Negative-ion electrospray ionization tandem mass spectrometry of N-phosphoryl amino acids and dipeptides

The negative-ions of N-phosphoryl amino acids were studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS). The negative-ion ESI-MS/MS of N-phosphoryl amino acids showed characteristic fragmentation patterns different from those observed in the corresponding positive-ion ESI-MS/MS and negative-ion fast-atom bombardment mass spectra. For negative-ion ESI-MS/MS, a unique fragmentation from the N-terminal of N-phosphoryl amino acids or peptides containing a free beta-OH or CO(2)H group was observed to yield the characteristic fragment ion (RO)(2)P(O)O(-). The ease of the rearrangement depended on the position of the hydroxyl group in amino acids or peptides, and the N --> O rearrangement mechanism was proposed to involve the participation of the hydroxyl group. From previous solution-phase experiments and theoretical calculations, it was found that the beta-OH group was more active than gamma-OH, and the corresponding difference in negative-ion ESI-MS/MS was consistent with those previous findings.     

Fragmentation of deprotonated cyclic dipeptides by electrospray ionization mass spectrometry

The fragmentation pathways of deprotonated cyclic dipeptides have been studied by electrospray ionization multi‐stage mass spectrometry (ESI‐MSⁿ) in negative mode. The results showed that the fragmentation pathways of deprotonated cyclic dipeptides depended significantly on the different substituents, the side chains of amino acid residues at the diketopiperazine ring. In the spectra of deprotonated cyclic dipeptides, the ion [M? H? substituent radical]^? was firstly observed in the ESI mode. The characteristic fragment ions [M? H? substituent radical]^? and [M? H? (substituent? H)]^? could be used as the symbols of particular cyclic dipeptides. The hydrogen/deuterium (H/D) exchange experiment, the high‐resolution mass spectrometry (Q‐TOF) and theoretical calculations were used to rationalize the proposed fragmentation pathways and to verify the differences between the fragmentation pathways. The relative Gibbs free energies (ΔG) of the product ions and possible fragmentation pathways were estimated using the B3LYP/6–31++G(d, p) model. The results have some potential applications in the structural elucidation and interpretation of the mass spectra of homologous compounds and will enrich the gas‐phase ESI‐MS ion chemistry of cyclic dipeptides. Copyright © 2009 John Wiley & Sons, Ltd.     

Discrimination of cyclic peptide diastereomers by electrospray ionization tandem mass spectrometry

In this research, the characteristic ions' abundance ratio between two isomers A and B in MS/MS mass spectra was defined as a parameter for discriminating diastereomers. Through this ratio, the discrimination of four pairs of cyclic peptide (CP) diastereomers was successfully achieved. Furthermore, in the analysis of diastereomers' mixtures, both calibration curve and calculational methods were substantiated to have high precision and accuracy. The average absolute errors of the two methods were 2.0 and 2.5% in the 48 measurements of 16 samples, respectively. This research provided a promising approach for the analysis of the CP diastereomers in the fields of asymmetrical synthesis, chiral natural products and structural biology by ESI‐MS/MS. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of cyclic structure for polydithiane using electrospray ionization mass spectrometry

Disulfide polymers obtained by ring‐opening polymerization have been considered to have a possibility of a cyclic catenane structure as judged from their test properties on the loss modulus and stress–strain. Electrospray ionization mass spectrometry (ESI‐MS) was used to detemine molecular structure of polydithiane and polyoxyethylene to find whether they are present as a cyclic or as a linear structure. The results indicated that the polydithiane possesses the cyclic structure, and analysis of the isotope distribution of the spectral ions further showed that the polymer consists entirely of the cyclic structure without contamination of a linear polymer. A linear chain polymer with a benzylmercaptan end group was synthesized, and the ESI‐MS analysis revealed that the polymer was a mixture of both the cyclic and the linear polymer. The cyclic polymer is probably formed by back‐biting of the highly reactive sulfur radicals that had been formed during the polymerization reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4403–4406, 2000     

Synchronized dual-polarity electrospray ionization mass spectrometry

This work describes the synchronized dual-polarity (DP) electrospray ionization (ESI) method and demonstrates the first DP ESI mass spectra obtained using two mass spectrometers. Stable double Taylor cones were produced by applying two counter electric voltages with opposite polarities to one electrosprayer. The development of double Taylor cones required higher extraction voltages than conventional ESI, but DP ESI worked effectively at liquid flow rate range three times wider than conventional ESI. Using pure methanol, the emission currents of the two cones were neutralized and no current was drawn from the sprayer. Synchronized DP mass spectra were obtained using electrospray calibrants dissolved in methanol solution of low water content. For bovine insulin with conventional electrospray solution, the gas-assisted electrospray delivered satisfactory sensitivity and stability for routine mass analyses.     

