Interpretation Automatique des Spectres de Masse de Composes Organiques. Mise au Point

Computerized methods for interpretation of mass spectra are increasing and many papers have now been published. Three different approaches can be used to identify an unknown compound. These methods are reviewed and discussed. First, the heuristic technique which tries to simulate the reasoning of the chemist and deduce the formulae. The only information needed are the mass spectra, the empirical formulae and the n.m.r. spectra. Some promising results have been obtained but the method has important restrictions. Another approach represents the mass spectra as points in a hyperspace. By developing decision surfaces it is possible to classify an unknown compound. This is called pattern recognition and the different empirical methods for recognizing patterns in mass spectral data are explained and reported. The last and simplest technique is to match an unknown spectrum against a library of standard references. When the unknown spectrum is in the library, this approach gives the best results.     

Étude D'une Série de Phénoxazines et D'azaphénoxazines par Spectrométrie de Masse

Mass spectra of substituted phenoxazines and azaphenoxazines have been determined and are discussed here. These compounds are characterised by hydrogen on the heterocyclic nitrogen and aromatic rings with nitro groups and chlorine as substituents. The fragmentation patterns are explained by known mechanisms. An interpretation of almost all the peaks is offered. The presence of the substituents causes a complete change in the fragmentation patterns compared with the unsubstituted phenoxazine. This is due to changes in charge localisation, caused by the substituents. Isomerisation of the nitro group to the nitroso group gives rise to a particular fragmentation route.     

An algorithm for interpretation of low-energy collision-induced dissociation product ion spectra for de novo sequencing of peptides

An algorithm for interpretation of product ion spectra of peptides generated from ion trap mass spectrometry is developed for de novo amino acid sequencing of peptides for the purpose of protein identification. It is based on a multi-pass analysis of product ion data using a rigorous data extraction and sequence interpretation protocol in the initial pass. The extraction/interpretation algorithm becomes more relaxed in subsequent passes, considering more of the fragment ions, and potentially more sequence candidates. The possible peptide sequences generated by the algorithm are scored according to those sequences which best explain the fragment ion spectrum. These sequences are searched against a protein database using a BLAST search engine to find likely protein candidates. The method is also suitable for locating and determining protein modifications, and can be applied to de novo interpretation of peptide fragment ions in the tandem mass (MS/MS) spectrum produced from a mixture of two peptides having similar nominal mass, but different sequences. Using a known protein, bovine serum albumin, as an example, it is illustrated that this method is rapid and efficient for MS/MS spectral interpretation. This method combined with BLAST programs is then applied to search homologies and to generate information on post-translational modifications of an unknown protein isolated from shark cartilage that does not have a complete genome or proteome database.     

Constantes de couplage de vinylallènes

In order to define the influence of an allenic type conjugation, the NMR spectra of vinylallene and of some of its methyl derivatives are analysed The signs of the coupling constants are also determined. These compounds are characterised by a planar s-trans structure of the two noncumulated double bonds. A discussion of the parameters, and especially of the methyl replacement effects, evidences the importance of the π type contributions. The theoretical calculations of the coupling constants by the methods of finite perturbation CNDO and INDO corroborate this interpretation.     

Quantum chemical electron impact mass spectrum prediction for de novo structure elucidation: Assessment against experimental reference data and comparison to competitive fragmentation modeling

We investigate the success of the quantum chemical electron impact mass spectrum (QCEIMS) method in predicting the electron impact mass spectra of a diverse test set of 61 small molecules selected to be representative of common fragmentations and reactions in electron impact mass spectra. Comparison with experimental spectra is performed using the standard matching algorithms, and the relative ranking position of the actual molecule matching the spectra within the NIST‐11 library is examined. We find that the correct spectrum is ranked in the top two matches from structural isomers in more than 50% of the cases. QCEIMS, thus, reproduces the distribution of peaks sufficiently well to identify the compounds, with the RMSD and mean absolute difference between appropriately normalized predicted and experimental spectra being at most 9% and 3% respectively, even though the most intense peaks are often qualitatively poorly reproduced. We also compare the QCEIMS method to competitive fragmentation modeling for electron ionization, a training‐based mass spectrum prediction method, and remarkably we find the QCEIMS performs equivalently or better. We conclude that QCEIMS will be very useful for those who wish to identify new compounds which are not well represented in the mass spectral databases.     

