The in-source decay (ISD) phenomenon, the fragmentation at an N–Cα bond of a peptide backbone that occurs within several tens of nanoseconds in the ion-source in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), is discussed from the standpoints of the discovery and early publications dealing with MALDI-ISD, the formation of c-ions in energy-sudden desorption/ionization methods, the formation of radical species in a MALDI, model construction for ISD, and matrix materials that are suitable for use in MALDI-ISD. The formation of c-ions derived from peptides and proteins in MALDI-ISD can be rationalized by a mechanism involving intermolecular hydrogen transfer, denoted as the “Takayama’s model” by De Pauw’s group (Anal. Chem. 79: 8678–8685, 2007). It should be emphasized that the model for MALDI-ISD was constructed on the basis of X-ray crystallography and scanning probe microscopy (SPM) analyses of matrix crystals, as well as the use of isotopically-labelled peptides.
The aim of this paper is to propose a stochastic method for estimating the detection limits (DLs) and quantitation limits (QLs) of compounds registered in a database of a GC/MS system and prove its validity with experiments. The approach described in ISO 11843 Part 7 is adopted here as an estimation means of DL and QL, and the decafluorotriphenylphosphine (DFTPP) tuning and retention time locking are carried out for adjusting the system. Coupled with the data obtained from the system adjustment experiments, the information (noise and signal of chromatograms and calibration curves) stored in the database is used for the stochastic estimation, dispensing with the repetition measurements. Of sixty-six pesticides, the DL values obtained by the ISO method were compared with those from the statistical approach and the correlation between them was observed to be excellent with the correlation coefficient of 0.865. The accuracy of the method proposed was also examined and concluded to be satisfactory as well. The samples used are commercial products of pesticides mixtures and the uncertainty from sample preparation processes is not taken into account.
A workflow based on liquid chromatography/high-resolution mass spectrometry (LC/HR-MS) was applied for the identification of compounds in urban environments. Substances extracted by solid-phase extraction from river water were wholly analyzed by LC/HR-MS without any purification. Fragmentation in collision-induced dissociation was manually studied for the 20 most intense ions in positive- and negative-ion electrospray ionization with accurate mass determination at a resolution of 100,000. Sixteen anthropogenic compounds in the extract were identified and confirmed using standard reference reagents. These compounds consisted of pharmaceuticals, surfactants, flame retardants, and industrial intermediates. The majority of the compounds are common in our daily life. In the identification process, two automated methods, MAGMa and MetFrag/MetFusion, for reading fragmentation were evaluated for the sixteen compounds. Although automated methods could be used to retrieve the correct molecular structures in most cases, they could not always be promoted to the top rank. Automated methods have yet to be a complete solution for identifying chemical compounds, but will considerably reduce the burden for humans in reading fragmentation.
The demand for leather goods has grown globally in recent years. Industry revenue is forecast to reach $91.2 billion by 2018. There is an ongoing labelling problem in the leather items market, in that it is currently impossible to identify the species that a given piece of leather is derived from. To address this issue, we developed a rapid and simple method for the specific identification of leather derived from cattle, horses, pigs, sheep, goats, and deer by analysing peptides produced by the trypsin-digestion of proteins contained in leather goods using liquid chromatography/mass spectrometry. We determined species-specific amino acid sequences by liquid chromatography/tandem mass spectrometry analysis using the Mascot software program and demonstrated that collagen α-1(I), collagen α-2(I), and collagen α-1(III) from the dermal layer of the skin are particularly useful in species identification.
Periodontitis is one of the most prevalent threats to oral health as the most common cause of tooth loss. In order to perform effective treatment, a clinical test that detect sites where disease activity is high and predicts periodontal tissue destruction is strongly desired, however, it is still difficult to prognose the periodontal tissue breakdown on the basis of conventional methods. The aim of this study is to examine the usefulness of gas chromatography/mass spectrometry (GC/MS), which could eventually be used for on-site analysis of metabolites in gingival crevicular fluid (GCF) in order to objectively diagnose periodontitis at a molecular level. GCF samples were collected from two diseased sites (one site with a moderate pocket and another site with a deep pocket) from each patient and from clinically healthy sites of volunteers.Nineteen metabolites were identified using GC/MS. Total ion current chromatograms showed broad differences in metabolite peak patterns between GCF samples obtained from healthy sites, moderate-pocket sites, and deep-pocket sites. The intensity difference of some metabolites was significant at sites with deep pockets compared to healthy sites. Additionally, metabolite intensities at moderate-pocket sites showed an intermediate profile between the severely diseased sites and healthy sites, which suggested that periodontitis progression could be observed with a changing metabolite profile. Principal component analysis confirmed these observations by clearly delineating healthy sites and sites with deep pockets. These results suggest that metabolomic analysis of GCF could be useful for prediction and diagnosis of periodontal disease in a single visit from a patient and provides the groundwork for establishing a new, on-site diagnostic method for periodontitis.
The formation of monomeric and dimeric ions of seven different aminonaphthols (ANLs) has been studied by using laser desorption/ionization (LDI) with a nitrogen laser. The positive-ion data of all the ANLs merely showed molecular ion M·+ without protonated molecule [M+H]+, while 1-amino-2-naphthol (1,2-ANL) and 2-amino-1-naphthol (2,1-ANL) showed an intense dimeric ion [2 M−2H2O+H]+. The negative-ion data showed deprotonated molecule [M−H]− in common, while the spectra of 1,2-ANL, 2,1-ANL and 8-amino-2-naphthol (8,2-ANL) accompanied an intense peak corresponding to negative molecular ion M·− and the 8,2-ANL and 4-amino-1-naphthol (4,1-ANL) accompanied dehydrogenated anion [M−2H]·−. The formation of monomeric ions was discussed from the standpoints of thermochemical properties such as ionization energy, gas-phase acidity, electron affinity, and bond dissociation energy. The formation of dimeric ions [2 M−2H2O+H]+ observed in the 1,2-ANL and 2,1-ANL could be explained by the radical combination in the amino groups. An isomer 5-amino-1-naphthol (1-ANL) did not give any dimeric ions in the both positive- and negative-ion spectra. The influence of laser fluence upon the appearance of the monomeric ions such as M·+, [M+H]+, [M−H]− and [M−2H]·− of the 5,1-ANL has been examined.
