Pyranose rings fragmentation

Early work had demonstrated [12, 13] that modification of either substituent or stereochemistry at positions 1, 2, 3, or 4 of the pyranose ring of the carbohydrate fragment resulted in drastic reduction of antibacterial activity. 

Linkage Analysis from a CID-MS/MS Spectrum Linkage information can be obtained from the cross-ring cleavages around the linkage position. These fragments involve the rupture of two bonds in the pyranose ring. The ions thus... 

Fig. 11 shows the structure of permethylated MDP with the principal fragmentations (dotted lines) Fig. 12 shows the corresponding mass spectrum. The molecular ion peak is at m/e 618 and a sequence peak at m/e 417, accompanied by m/e 389 (loss of CO) and 357 (loss of MeOH). Loss of the entire pyranose ring gives the ion at m/e 342. The complementary C-terminal fragment to m/e 417 is at m/e 203. Peaks at m/e 260 and 228 (260-MeOH) contain the intact pyranose ring and are known to occur in the spectra of permethylated-N-acetyl hexosamines. In these fragments the entire lactyl peptide chain has been eliminated. 

S-hydroxv acid fragment of the pyranose form was rejected because it implies formation of a six-membered chelate ring, which it is not a favorable situation. The signal at yiso = 1.9788 may stem from a bischelate pentacoordinate oxo-Cr(V) species, bound to the carboxylate group and the ring oxygen atom (Scheme 13, complexes III and IV). 

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