Derivatization of peptides as quaternary ammonium salts for sensitive detection by ESI-MS

Peptidyl-Resin Substrates as a Tool in the Analysis of Caspase Activity

Caspases, proteolytic enzymes belonging to the group of cysteine proteases, play a crucial role in apoptosis. Understanding their activity and substrate specificity is extremely important. Fluorescence-based approaches, including fluorogenic substrates, are generally used to confirm cleavage preferences. Here we present a new method of substrate specificity and activity analysis based on the application of fix-charge tagged peptides located on the resin. The proteolysis of peptide bond on the resin, occurring even with low efficiency, results in the formation of N-terminal fragments of model peptide containing ionization enhancers in the form of quaternary ammonium groups, allowing for ultrasensitive and reliable analysis by LC-MS/MS. The possibility of application of the proposed solution was tested through the analysis of substrate specificity and activity of caspase 3 or 7. The obtained results confirm the known substrate specificity of executioner caspases. Our solution also allowed us to observe that caspases can hydrolyze peptides shorter than those presented to date in the scientific literature.

Quantitative Site-Specific Chemoproteomic Profiling of Protein Lipoylation

Protein lipoylation is an evolutionarily conserved post-translational modification from prokaryotes to eukaryotes. Lipoylation is implicated with several human diseases, including metabolic disorders, cancer, and Alzheimer's disease. While individual lipoylated proteins have been biochemically studied, a strategy for globally quantifying lipoylation with site-specific resolution in proteomes is still lacking. Herein, we developed a butyraldehyde-alkynyl probe to specifically label and enrich lipoylations in complexed biological samples. Combined with a chemoproteomic pipeline using customized tandem enzyme digestions and a biotin enrichment tag with enhanced ionization, we successfully quantified all known lipoylation sites in both Escherichia coli (E. coli) and human proteomes. The strategy enabled us to dissect the dependence of three evolutionarily related lipoylation sites in dihydrolipoamide acetyltransferase (ODP2) in E. coli and evaluated the functional connection between the de novo lipoylation synthetic pathway and the salvage pathway. Our chemoproteomic platform provides a useful tool to monitor the state of lipoylation in proteome samples, which will help decipher molecular mechanisms of lipoylation-related diseases.

Veni, Vidi, Vici: Immobilized Peptide-Based Conjugates as Tools for Capture, Analysis, and Transformation

Analysis of peptide biomarkers of pathological states of the organism is often a serious challenge, due to a very complex composition of the cell and insufficient sensitivity of the current analytical methods (including mass spectrometry). One of the possible ways to overcome this problem is sample enrichment by capturing the selected components using a specific solid support. Another option is increasing the detectability of the desired compound by its selective tagging. Appropriately modified and immobilized peptides can be used for these purposes. In addition, they find application in studying the specificity and activity of proteolytic enzymes. Immobilized heterocyclic peptide conjugates may serve as metal ligands, to form complexes used as catalysts or analytical markers. In this review, we describe various applications of immobilized peptides, including selective capturing of cysteine-containing peptides, tagging of the carbonyl compounds to increase the sensitivity of their detection, enrichment of biological samples in deoxyfructosylated peptides, and fishing out of tyrosine–containing peptides by the formation of azo bond. Moreover, the use of the one-bead-one-compound peptide library for the analysis of substrate specificity and activity of caspases is described. Furthermore, the evolution of immobilization from the solid support used in peptide synthesis to nanocarriers is presented. Taken together, the examples presented here demonstrate immobilized peptides as a multifunctional tool, which can be successfully used to solve multiple analytical problems.

Analysis of Fragmentation Pathways of Peptide Modified with Quaternary Ammonium and Phosphonium Group as Ionization Enhancers

Peptide modification by a quaternary ammonium group containing a permanent positive charge is a promising method of increasing the ionization efficiency of the analyzed compounds, making ultra-sensitive detection even at the attomolar level possible. Charge-derivatized peptides may undergo both charge remote (ChR) and charge-directed (ChD) fragmentation. A series of model peptide conjugates derivatized with N,N,N-triethyloammonium (TEA), 1-azoniabicyclo[2.2.2]octane (ABCO), 2,4,6-triphenylopyridinium (TPP) and tris(2,4,6-trimetoxyphenylo)phosphonium (TMPP) groups were analyzed by their fragmentation pathways both in collision-induced dissociation (CID) and electron-capture dissociation (ECD) mode. The effect of the fixed-charge tag type and peptide sequence on the fragmentation pathways was investigated. We found that the aspartic acid effect plays a crucial role in the CID fragmentation of TPP and TEA peptide conjugates whereas it was not resolved for the peptides derivatized with the phosphonium group. ECD spectra are mostly dominated by c_n ions. ECD fragmentation of TMPP-modified peptides results in the formation of intense fragments derived from this fixed-charge tag, which may serve as reporter ion.

