Sugars, Neutral

Macrolides containing only neutral sugars were obtained from a platenomycin-producing organism (217,218). Four demycarosyl derivatives of platenomycins were isolated from biosyntheticaHy blocked mutants of S. platensis, two of these possessed a methyl group rather than an aldehyde (219). A pair of novel compounds related to carbomycin were isolated in which one contained an unusual 10,ll-dihydro-12,13-diol moiety, the other a 14-hydroxy-epoxyenone moiety (220). 

Hydrolysis of both neutral sugars from tylosin yields S-O-mycaminosyltylonolide (OMT) (51, R = CHO, R = R" = OH) (238). However, this compound is more conveniendy obtained by hydrolysis of mycarose from the fermentation-derived demycinosyltylosin. Analogous hydrolyses of mycarose from DMOT, GS-77-3, and GS-77-1 yield, respectively, 23-deoxy-5-0-mycaminosyltylonohde (DOMT), 20-deoxo-5-O-mycaminosyltylonohde (GS-77-4), and 5-0-mycaminosyltylactone (GS-77-2), all given in Table 9 (246,250,280). 

A few neutral 16-membered macroHdes have been isolated. The stmcture of chalcomycin (54, R = CH3, R = OH), produced by S. bikiniensis was estabhshed as a 16-membered lactone having two neutral sugars, D-chalcose (7) and D-mycinose (9, R = R = OH3) (293—295). Its aglycone resembles that... 

Other macrohdes have been prepared which represent hybrids of stmctures within the 14-membered family, within the 16-membered family, or between the two families. These hybrids have been made by chemical, bioconversion, or genetic manipulations. The 9-0-[(2-methoxyethoxy)methyl]oxime of tylosin (Table 7) was synthesized, using the oxime found in roxithromycin (33) (369). 3-0-Cladinosyl derivatives of 16-membered macrohdes were synthesized in which the neutral sugar of erythromycin (Table 3) was attached to tylosin derivatives at thek 3-hydroxyl group, analogous to its position in erythromycin... 

Figure 2 - Size-exclusion chromatography on Sephacryl S200 of MHR population A after degradation with RGase ------------, uronic acids —, neutral sugars [39].

Pectins is a general term for a group of natural polymers based on polymerized galacturonic acid partly esterified with methanol. In addition these polymers must be considered as copolymers due to existence of neutral sugar branched zones. [1]. Some uronic acid units may also be esterified on 0-2 or 0-3 position with acetic acid. The pectins occur in the cell wall of higher plants and control at least partly the mechanical properties, the ion exchange properties and the swelling of the cell walls. 

This paper will not described the chemical structure of pectins which is a difficult problem [1] even if the physical properties in solution and ability to form gel must be directly related with the distribution of the units along the chain. The functional properties of pectins are not only related to the neutral sugar content (up to 15 %) but also to the distribution of structural blocks having very different contibutions. 

The HM and LM pectins give two very different types of gels the mechanisms of stabilization of the junction zones in the two cases are described and few characteristics given. The different molecular characteristics (DE, distribution of methoxyl or acetyl substituents, neutral sugar content or rhamnose content) play an important role on the kinetic of gelation, mechanical properties of the gel formed and also on the experimental conditions to form the stronger gels. All these points were briefly discussed. 

Fractions were pooled as indicated in the figure and analyzed for their sugar compositions (Table 3). The sugar compositions clearly show that most of the neutral sugars were concentrated in the large fragments that eluted close to the void volume of the column (Fraction... 

Table II. Carbohydrate compositions (weight percentage) of individual oligomer peaks purified (QAE-Sephadex or HPLC ion-exchange separation, respectively) from mixtures of citrus pectin oligomers or B fruit extracts Compositions shown are for peaks whose biological activity is described in Figure 4. Uronic acid values are based on colorimetric assay. Proportions of neutral sugars were determined by GC and adjusted so that totals equal 100%. In fact, some oligomers (G7 peaks 8, 9 and 10. B extract peak 10) produced small (less than 1 % of the total integrated area), unknown peaks in the GC chromatograms.

Matsuhashi S, Inoue S-1, Hatanaka C (1992) Simultaneous measurement of the galacturonate and neutral sugar contents of pectic substances by an enzymic-HPLC method. Biosci Biotech Biochem 56 1053-1057... 

However, the aqueous extract has contents in galacturonic acids and in neutral sugar slightly lower and higher than the acid extract, respectively. 

Figure 2 Ion-exchange chromatography on DEAE-Sepharose CL-6B (elution by acetate buffer pH 4.8) of (a) dialysed water-soluble pectins from extruded citrus fibres (SME = 250 kWh/t) and (b) dialysed acid extracted pectins from the same raw material, (empty symbols neutral sugars full symbols= galacturonic acids)...

One peak is eluted at the void volume of the column and contained the bulk of neutral sugars and few galacturonic acid residues while the peak at the total volume contained almost pure oligogalacturonides. Therefore the same model of "hairy" regions and "smooth" regions also applied to these pectins. 

---