Etude de la réactivité de la fonction carbonyle avec le cétène en présence d'un alcoxyde de titane. 2e communication. A propos de l'interprétation de quelques spectres RMN de β-hydroxyesters formés

The NMR. spectra of a serie of β-hydroxyesters have been studied. It has been found that the methylene protons are magnetically nonequivalent only when the substituents on the center of asymmetry of I or II are very different. The magnetically non-equivalence of the isopropylmethylprotons arise when the β-hydroxyesters contain an aromatic or aromatic conjugated group directly bonded to the asymmetric carbon. The interpretation of this finding is proposed.     

Fourier transform Raman spectroscopy of 1,3-dithiole-2-thione and related compounds

The Raman spectra of 1,3-dithiole-2-thione and a series of related compounds were recorded on a fourier transform Raman spectrometer using near-infrared laser excitation. The spectra proved, with limited exception, to be of high quality. With the help of measured depolarisation ratios it has proved possible to make assignments for the stronger peaks in the spectrum of 1,3-dithiole-2-thione, thereby resolving ambiguities and disagreements in the results of previous workers. These assignments have been used to aid in the interpretation of the very characteristic spectra of substituted thiones. The value of these spectra for analytical purposes is discussed.     

Computational mass spectrometry for metabolomics: Identification of metabolites and small molecules

The identification of compounds from mass spectrometry (MS) data is still seen as a major bottleneck in the interpretation of MS data. This is particularly the case for the identification of small compounds such as metabolites, where until recently little progress has been made. Here we review the available approaches to annotation and identification of chemical compounds based on electrospray ionization (ESI-MS) data. The methods are not limited to metabolomics applications, but are applicable to any small compounds amenable to MS analysis. Starting with the definition of identification, we focus on the analysis of tandem mass and MS(n) spectra, which can provide a wealth of structural information. Searching in libraries of reference spectra provides the most reliable source of identification, especially if measured on comparable instruments. We review several choices for the distance functions. The identification without reference spectra is even more challenging, because it requires approaches to interpret tandem mass spectra with regard to the molecular structure. Both commercial and free tools are capable of mining general-purpose compound libraries, and identifying candidate compounds. The holy grail of computational mass spectrometry is the de novo deduction of structure hypotheses for compounds, where method development has only started thus far. In a case study, we apply several of the available methods to the three compounds, kaempferol, reserpine, and verapamil, and investigate whether this results in reliable identifications.     

Etude des fragmentations en spectrographie de masse de dithiolannes-1,3 substitués en 2 par des groupements insaturés

The mass spectra of the title compounds always show an intense molecular ion. Some fissions involve loss of the ethylenic group, while others are related to cleavage of the dithiolane ring, eventually followed by recyclizations involving the ethylenic chain, due to which resonance structures can be written. If a phenyl group is present in the ethylenic chain, the spectra are modified due to the formation of the very stable benzothiopyrylium ion.     

Software algorithm for automatic interpretation of mass spectra of glycerolipids

A new software algorithm for automatic interpretation of mass spectra of glycerolipids has been developed. The algorithm utilizes a user-specified list of parameters needed to process the spectra. The compounds in mass spectra are identified according to range of measured m/z values, after which the spectra are automatically corrected by the content of naturally occurring isotopes and ion intensities of identified compounds by response correction factors. Automatic processing of the spectra was shown to be accurate and reliable by testing with numerous spectra of glycerophospholipids obtained by liquid chromatography/electrospray ionization mass spectrometry and by comparing the results with manual interpretation of the spectra. If quantitative analysis using internal standards is performed, all the identified compounds in the sample are quantified automatically. A dilution factor may be defined for each sample and is applied to correct the alterations in sample concentration during sample preparation. Processing of several replicate spectra simultaneously produces mean results with standard deviations. The software may also be used to subtract the results of two analyses and to calculate the mean result of replicate subtractions. The algorithm was shown to save time and labor in repetitive processing of mass spectra of similar type. It may be applied to processing of spectra obtained by various mass spectrometric methods.     