Programmed Formation of HCN Oligomers through Organosulfur Catalysis

An efficient, inexpensive, and reliable synthesis of diaminomaleonitrile (DAMN, 1) is described starting from readily available acetone cyanohydrin as the source of hydrogen cyanide (HCN). Diaminomaleonitrile (DAMN) is known to be an important intermediate in heterocyclic and medicinal chemistry as well as being a possible precursor for the origin of life's hypothesis within prebiotic chemistry. The mechanism of its formation through organosulfur catalysis has been investigated by electrospray ionization mass spectrometry (ESI-MS) using two newly synthesized cationic "marker" molecules as a tool that allows for sensitive detection. As a result, the proposed mechanism of a thiocyanate-mediated synthesis of the HCN tetramer DAMN starting from organic disulfides was confirmed.

Application of Safirinium N-Hydroxysuccinimide Esters to Derivatization of Peptides for High-Resolution Mass Spectrometry, Tandem Mass Spectrometry, and Fluorescent Labeling of Bacterial Cells

Mass spectrometry methods are commonly used in the identification of peptides and biomarkers. Due to a relatively low abundance of proteins in biological samples, there is a need for the development of novel derivatization methods that would improve MS detection limits. Hence, novel fluorescent N–hydroxysuccinimide esters of dihydro-[1,2,4]triazolo[4,3-a]pyridin-2-ium carboxylates (Safirinium P dyes) have been synthesized. The obtained compounds, which incorporate quaternary ammonium salt moieties, easily react with aliphatic amine groups of peptides, both in solution and on the solid support; thus, they can be applied for derivatization as ionization enhancers. Safirinium tagging experiments with ubiquitin hydrolysate revealed that the sequence coverage level was high (ca. 80%), and intensities of signals were enhanced up to 8-fold, which proves the applicability of the proposed tags in the bottom–up approach. The obtained results confirmed that the novel compounds enable the detection of trace amounts of peptides, and fixed positive charge within the tags results in high ionization efficiency. Moreover, Safirinium NHS esters have been utilized as imaging agents for fluorescent labeling and the microscopic visualization of living cells such as E. coli Top10 bacterial strain.

One‐Step Azolation Strategy for Site‐ and Chemo‐Selective Labeling of Proteins with Mass‐Sensitive Probes

The chemical modification of proteins in a site-selective manner leads to many advances in various scientific fields. The major challenges with conventional N-terminal bioconjugation techniques are the lack of universal sequence compatibility and poor mass-detection sensitivity of the resulting bioconjugates. This approach efficiently analyzes proteolytic fragments and native proteins in a complex mixture. Multiple chemical steps are usually required for the site-selective synthesis of bioconjugates with enhanced mass-detection sensitivity. We present a single-step, versatile strategy for the selective modification of protein N-termini with mass boosters. The chemical tag enhances the peptide detection by multiple orders thus leading to the unambiguous analysis of the resulting bioconjugates. We demonstrate that tagging proteolytic fragments with mass sensitivity probes in a complex mixture improves the detection of resulting bioconjugates.

