Structural high resolution mass spectrometry

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

Both compounds were non-fluorescent. Compound KM-1 was similar to PMs in the absorption maximum (A.max 488 nm) but not in the spectral shape, whereas KM-2 was similar to PMs in the spectral shape but not in the absorption maximum at 515 nm (Fig. 9.10). The chemical structures of KM-1 and KM-2 have been determined by high-resolution mass spectrometry and NMR spectrometry (Fig. 9.11 Stojanovic, 1995). 

Structure determination of luciferin. Once a luciferin is obtained in a sufficient purity, the determination of luciferin structure should be attempted most of the important properties of luciferin are usually already obtained during the course of purification as a necessity. The structural study is considerably more straightforward than the extraction and purification, due to the availability of advanced methods, such as high-resolution mass spectrometry and various NMR techniques. If help or collaboration is needed in structure determination, the attractiveness of a luciferin will make it easy to find a good collaborator. However, the purified luciferin is usually an extremely precious material considering the effort spent in preparing it. To avoid accidental loss of the purified material, the chosen collaborator must have solid knowledge and experience in structure determination a criterion to be considered is that the person has successfully done the structure determination of at least one new natural product. 

The following is a procedure recommended for elucidating the structure of complex organic molecules. It uses a combination of different NMR and other spectroscopic techniques. It assumes that the molecular formula has been deduced from elemental analysis or high-resolution mass spectrometry. Computer-based automated or interactive versions of similar approaches have also been devised for structural elucidation of complex natural products, such as SESAMI (systematic elucidation of structures by using artificial machine intelligence), but there is no substitute for the hard work, experience, and intuition of the chemist. 

Figure 7.39 Structural assignments of a benzophenone derivative based on low- and high-resolution mass spectrometry. After Squirrell [258], From D.C.M. Squirrell, Analyst, 106, 1042-1056 (1981). Reproduced by permission of The Royal Society of Chemistry...

Jacobsen, R.B., Sale, K.L., Ayson, M.J., Novak, P., Hong, J., Lane, P., Wood, N.L., Kruppa, G.H., Young, M.M., and Schoeniger, J.S. (2006) Structure and dynamics of dark-state bovine rhodopsin revealed by chemical cross-linking and high-resolution mass spectrometry. Protein Sci. 15, 1303-1317. 

A systematic investigation of the free amino acids of the Leguminosae led to the isolation of a novel ninhydrin-positive compound from the leaves of Derris elliptica Benth. (Papilionidae) (93). This substance was analyzed as C6H,3N04 (microanalysis and high resolution mass spectrometry) and was shown to be an amino alcohol. The absence of a carbonyl in the 1R, the loss of 31 mass units in the mass spectrum, and a positive periodate cleavage reaction were best embodied into a dihydroxydihydroxymethylpyrrolidine structure. The relative simplicity of the NMR spectra (three peaks in the 13C spectrum four spin-system in the H spectrum) pointed out a symmetrical structure. Inasmuch as the material was optically active ([a]D 56.4, c = 7, H20), meso structures were ruled out, and the 2R, 3R, 4R, 5R relative configuration was retained (93). This structure (53) was further confirmed by an X-ray determination (94). 

The structure of the 3-oxo derivative 65 was determined by high resolution mass spectrometry, which demonstrated that a single oxygen atom had been incorporated into the alkaloid skeleton. Peaks in the mass spectrum at mte 174 and 188 provided evidence that the additional oxygen atom was not in the dihydroindole portion of the molecule, while a peak at mte 138 supported the presence of an oxygenated piperdine ring. This metabolite was also chemically compared with authentic oxodihydrovindoline derivatives previously prepared and provided for comparison by J. P. Kutney. 

The Most Intense Peaks. It is not so easy to extract valuable information dealing with the most intense peaks in mass spectra. In contrast to other physicochemical methods (IR, NMR, UV), registration of an ion peak of an integer m/z value does not provide an unequivocal identification of its structure or even composition. Even accurate mass measurements (high resolution mass spectrometry) defining the elemental composition of an ion does not establish its structure, as it could be formed directly from the M+, with minimal distortion of the authentic structure, or as a result of numerous rearrangement processes. 

Structure elucidation of semiochemicals by modern NMR-techniques (including HPLC/NMR) is often hampered by the very small amounts of available material and problems in the isolation of pure compounds from the complex mixtures they are embedded in. Thus, the combination of gas chromatography and mass spectrometry, GC/MS, is frequently the method of choice. Determination of the molecular mass of the target compound (by chemical ionisation) and its atomic composition (by high resolution mass spectrometry) as well as a careful use of MS-Ubraries (mass spectra of beetle pheromones and their fragmentation pattern have been described [27]) and gas chromatographic retention indices will certainly facihtate the identification procedure. In addition, the combination of gas chromatography with Fourier-transform infrared spec-... 

