Neu5Ac anomers

H NMR spectroscopic investigations have shown that only the a-anomer of Neu5Ac is consumed by the C. perfringens lyase, yielding a-ManNAc, as described in detail in section 5.3.5. From this observation, the chemical modification experiments, and models drawn in refs. [33,892], a reaction scheme for the heterolytic fragmentation of cyclic sialic acids into pyruvic acid and A-acylmannosamines is delineated (Fig. 20, overleaf). 

Fig. 2. Resolution-enhanced 500 MHz iH-NMR spectrum of Neu5Ac dissolved in 2H2O, recorded at p2H 1.4 and 27 °C. The intensity ratio of the signals from corresponding protons reflects the molar ratio of the a- and p-anomer under the applied measuring conditions to be 7 93. Detailed splitting patterns are indicated for all resonances from the (3-, and for the greater part of those from the a-anomer.

Fig. 13. Proton-noise-decoupled 25 MHz i C-NMR spectrum of Neu5Ac dissolved in 2H2O, recorded at p H 2 and 25 °C. Only the signals stemming from the p-anomer are...

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). 

Fig. 16. A Structure of NeuSAc a- and (3-anomers and the torsion angles between Cl and the H3 atoms (R = H or aglycon). B Carbonyl carbon resonances in the undecoupled 25 MHz i C-NMR spectrum of aNeu5Ac2Me, dissolved in 2H2O, recorded at p2H 7 and 25 °C. C Carbonyl carbon resonances in the undecoupled 25 MHz i C-NMR spectrum of Neu5Ac, dissolved in 2H2O, recorded at p H 7 and 25 °C. Only the signals from the p-...

The nature of the cleavage mechanism has been studied using n.m.r.-spectroscopy leading to the proposal that the a-glycoside is cleaved to yield a-Neu5Ac which then leaves the active site of the sialidase molecule and mutarotates in water to give over 90% of the p-anomer (Holmquist and Ostman 1975, Friebolin et al 1980). 

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