Trends in the Design of New Isobaric Labeling Reagents for Quantitative Proteomics

Modern mass spectrometry is one of the most frequently used methods of quantitative proteomics, enabling determination of the amount of peptides in a sample. Although mass spectrometry is not inherently a quantitative method due to differences in the ionization efficiency of various analytes, the application of isotope-coded labeling allows relative quantification of proteins and proteins. Over the past decade, a new method for derivatization of tryptic peptides using isobaric labels has been proposed. The labels consist of reporter and balanced groups. They have the same molecular weights and chemical properties, but differ in the distribution of stable heavy isotopes. These tags are designed in such a way that during high energy collision induced dissociation (CID) by tandem mass spectrometry, the isobaric tag is fragmented in the specific linker region, yielding reporter ions with different masses. The mass shifts among the reporter groups are compensated by the balancing groups so that the overall mass is the same for all forms of the reagent. Samples of peptides are labeled with the isobaric mass tags in parallel and combined for analysis. Quantification of individual peptides is achieved by comparing the intensity of reporter ions in the tandem mass (MS/MS) spectra. Isobaric markers have found a wide range of potential applications in proteomics. However, the currently available isobaric labeling reagents have some drawbacks, such as high cost of production, insufficient selectivity of the derivatization, and relatively limited enhancement of sensitivity of the analysis. Therefore, efforts have been devoted to the development of new isobaric markers with increased usability. The search for new isobaric markers is focused on developing a more selective method of introducing a tag into a peptide molecule, increasing the multiplexicity of markers, lowering the cost of synthesis, and increasing the sensitivity of measurement by using ionization tags containing quaternary ammonium salts. Here, the trends in the design of new isobaric labeling reagents for quantitative proteomics isobaric derivatization strategies in proteomics are reviewed, with a particular emphasis on isobaric ionization tags. The presented review focused on different types of isobaric reagents used in quantitative proteomics, their chemistry, and advantages offer by their application.

Quaternary ammonium isobaric tag for a relative and absolute quantification of peptides

Isobaric labeling quantification of peptides has become a method of choice for mass spectrometry-based proteomics studies. However, despite of wide variety of commercially available isobaric tags, none of the currently available methods offers significant improvement of sensitivity of detection during MS experiment. Recently, many strategies were applied to increase the ionization efficiency of peptides involving chemical modifications introducing quaternary ammonium fixed charge. Here, we present a novel quaternary ammonium-based isobaric tag for relative and absolute quantification of peptides (QAS-iTRAQ 2-plex). Upon collisional activation, the new stable benzylic-type cationic reporter ion is liberated from the tag. Deuterium atoms were used to offset the differential masses of a reporter group. We tested the applicability of QAS-iTRAQ 2-plex reagent on a series of model peptides as well as bovine serum albumin tryptic digest. Obtained results suggest usefulness of this isobaric ionization tag for relative and absolute quantification of peptides.

New ionization tags based on the structure of the 5-azoniaspiro[4.4]nonyl tag for a sensitive peptide sequencing by mass spectrometry

Quaternary ammonium salts (QAS), both linear and bicyclic, are often utilized to improve the mass spectrometry (MS) analysis of peptides by fixing a permanent positive charge on the analyzed molecule. However, during collision-induced dissociation (CID) experiments, QAS undergo unwanted side reactions-Hofmann elimination as well as a tertiary amine loss- rendering the data interpretation complicated. In this work, we present 2-thia- and 2-oxa-5-azoniaspiro[4.4]nonyl groups as heterocyclic derivatives of the highly stable ionization group, 5-azoniaspiro[4.4]nonyl, for a sensitive peptide analysis by MS. Due to the permanent positive charge, labeled peptides are characterized by enhanced ionization efficiency during electrospray mass spectrometry (ESI-MS) conditions. Moreover, interpretation of the CID fragmentation of labeled peptides is facilitated since a series of generated fragmentation ions enable a complete sequence coverage. Introduction of a heteroatom into the 5-azoniaspiro[4.4]nonyl scaffold allows for liberation of a stable reporter ion which could be used in selected reaction monitoring (SRM)-targeted quantification experiments. Additionally, we synthesized a deuterated analog of the tag for LC-SRM-targeted quantitative analysis. The obtained results indicate the general usefulness of the proposed heterocyclic quaternary ammonium ionization tag for sequencing and quantification of peptides. Graphical abstract New reagents based on the structure of the 5-azoniaspiro[4.4]nonyl tag for peptide analysis by tandem mass spectrometry.

Podocyturia as an early diagnostic marker of preeclampsia: a literature review

CONTEXT

Preeclampsia (PE) is a pregnancy-related disease, and it is a leading cause of maternal and neonatal morbidity and mortality. It is characterized by the new onset of hypertension after 20 weeks of gestation together with signs of organ damage, most commonly the kidneys. The treatment of PE is symptomatic and final intervention requires delivery, regardless of the gestational age of the foetus. Furthermore, PE is a risk factor for developing cardiovascular disease and chronic kidney disease - even many years after the delivery.

