Positive Spectral Interpretation

The spectrum should be interpreted by (a) seeing which absorption bands are absent - negative spectral interpretation - and (b) examining those bands present - positive spectral interpretation. 

The technique of negative interpretation should, of course, be used in conjunction with the positive approach. It is important to be aware that correlation tables give the positions and intensities of bands characteristic of a large 

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For the spectral interpretation, the oligosaccharides are conceived as extensions of 25 and 26, with a Gal residue /3-(l— 4)-linked to GlcNAc-5 or -5, respectively. As already described, this attachment almost exclusively influences the chemical shift of H-l of the penultimate GlcNAc residue (A8 0.027 p.p.m.). The resonance positions of the H-l signals of the terminal Gal residues are in complete accord with those of the diantennary, asialo oligosaccharide 7. 

The H-l and H-2 signals of Man-4 are found at 8 4.87 and —4.16, respectively these positions are in accord with those of the corresponding protons in 61—63, also possessing a Man-4 residue disubsti-tuted by A and B. Thus, for the spectral interpretation, the lower-branch part of 67 can be conceived of as an extension of 61-63 with two a-(l— 2)-linked Man groups, D2 and D3. As is usual for terminal,... 

In individuals with a deficiency in methylacetoacetyl CoA thiolase (MACT), the conversion of 2-methylacetoacetyl-CoA into acetyl-CoA and propionyl-CoA is inhibited and an accumulation of abnormal catabolic products is observed in the urine. Williams etal. have successfully used both ID and 2D H NMR spectroscopy to investigate the urinary metabolites of two unrelated patients with this disorder. The urine spectra from both patients clearly showed the presence of both 2-methyl-3-hydroxybutyrate and tiglylglycine, which is characteristic for MACT deficiency due to the build-up of metabolites close to the position of enzyme deficiency. However, at 360 MHz, the H NMR spectra are quite crowded and difficult to interpret fully. This difficulty was overcome by the use of the 2D JRES H NMR experiment, which resolved all the proton-proton couplings into the second dimension, allowing much clearer spectral interpretation. In addition to the spin-echo and 2D H J-resolved NMR spectra, 2D H COSY NMR spectroscopy was used to determine the spin-spin coupling connectivities between the protons in the various urinary components. 

IRMPD offers a number of advantages over SORI-CID. The principal advantage is that the introduction of gas to the ICR cell is obviated. There is no requirement for a pump-down delay, hence the speed of analysis is greater and the method is compatible with on-line separation techniques [61]. Unlike SORI-CID, blind-spots in the MS/MS spectrum do not occur because there is no resonant excitation of the product ions. All product ions are formed on-axis so there is no loss of resolution and it is possible to undertake further stages of MS/MS. A consequence of the on-axis position is the potential for secondary fragmentation, that is, the product ions can be photon activated also. Fragmentation of product ions can complicate spectral interpretation. 

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