Analyse des spectres de vibration infrarouge et Raman de benzothiazolines et de 2H-chromènes, éléments constitutifs de spiropyrannes benzothiazoliniques. Aspect synthétique

Study of infrared and Raman spectra of benzothiazolines and 2H-chromenes, constitutive parts of benzothiazolinic spiropyrans. Synthetic aspect. We have synthesized some benzothiazolines and some substituted 2 H-chromenes as models of the two halves of benzothiazolinic spiropyrans with the aim of interpretation of infrared and Raman spectra of spiropyranic structure. While doing that, we have been able to improve the synthesis method of chromenes obtained from coumarins and we have prepared new nitro-chromenes. By studying IR. and Raman spectra of the two halves of spiropyranic molecule, we could propose a rather good attribution of fondamental modes of vibration. This attribution will used as base for the interpretation of IR. and Raman spectra of a series a benzothiazolinic spiropyrans.     

Composés de solvatation des halogenures de mercure II avec quelques amines aliphatiques

Addition compounds of anhydrous mercury(II) halides were prepared with propylamine, isopropylamine, butylamine and isobutylamine. These solyates have the general formula HgX₂, 2RNH₂. A study of their thermal stability by TG and DTA methods has shown that some stable intermediates are produced in the course of decomposition. The standard enthalpies of formation have been determined by dissolution in acidic medium and the variation of enthalpy based on the evolution of the molecules of amine was determined by this way and by means of a differential enthalpic analyser. The interpretation of their infrared spectra is discussed.     

Comparing similar spectra: From similarity index to spectral contrast angle

We investigated a spectral-contrast-angle (theta) method to determine whether mass spectra of structural isomers are the same or significantly different. This method represents collisionally activated dissociation (CAD) spectra as vectors in space. Mass spectra of different isomers are represented as different vectors, having characteristic lengths and direction. The derived spectral contrast angle, which is a measure of the angle between two vectors corresponding to two closely related spectra, is a measure of whether the mass spectra are the same or significantly different. We compare this method with the similarity index (SI) method and show that the spectral contrast angle method is superior and can differentiate between very similar spectra in cases where the SI cannot. Both methods can be implemented simply in situations where the analyst is called on to decide, on the basis of mass or product-ion spectra, whether reference and unknown compounds are the same or to evaluate the reproducibility of spectra comprised of many peaks.     

Etude de la fragmentation de semi-carbazones terpeniques

Terpene semicarbazones have been investigated using high resolution mass spectrometry. We have studied correlations between rings and functional semicarbazone degradations. The mass spectra are more complicated than the mass spectra of oximes, methyloximes and hydrazones. Cleavages are explained by known mechanisms. An interpretation of almost all the peaks is offered. The fragmentation of the semicarbazone skeleton is less marked than in the oximes. Fragmentation processes induced by the functional group are numerous. Ions obtained are concerned in secondary cleavages. Thus it is not surprising that the predominant feature is localisation of charge upon the imino nitrogen atom. Other centres may, however, be responsible for the appearance of several peaks.     

Spectres de masse de composes alleniques—I : Transposition de type McLafferty de groupes heteroatomiques et de phenyle sur le carbone allenique central

Mass spectra of allenic compounds substituted with a hydrocarbon chain bearing a heteroatomic group (hydroxy, alcoxy, halogen, dialkylamino) or a phenyl in the γ position exhibits a strong peak corresponding to the loss of C₂H₄ (28a.m.u.) from the molecular ion. This is commonly the base peak of the spectra and due to a McLafferty type transfer of the heteroatom or phenyl group to the central allenic carbon atom. The methyl group shows a lower migratory aptitude in such a process. This type of fragmentation involving the migration of heteroatomic groups is not observed in the spectra of γ-halogen and γ-hydroxyketones, alkenes, alkynes or arenes and seems to be characteristic of the allenic linkage. A nucleophilic attack by heteroatom group (or phenyl) on the central allenic carbon atom is proposed.     