We have previously proposed a rapid identification method for bacterial strains based on the profiles of their ribosomal subunit proteins (RSPs), observed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). This method can perform phylogenetic characterization based on the mass of housekeeping RSP biomarkers, ideally calculated from amino acid sequence information registered in public protein databases. With the aim of extending its field of application to medical mycology, this study investigates the actual state of information of RSPs of eukaryotic fungi registered in public protein databases through the characterization of ribosomal protein fractions extracted from genome-sequenced Aspergillus fumigatus strains Af293 and A1163 as a model. In this process, we have found that the public protein databases harbor problems. The RSP names are in confusion, so we have provisionally unified them using the yeast naming system. The most serious problem is that many incorrect sequences are registered in the public protein databases. Surprisingly, more than half of the sequences are incorrect, due chiefly to mis-annotation of exon/intron structures. These errors could be corrected by a combination of in silico inspection by sequence homology analysis and MALDI-TOF MS measurements. We were also able to confirm conserved post-translational modifications in eleven RSPs. After these verifications, the masses of 31 expressed RSPs under 20,000 Da could be accurately confirmed. These RSPs have a potential to be useful biomarkers for identifying clinical isolates of A. fumigatus.
The degradation routes of poly(vinyl pyrrolidone) (PVP) exposed to sodium hypochlorite (bleach) have been previously investigated using chemical analyses such as infrared spectroscopy. So far, no reports have proposed mass spectrometry (MS) as an alternative tool despite its capability to provide molecular and structural information using its single stage electrospray (ESI) or matrix assisted laser desorption ionization (MALDI) and multi stage (MSn) configurations, respectively. The present study thus reports on the characterization of PVP after its exposure to bleach by high resolution MALDI spiralTOF-MS and Kendrick mass defect analysis providing clues as to the formation of a vinyl pyrrolidone/vinyl succinimide copolymeric degradation product. A thorough investigation of the fragmentation pathways of PVP adducted with sodium and proton allows one main route to be described—namely the release of the pyrrolidone pendant group in a charge remote and charge driven mechanism, respectively. Extrapolating this fragmentation pathway, the oxidation of vinyl pyrrolidone into vinyl succinimide hypothesized from the single stage MS is validated by the detection of an alternative succinimide neutral loss in lieu of the pyrrolidone release in the ESI-MSn spectra of the aged PVP sample. It constitutes an example of application of multi-stage mass spectrometry for the characterization of the degradation of polymeric samples at a molecular level.
De novo sequencing is still essential in the identification of peptides and proteins from unexplored organisms whose sequence information is not available. One of the remaining problems in de novo sequencing is discrimination between Leu and Ile residues. The discrimination is possible based on differences in side chain fragmentation between Leu and Ile under high-energy collision-induced dissociation (HE-CID) conditions. However, this is observed only when basic residues, such as Arg and Lys, are present near the N- or C-terminal end. It has been shown that the charge derivatization at the N-terminal end by a quarternary ammonium or phosphonium moiety facilitates the side chain fragmentation by HE-CID. However, the effective backbone fragmentation by low-energy CID (LE-CID) is often hampered in those derivatives with a fixed charge. Previously, we demonstrated that the N-terminal charge derivatization with the structures having high proton affinity induced the preferential formation of b-ions under LE-CID conditions, allowing straightforward interpretation of product ion spectra. In the present study, we further investigated whether the same derivatization approach is also effective for discrimination between Leu and Ile under HE-CID conditions. Consequently, the side chain fragmentation of Leu and Ile residues was most effectively enhanced by the N-terminal derivatization with 4-(guanidinomethyl)benzoic acid among the tested structures. This derivatization approach, which is compatible with both HE- and LE-CID analysis, offers a straightforward and unambiguous de novo peptide sequencing method.
To improve the durability of organic materials in electronic devices, an analytical method that can obtain information about the molecular structure directly from specific areas on a device is desired. For this purpose, laser desorption/ionization mass spectrometry imaging (LDI-MSI) is one of the most promising methods. The high spatial resolution stigmatic LDI-MSI with MULTUM-IMG2 in the direct analysis of organic light-emitting diodes was shown to obtain a detailed mass image of organic material in the degraded area after air exposure. The mass image was observed to have a noticeably improved spatial resolution over typical X-ray photoelectron spectroscopy, generally used technique in analysis of electronic devices. A prospective m/z was successfully deduced from the high spatial resolution MSI data. Additionally, mass resolution and accuracy using a spiral-orbit TOF mass spectrometer, SpiralTOF, were also investigated. The monoisotopic mass for the main component, N,N′-di-1-naphthalenyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (m/z 588), was measured with a mass resolution of approximately 80,000 and a mass error of about 5 mDa using an external calibrant. This high mass resolution and accuracy data successfully deduced a possible elemental composition of partially remained material in the degraded area, C36H24, which was determined as anthracene, 9-[1,1′-biphenyl]-4-yl-10-(2-naphthalenyl) by combining structural information with high-energy CID data. The high spatial resolution of 1 μm in LDI-MSI along with high mass resolution and accuracy could be useful in obtaining molecular structure information directly from specific areas on a device, and is expected to contribute to the evolution of electrical device durability.
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