Investigation of the interaction between HIV-1 DNA and cyclic peptides by electrospray ionization mass spectrometry

The interaction between HIV-I DNA and five cyclic peptides (CPI-CP5) was investigated using electrospray ionization mass spectrometry (ESI-MS). It revealed that CPI [c(Ala-Tyr-Leu-Ala-Gly)] and CP4 [c(Pro-D-Tyr-Leu-D-Ala-Gly)] have the higher binding affinity with the duplex DNA among the five cyclic peptides.     

Determination of micelle mass by electrospray ionization mass spectrometry

By electrospray ionization (ESI) mass spectrometry, micelle solutions of sodium cholate were investigated in detail in the presence and absence of ethanol. The average aggregation number could be evaluated from the spectra acquired under conditions where soft collisions adequate to measure the micelle solution were induced, and the value agreed well with that obtained previously by other methods. From the dependence on ethanol content, it was also found that the average aggregation number in aqueous solution without organic solvent could be reliably estimated. The ESI method proved to be a useful tool for determining the micelle mass in the original aqueous phase.     

Infrared laser-assisted desorption electrospray ionization mass spectrometry

We have used an infrared laser for desorption of material and ionization by interaction with electrosprayed solvent. Infrared laser-assisted desorption electrospray ionization (IR LADESI) mass spectrometry was used for the direct analysis of water-containing samples under ambient conditions. An ion trap mass spectrometer was modified to include a pulsed Er:YAG laser at 2.94 microm wavelength coupled into a germanium oxide optical fiber for desorption at atmospheric pressure and a nanoelectrospray source for ionization. Analytes in aqueous solution were placed on a stainless steel target and irradiated with the pulsed IR laser. Material desorbed and ablated from the target was ionized by a continuous stream of charged droplets from the electrosprayed solvent. Peptide and protein samples analyzed using this method yield mass spectra similar to those obtained by conventional electrospray. Blood and urine were analyzed without sample pretreatment to demonstrate the capability of IR LADESI for direct analysis of biological fluids. Pharmaceutical products were also directly analyzed. Finally, the role of water as a matrix in the IR LADESI process is discussed.     

Negative-ion electrospray and fast atom bombardment mass spectrometry of esters of boron acids

It is shown that both electrospray and fast atom bombardment mass spectrometry provide excellent negative-ion mass spectra of the anionic esters of boric, boronic and borinic acids. For electrospray, contact of the esters with water causes some hydrolysis but, in most cases, spectra of the intact molecular anions are readily obtained. For fast atom bombardment, solvents that chelate with the boron esters must be avoided, Tetraethylene glycol diethyl ether, pentaethylene glycol dimethyl ether or hexaethylene glycol dimethyl ether are suitable solvents. Negative-ion electrospray mass spectra showed few, if any, fragment ions, whereas fast atom bombardment generally produced abundant M^? ions and several fragment ions of low abundance. It is shown that a simple reaction with dibenzene-borinic acid converts diols such as monoglycerides and monoalkyl glyceryl ethers into anionic borinate esters as a pre-ionization procedure for analysis by electrospray or fast atom bombardment mass spectrometry.     

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this month's Special Feature. We particularly hope that younger scientists will find value in this month's Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.     

Sequencing cyclic peptide inhibitors of mammalian ribonucleotide reductase by electrospray ionization mass spectrometry.

Mammalian ribonucleotide reductase (mRR) is a potential target for cancer intervention. A series of lactam-bridged cyclic peptide inhibitors (1-9) of mRR have been synthesized and tested in previous work. These inhibitors consist of cyclic and linear regions, causing their mass spectral characterization to be a challenge. We determined the fragmentation mechanism of cyclic peptides 1-9 using an ion-trap mass spectrometer equipped with an ESI source. Low-energy collision-induced dissociation of sodiated cyclic peptides containing linear branches follows a general pathway. Fragmentation of the linear peptide region produced mainly a and b ions. The ring peptide region was more stable and ring opening required higher collision energy, mainly occurring at the amide bond adjacent to the lactam bridge. The sodium ion, which bound to the carbonyl oxygen of the lactam bridge, acted as a fixed charge site and directed a charge-remote, sequence-specific fragmentation of the ring-opened peptide. Amino acid residues were cleaved sequentially from the C-terminus to the N-terminus. Our findings have established a new way to sequence cyclic peptides containing a lactam bridge based on charge-remote fragmentation. This methodology will permit unambiguous identification of high-affinity ligands within cyclic peptide libraries.     