Hierarchical clustering of infrared spectra

The generation of a hierarchical tree of 500 infrared spectra, using the recently proposed fractal or 3-distances-clustering method is described and discussed. The objects of clustering are infrared spectra of polymer compounds which are represented as sets of 80 complex Fourier coefficients, obtained by fast Fourier transformation of digitized absorbance spectra. The generated hierarchical tree, with a maximum height of 20 and an average height of 12 levels, yields a very satisfactory clustering scheme with respect to the structure of the compounds involved. In addition to very good clustering, a 100% retrieval (prediction) ability was obtained. This was achieved by the use of an iterative procedure after the initial tree had been generated. Additionally, the tree was tested with 240 infrared spectra of different compounds which were taken into account during the generation of the tree. The retrieval success of these test runs is discussed with respect to the structural similarity of the compounds to which the “unknown” spectra were linked.     

Interpretation Automatique du Spectre de Masse de Complexes Metalliques π Incluant des Derives du Silacyclopentadiene

The programme described previously for the interpretation of mass spectra of heterocyclic derivatives including one group IV element has been extended to -n metallic complexes. Their structure may be determined in few seconds by this method.     

Etude des fragmentations en spectrographie de masse de dithiannes-1,3 substitués en 2 par des groupements insaturés

The mass spectra of the title compounds always show an intense molecular ion. Some fissions involve loss of the ethylenic group, while others are related to cleavage of the dithiane ring, eventually followed by recyclizations involving the ethylenic chain stabilized by the resonance possibilities. If a phenyl group is present in the ethylenic chain, the spectra are modified by the formation of the very stable benzothiopyrylium ion.     

Evaluation of charge derivatization of a proteolytic protein digest for improved mass spectrometric analysis: de novo sequencing by matrix-assisted laser desorption/ionization post-source decay mass spectrometry.

A simple mass spectrometric method to sequence a recombinant phosphoenolpyruvate carboxykinase of known structure and a novel variant of unknown structure isolated from Anaerobiospirillum succiniciproducens and Actinobacillus succinogenes 130Z, respectively, was evaluated. The proteolytic digests of the proteins were each chemically derivatized at the N-terminus by addition of a tris(trimethoxyphenyl)phosphoniumacetyl (TMPP(+)-Ac) group to produce peptides with a fixed positive charge. The derivatized digests were then partially separated by reversed-phase high-performance liquid chromatography. The fractions collected were subjected to matrix-assisted laser desorption/ionization post-source decay (MALDI/PSD) mass spectrometric analysis. The resulting spectra are sufficiently simple to allow the sequence to be read directly without extensive interpretation. This is in contrast to spectra of underivatized peptides obtained by MALDI/PSD or conventional tandem mass spectrometry, where full sequence interpretation can be challenging. Aided with a set of very simple established rules, it was shown that the sequence of TMPP(+)-Ac derivatives can be derived strictly from predictable fragment ion series. In most cases, this is sufficient to determine extensive, unambiguous, peptide sequences de novo. The partial sequence (35%) of the unknown phosphoenolpyruvate carboxykinase from Actinobacillus succinogenes 130Z was obtained entirely by the mass spectrometric method evaluated here, which provided the basis for evaluating homology and for the design of oligonucleotide probes for cloning the corresponding gene.     

Rearrangement thermique des adduits du tetracyanoethylene de 2H-pyrannes

The addition of tetracyanotehylene to 2H-pyrans 1 results in a mixture of two isomeric adducts 6. The thermal decomposition of the compounds 6 gives rise to both a small percentage of the retro Diels-Alder, and a new mode of evolution leading to a mixture of furans (E+Z) 7. A detailed interpretation of the ¹³C spectra of the compounds 7 was established by comparison with the ¹³C spectra of 2,5-diphenylfuran and trans-stilbene. The structure 7 is confirmed by chemical degradation.