Disaccharides defined

The side chains in the latter are flexible disaccharides on account of poor-quality diffraction patterns, their tentative molecular structures are known only from computer modeling.1" On the other hand, well-defined crystal structures are available for gellan and welan, and they can be correlated with the physical properties of the polysaccharides the details are presented here. Their conformation angles are listed in Table VI. 

As these results and Fig. 2 show, three structural components may be defined in lipid A (/) the lipid A backbone consisting of a pyranosidic HexN disaccharide and phosphate groups, (ii) substituents of the backbone phosphate residues (polar head groups), and (iii) fatty acids. Therefore, lipid A of different bacteria may be classified according to the nature of the backbone constituents (GlcpN or GlcpN3N), the type and nature of the polar head groups, and features of the acylation pattern. In a few instances, other backbone substituents have been encountered. These will be described later in conjunction with individual lipid A forms. 

In the case of vancomycin [72], an original study was performed to obtain a well-defined stationary phase structure, since it was reasonably assumed that the antibiotic is randomly linked to the silica by one or both of its amino groups, one belonging to the disaccharide portion (primary), and the other one to the heptapeptide core (secondary). Thus, alternate fluorenylmethyloxycarbonyl (FMOC)-amino-protected derivatives were prepared and immobilized in a packed column, and then vancomycin was recovered by cleavage of the protecting groups. The two defined CSPs obtained, when compared with the CSP produced from native randomly linked vancomycin, showed lower retention and enantioselectivity, also if they still separated the same compounds. Thus, no advantages could be found to choose these phases as an alternative to the native vancomycin CSP. 

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