OBJECTIVE

Current research of PE has revealed that detection of podocytes in urine (podocyturia) could be a useful method for both confirmation of PE diagnosis and for the prediction of the severity of the disease.

CONCLUSION

The main aim of this review is to summarize the current state of available methods for podocyte detection and to discuss their relevance in clinical practice.

Peptides Labeled with Pyridinium Salts for Sensitive Detection and Sequencing by Electrospray Tandem Mass Spectrometry

Mass spectrometric analysis of trace amounts of peptides may be problematic due to the insufficient ionization efficiency resulting in limited sensitivity. One of the possible ways to overcome this problem is the application of ionization enhancers. Herein we developed new ionization markers based on 2,4,6-triphenylpyridinium and 2,4,6-trimethylpyridinium salts. Using of inexpensive and commercially available pyrylium salt allows selective derivatization of primary amino groups, especially those sterically unhindered, such as ε-amino group of lysine. The 2,4,6-triphenylpyridinium modified peptides generate in MS/MS experiments an abundant protonated 2,4,6-triphenylpyridinium ion. This fragment is a promising reporter ion for the multiple reactions monitoring (MRM) analysis. In addition, the fixed positive charge of the pyridinium group enhances the ionization efficiency. Other advantages of the proposed ionization enhancers are the simplicity of derivatization of peptides and the possibility of convenient incorporation of isotopic labels into derivatized peptides.

Fabrication of transparent quaternized PVA/silver nanocomposite hydrogel and its evaluation as an antimicrobial patch for wound care systems

Grafting of quaternary nitrogen atoms into the backbone of polymer is an efficient way of developing new generation antimicrobial polymeric wound dressing. In this study, an elastic, non-adhesive and antimicrobial transparent hydrogel based dressing has been designed, which might be helpful for routine observation of wound area without removing the dressing material along with maintaining a sterile environment for a longer period of time. Green synthesized silver nanoparticles have been loaded into the quaternized PVA hydrogel matrix to improve its antimicrobial property. Silver nanoparticles loaded quaternized PVA hydrogel showed enhanced mechanical and swelling properties compared to native quaternized PVA hydrogel. Release kinetics evaluated by atomic absorption spectroscopy revealed that the release mechanism of silver nanoparticles from the hydrogel follows Fickian diffusion. Antimicrobial efficacy of the hydrogels was evaluated by disk diffusion test on Pseudomonas aeruginosa, Staphylococcus aureus and Escherichia coli. After 96 h of release in phosphate buffer, the growth inhibition zone created by silver nanoparticless loaded quaternized PVA hydrogel is comparable to that created by ampicillin. These observations assert that the silver nanoparticles loaded quaternized PVA hydrogel acts as a reservoir of silver nanoparticles, which helps in maintaining a sterile environment for longer time duration by releasing Ag nanocrystallite in sustained manner.

Mass spectrometry for the elucidation of the subtle molecular structure of biodegradable polymers and their degradation products

Contemporary reports by Polish authors on the application of mass spectrometric methods for the elucidation of the subtle molecular structure of biodegradable polymers and their degradation products will be presented. Special emphasis will be given to natural aliphatic (co)polyesters (PHA) and their synthetic analogues, formed through anionic ring-opening polymerization (ROP) of β-substituted β-lactones. Moreover, the application of MS techniques for the evaluation of the structure of biodegradable polymers obtained in ionic and coordination polymerization of cyclic ethers and esters as well as products of step-growth polymerization, in which bifunctional or multifunctional monomers react to form oligomers and eventually long chain polymers, will be discussed. Furthermore, the application of modern MS techniques for the assessment of polymer degradation products, frequently bearing characteristic end groups that can be revealed and differentiated by MS, will be discussed within the context of specific degradation pathways. Finally, recent Polish accomplishments in the area of mass spectrometry will be outlined.

