of a set of doubly protonated pentapeptides, all composed of one lysine (K) and either four glycine (G) or four alanine (A) residues, as a function of the sequence of these building blocks. Thereby the separation of the two charges, sequestered on the N-terminal amino group and the lysine side chain, is varied. The characteristic cleavage of N-C(α) bonds is observed for all peptides over the whole backbone length, with the charge carrying fragments always containing K. The resulting fragmentation patterns are very similar if G is replaced by A. In the case of [XKXXX+2H](2+) (X=A or G), a distinct feature is observed in the distribution of backbone cleavage fragments and the probability for ammonia loss is drastically reduced. This may be due to an isomer with an amide oxygen as protonation site giving rise to the observed increase in breakage at a specific site in the molecule. For the other peptides, a correlation with the distance between amide oxygen and the charge at the lysine side chain has been found. This may be an indication that it is only the contribution from this site to the charge stabilization of the amide π(*) orbitals which determines relative fragment intensities. For comparison, complexes with two crown ether molecules have been studied as well. The crown ether provides a shielding of the charge and prevents the peptide from folding and internal hydrogen bonding, which leads to a more uniform fragmentation behavior.     

The Competition of Charge Remote and Charge Directed Fragmentation Mechanisms in Quaternary Ammonium Salt Derivatized Peptides—An Isotopic Exchange Study

Derivatization of peptides as quaternary ammonium salts (QAS) is a promising method for sensitive detection by electrospray ionization tandem mass spectrometry (Cydzik et al. J. Pept. Sci. 2011, 17, 445–453). The peptides derivatized by QAS at their N-termini undergo fragmentation according to the two competing mechanisms – charge remote (ChR) and charge directed (ChD). The absence of mobile proton in the quaternary salt ion results in ChR dissociation of a peptide bond. However, Hofmann elimination of quaternary salt creates an ion with one mobile proton leading to the ChD fragmentation. The experiments on the quaternary ammonium salts with deuterated N-alkyl groups or amide NH bonds revealed that QAS derivatized peptides dissociate according to the mixed ChR-ChD mechanism. The isotopic labeling allows differentiation of fragments formed according to ChR and ChD mechanisms.