Positive ion electrospray ionization mass spectrometry of oligonucleotides

Positive ion electrospray ionization mass spectra have been obtained of deoxyribonucleic acids (DNA) and ribonucleic acids (RNA), including transfer RNAs (77-mer, ～ 25 kDa). For several different solution conditions, the charge state distributions of DNA and RNA molecules were determined. It is postulated that the production of the multiply charged positive ions results from gas phase dissociation of complexes between nitrogen-containing bases and oligonucleotides.     

Desorption electrospray ionization mass spectrometry of intact bacteria

Desorption electrospray ionization (DESI) mass spectrometry (MS) was used to differentiate seven bacteria species on the basis of their measured DESI-mass spectral profile. Both gram-positive and gram-negative bacteria were tested and included Escherichia coli, Staphyloccocus aureus, Enterococcus sp., Bordetella bronchiseptica, Bacillus thuringiensis, Bacillus subtilis and Salmonella typhimurium. Distinct DESI-mass spectra, in the mass range of 50-500 u, were obtained from whole bacteria in either positive or negative ion modes in less than 2 mins analysis time. Positive ion DESI-mass spectral fingerprints were compared using principal components analysis (PCA) to investigate reproducibility for the intraday and the day-to-day measurements and the method selectivity to differentiate the bacteria studied. Detailed study of variances in the assay revealed that a large contribution to the DESI-mass spectral fingerprint variation was the growth media preparation procedure. Specifically, experiments conducted with the growth media prepared using the same batch yielded highly reproducible DESI-mass spectra, both in intraday and in day-to-day analyses (i.e. one batch of growth media used over a 3-day period versus a new batch every day over the same 3-day period). Conclusions are drawn from our findings in terms of strategies for rapid biodetection with DESI-MS.     

Characterization of linear and cyclic polylactic acids and their solvolysis products by electrospray ionization mass spectrometry

Linear and cyclic polylactic acids (PLAs) were characterized using electrospray ionization mass spectrometry (ESI-MS) as part of our ongoing investigation of the hydrolysis mechanism of biodegradable polymers. The condensation oligomers of linear polylactic acid (LPLA) were synthesized by thermal dehydration of L-lactic acid. The trimer and tetramer base polymers of cyclic polylactic acid (CPLA) were obtained by cyclization reactions of lactic acid trimers and tetramers, respectively. In the ESI-MS/MS measurement, LPLA yielded three types of product ion series, while CPLA yielded only one type, from which the repeated units of CPLA were removed. The MS/MS spectrum of the NH4+ adduct ion for both cyclic and linear PLA showed loss of one ammonia molecule. The postsource decay (PSD) spectrum of CPLA by matrix-assisted laser desorption ionization (MALDI) mass spectrometry was similar to the ESI-MS/MS spectrum, while that of LPLA was different. In addition, the degradation of cyclic and linear PLAs by solvolysis was investigated. Solvolysis with anhydrous MeOH was quite feasible, but did not readily occur in the presence of even a small amount of water in the MeOH solvent.     

Aspartame degradation study using electrospray ionization mass spectrometry

Electrospray mass spectrometry was used to simultaneously determine aspartame (APM) and five of its degradation products; aspartic acid, aspartylphenylalanine, 5-benzyl-3,6-dioxo-2-piperazieacetic acid (diketopiperazine), phenylalanine, and phenylalanine methyl ester. Under the ionization conditions used, there was no interfering fragmentation for any of the six compounds, i.e., no fragmentation of the compound being tested into other species also being monitored. A study of APM degradation in solution at various pH's and at various temperatures using this method was performed.     

Multi-anode detection in electrospray ionization time-of-flight mass spectrometry

An electrospray ionization ion source coupled to a time-of-flight mass analyzer incorporating a multi-anode time-to-digital converter is described. High-speed data acquisition (kHz mass spectral acquisition) rates are achieved. The four-anode detector produces a significant increase in detection/counting efficiency over that for a single-anode detector. In this work a 2.5 times increase in detection efficiency is demonstrated. The multi-anode detector is also used as a diagnostic tool to optimize transmission of the ion optics.     

Some tenets pertaining to electrospray ionization mass spectrometry

The field of electrospray ionization mass spectrometry is reviewed with emphasis placed upon advances in the elucidation of fundamental mechanistic aspects of the ionization process that have been reported over the past 10 years. The analytical consequences of these findings are also examined. Eight central conclusions or 'tenets' are presented, as deduced from the body of work contained in 80 references.