Synthesis and mass spectrometry analysis of quaternary cryptando-peptidic conjugates

The bicyclic amines in the form of cryptands, the crown ether analogs, were used in the synthesis of cryptando-peptidic conjugates with simultaneous formation of quaternary ammonium nitrogen moiety. A series of model cryptando-peptidic conjugates at the peptide N-terminus was efficiently prepared by the standard Fmoc solid phase synthesis. Tandem mass spectrometric analysis of the obtained conjugates has shown the specific fragmentation pattern during MS/MS experiment. The obtained cryptandic quaternary ammonium group undergoes the Hofmann elimination during collision-induced dissociation fragmentation followed by the ethoxyl group elimination. The presented quaternization of cryptands by iodoacetylated peptides is relatively easy and compatible with standard solid-phase peptide synthesis. Additionally, the applicability of such peptide derivatives and their isotopologues selectively deuterated at the α-carbon in the quantitative LC-MS analysis was analyzed.

Convenient preparation of deuterium-labeled analogs of peptides containing N-substituted glycines for a stable isotope dilution LC-MS quantitative analysis

N-substituted glycines constitute mimics of natural amino acids that are of great interest in the peptide-based drug development. Peptoids-oligo(N-substituted glycines) have been recently demonstrated to be highly active peptidomimetics in biological systems, resistant to proteolytic degradation. We developed a method of the deuterium labeling of peptidomimetics containing N-substituted glycine residues via H/D exchange of their α-carbon hydrogen atoms. The labeling was shown to be easy, inexpensive, and without the use of derivatization reagents or the need for a further purification. The deuterons introduced at the α-carbon atoms do not undergo a back exchange under acidic conditions during liquid chromatography mass spectrometry (LC-MS) analysis. The LC-MS analysis of a mixture of isotopologues revealed a co-elution of deuterated and nondeuterated forms of the peptidomimetics, which may be useful in the quantitative isotope dilution analysis of peptoids and other derivatives of N-substituted glycines.

The development of organometallic OBOC peptide libraries and sequencing of N-terminal rhenium(I) tricarbonyl-containing peptides utilizing MALDI tandem mass spectrometry

The development of peptide-based imaging agents through screening of large peptide libraries is hindered by the additional requirement of a radionuclide−chelator complex that can negatively affect the binding properties of the peptide. Herein, we report N-terminal rhenium(I)tricarbonyl OBOC (one-bead, one-compound) peptide libraries for use in the direct screening of potential imaging agents. The rhenium(I) tricarbonyl is incorporated directly in the library as an imaging entity surrogate to account for the presence of a technetium-99m radionuclide chelate. The identification of unknown organometallic peptides on single beads is successfully accomplished through MALDI tandem mass spectrometry, preceded by a systematic investigation of the effects of a variety of N-terminal rhenium(I) tricarbonyl chelates on peptide fragmentation patterns.

Peptides derivatized with bicyclic quaternary ammonium ionization tags. Sequencing via tandem mass spectrometry

Improving the sensitivity of detection and fragmentation of peptides to provide reliable sequencing of peptides is an important goal of mass spectrometric analysis. Peptides derivatized by bicyclic quaternary ammonium ionization tags: 1-azabicyclo[2.2.2]octane (ABCO) or 1,4-diazabicyclo[2.2.2]octane (DABCO), are characterized by an increased detection sensitivity in electrospray ionization mass spectrometry (ESI-MS) and longer retention times on the reverse-phase (RP) chromatography columns. The improvement of the detection limit was observed even for peptides dissolved in 10 mM NaCl. Collision-induced dissociation tandem mass spectrometry of quaternary ammonium salts derivatives of peptides showed dominant a- and b-type ions, allowing facile sequencing of peptides. The bicyclic ionization tags are stable in collision-induced dissociation experiments, and the resulted fragmentation pattern is not significantly influenced by either acidic or basic amino acid residues in the peptide sequence. Obtained results indicate the general usefulness of the bicyclic quaternary ammonium ionization tags for ESI-MS/MS sequencing of peptides.

