N-Acetyl- and Glycolylneuraminic Acid

13C-NMR spectroscopy is another frequently used technique for structure analysis of biomolecules, however, requiring at least 10-100 times more substance. 

HC-NMR spectrum of Neu5Ac is shown in Fig. 13. This proton noise-decoupled spectrum shows 11 intense singlets. Assignment of the resonances was made by comparison with spectral data of carbon atoms in similar chemical environments of model compounds and by various proton decoupling techniques (Bhattacharjee et al. 1975). The relatively low intensities for the carbonyl carbons and for C2 are due to the restricted nuclear Overhauser enhancement as a result of the absence of proton substituents at these carbon atoms. In addition, the spectrum shows several small resonances (see Table 11) belonging to the aNeuSAc anomer which is present for 9% in the equilibrium mixture (Jaques et al 1977). 

It has to be noted that the spectrum of Fig. 13 has been recorded with TMS as an external standard. Addition of an internal standard, e.g. the sodium salt of 2,2,3,3-tetradeutero-3-(trimethyl)propionic acid as applied by Eschenfelder / al (1975) in a study of a series of monomeric aNeu5Ac and pNeuSAc derivatives (glycosides and esters), can seriously affect several of the carbon resonances if the 

Furthermore, significant chemical shift differences between a- and p-anomers are observed for C4 and C6 as a result of 1,3-diaxial interactions between the protons in these positions and the anomeric centre (Bhattacharjee et al. 1975). The resonance positions of C2, C3, C5, C7, and C9 show only minor differences between a- and p anomers, whereas Cl and C8 occupy intermediate positions in this respect. The i C resonances for pNeuSAc and pNeuSGc in aqueous solutions are compiled in Table 11. Hydroxylation of the CH3 group of the N-acetyl substituent results in two alterations in the i C-NMR spectrum a strong downfield shift for the resonance of the involved carbon and a small downfield shift for the neighbouring carbonyl carbon. 

The effect of 0-acetylation at various positions of the sialic acid skeleton has been studied for aqueous solutions (Bhattacharjee et al. 1975). The main effects induced by 0-acetylation at C9 of Neu5Ac are the downfield shift of 3.1 ppm for C9 and the upfield shift of -3.1 ppm for C8. In methanol as solvent the directions and magnitudes of these shifts are similar (Haverkamp et al. 1975) for pNeu4,5,9Ac3l,2Me2 O-acetylation at C4 causes a downfield shift of 2.9ppm for C4 and upfield shifts of -3.3 and -3.4ppm for C3 and C5, respectively. These 

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