Deoxyvinblastine (16) was first detected as an accompanying impurity of leurosine (11) 43). Initially, 16 was believed to be isomeric with leurosine (11) and was therefore named isoleurosine. However, high-resolution mass spectrometry proved that isoleurosine (16) corresponded to a molecular composition of C46H5gN40g and, in fact, contains two more hydrogen atoms and one less oxygen atom than 11. The structure of 4 -... 

Complete structure elucidation of individual resin glycoside constituents is now achieved readily by the use of a combination of high-resolution mass spectrometry and NMR spectroscopy. These methods are applicable to the isolated natural products or to their peracetylated and methylated derivatives. 

CjqH,7N on the basis of high-resolution mass spectrometry. The structure has been deduced from its spectra and an X-ray crystallographic analysis of the perchlorate, and confirmed by synthesis 113). 

Compound 165 was reduced with SDMA and the product was hydrolyzed with acid to effect ring contraction, affording 4-deoxy-4-C-[(jR,S)-ethylphosphinyl]-o ,/ -D-ribo- and -L-lyxo-furanoses (166), which were characterized by conversion into the peracetates. After separation by chromatography on silica gel, structures 167-170 were established for these peracetates by 400-MHz, -n.m.r. spectroscopy and high-resolution mass spectrometry the structures of these products, their probable conformations, and the yields from 165 are summarized in Scheme 7. 

Zhu, M., Ma, L., Zhang, H., and Humphreys, W. G. (2007). Detection and structural characterization of glutathione-trapped reactive metabolites using hquid chromatography-high-resolution mass spectrometry and mass defect filtering. Anal. Chem. 79 8333-8341. 

The early structural studies of the peptide alkaloids by chemical methods were difficult and were interpreted in structures which in some cases required later revision (13, 25). However, when high resolution mass spectrometry became available it was possible to elucidate the structures of many of these bases. 

Mass spectrometry remains of limited use for the characterization of dioxetanes however, numerous relatively stable 1,2-dioxetanes have been prepared over the last decade allowing not only for the detection of their parent ions but also allowing for high-resolution mass spectrometry (HRMS) measurements to be taken. See references associated with structures depicted in Figure 1 or Table 1. 

When analyzing any compound obtained as a result of subsequent chemical reactions, chemists always address three major issues identity (Did we synthesize what we intended ), quality (How pure is our compound What side product(s) do we have in our sample ), and quantity (How much did we synthesize What is the yield of the reactions ). In the process of analysis of organic compounds, the focus has always been on obtaining comprehensive information with a variety of analytical methods. The traditional scheme of analysis includes structure identification by NMR ( 11 and 13C) and MS (including high-resolution mass spectrometry), with additional confirmation of structure provided by IR spectroscopy, X-ray, etc. [5]. 

Mass spectrometry (MS) provides the molecular weight and valuable information about the molecular formula, using a very small sample. High-resolution mass spectrometry (HRMS) can provide an accurate molecular formula, even for an impure sample. The mass spectrum also provides structural information that can confirm a structure derived from NMR and IR spectroscopy. 

The structure was confirmed with spectroscopy j1]- NMR, UV, and IR), high-resolution mass spectrometry, and elemental analysis. 

In comparison with NMR, mass spectrometry is more sensitive and, thus, can be used for compounds of lower concentration. While it is easily possible to measure picomoles of compounds, detection limits at the attomole levels can be reached. Mass spectrometry also has the ability to identify compounds through elucidation of their chemical structure by MS/MS and determination of their exact masses. This is true at least for compounds below 500 Da, the limit at which very high-resolution mass spectrometry can unambiguously determine the elemental composition. In 2005, this could only be done by FTICR. Orbitrap appears to be a good alternative, with a more limited mass range but a better signal-to-noise ratio. Furthermore, mass spectrometry allows relative concentration determinations to be made between samples with a dynamic range of about 10000. Absolute quantification is also possible but needs reference compounds to be used. It should be mentioned that if mass spectrometry is an important technique for metabolome analysis, another key tool is specific software to manipulate, summarize and analyse the complex multivariant data obtained. 

Identification and structural characterization of an unknown class of substrates for FAAH. (A) By high-resolution mass spectrometry, the high-accuracy mass measurements of a compound of this class gives an exact mass of 446.3310 that corresponds to a molecular formula of C24H48N04S. (B) By MS/MS analysis, the structure of this compound is assigned as the C24 0 fatty acyl amide of taurine (NAT). Reproduced from Saghatelian A., Trauger S.A., Want E.J., Hawkins E.G., Siuzdak G. and Cravatt B.F., Biochemistry, 43,14332-14339, 2004, with permission. 

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