Hydrogen-deuterium exchange of α-carbon protons and fragmentation pathways in N-methylated glycine and alanine-containing peptides derivatized by quaternary ammonium salts

Recently, we developed a selective and efficient method of hydrogen-deuterium exchange (HDX) at the α-carbon (α-C) of sarcosine residue (N-methylglycine) in model peptides [Bąchor et al. J. Mass Spectrom. 2014, 49, 43]. Here, we report the influence of quaternary ammonium (QA) group on HDX at the α-C of sarcosine and N-methylalanine in peptides. The obtained results suggest a significant acceleration of the HDX in sarcosine residue caused by the presence of QA. The effect depends on the distance between the sarcosine residue and QA moiety. The deuterons, introduced at α-C, are resistant to the back-exchange in acidic aqueous solution. The collision induced dissociation of the deuterium-labeled analogs of QA-tagged oligosarcosine peptides without mobile hydrogen revealed the mobilization of the hydrogens localized at α-C of sarcosine residue.

Gas-phase fragmentation of oligoproline peptide ions lacking easily mobilizable protons

The fragmentation of peptides containing quaternary ammonium group, but lacking easily mobilizable protons, was examined with the aid of deuterium-labeled analogs and quantum-chemical modeling. The fragmentation of oligoproline containing quaternary ammonium group involves the mobilization of hydrogens localized at α- and γ- or δ-carbon atoms in the pyrrolidine ring of proline. The study of the dissociation pattern highlights the unusual proline residue behavior during MS/MS experiments of peptides.

New method of peptide cleavage based on Edman degradation

A straightforward cleavage method for N- acylated peptides based on the phenylthiohydantoin (PTH) formation is presented. The procedure could be applied to acid-stable resins, such as TentaGel HL–NH\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2. We designed a cleavable linker that consists of a lysine residue with the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upalpha $$\end{document}α-amino group blocked by Boc, whereas the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upvarepsilon $$\end{document}ε-amino group is used for peptide synthesis. After the peptide assembly is completed, the protecting groups in peptide side chains are removed using trifluoroacetic acid, thus liberating also the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upalpha $$\end{document}α-amino group of the lysine in the linker. Then the reaction with phenyl isothiocyanate followed by acidolysis causes an efficient peptide release from the resin as a stable PTH derivative. Furthermore, the application of a fixed charge tag in the form of 2-(4-aza-1-azoniabicyclo[2.2.2]octylammonium)acetyl group increases ionization efficiency and reduces the detection limit, allowing ESI-MS/MS sequencing of peptides in the subfemtomolar range. The proposed strategy is compatible with standard conditions during one-bead-one-compound peptide library synthesis. The applicability of the developed strategy in combinatorial chemistry was confirmed using a small training library of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upalpha $$\end{document}α-chymotrypsin substrates.

Sensitive electrospray mass spectrometry analysis of one-bead-one-compound peptide libraries labeled by quaternary ammonium salts

A rapid and straightforward method for high-throughput analysis of single resin beads from one-bead-one-compound combinatorial libraries with high resolution electrospray ionization tandem mass spectrometry (HR ESI-MS/MS) is presented. The application of an efficient method of peptide derivatization by quaternary ammonium salts (QAS) formation increases ionization efficiency and reduces the detection limit, allowing analysis of trace amounts of compounds by ESI-MS. Peptides, synthesized on solid support, contain a new cleavable linker composed of a Peg spacer (9-aza-3,6,12,15-tetraoxa-10-on-heptadecanoic acid), lysine with ɛ-amino group marked by the N,N,N-triethylglycine salt, and methionine, which makes possible the selective cleavage by cyanogen bromide. Even a small portion of peptides derivatized by QAS cleaved from a single resin bead is sufficient for sequencing by HR ESI-MS/MS experiments. The developed strategy was applied to a small training library of α chymotrypsin substrates. The obtained results confirm the applicability of the proposed method in combinatorial chemistry.

The hydrogen-deuterium exchange at α-carbon atom in N,N,N-trialkylglycine residue: ESI-MS studies

Derivatization of peptides as quaternary ammonium salts (QAS) is a known method for sensitive detection by electrospray ionization tandem mass spectrometry. Hydrogens at α-carbon atom in N,N,N-trialkylglycine residue can be easily exchanged by deuterons. The exchange reaction is base-catalyzed and is dramatically slow at lower pH. Introduced deuterons are stable in acidic aqueous solution and are not back-exchanged during LC-MS analysis. Increased ionization efficiency, provided by the fixed positive charge on QAS group, as well as the deuterium labeling, enables the analysis of trace amounts of peptides.

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.