Cell-wall polysaccharides from the marine green alga Ulva "rigida" (Ulvales, Chlorophyta)--NMR analysis of ulvan oligosaccharides

Glucuronic acid directly linked to galacturonic acid in the rhamnogalacturonan backbone of beet pectins

Sugar-beet pulp was de-esterified and submitted to 72 h hydrolysis by 0.1 M HCl at 80 degrees C. Oligomers containing a single glucuronic acid (GlcA) moiety in addition to n(>/= 2) repeats of the dimer -->4)-alpha-D-GalpA-(1-->2)-alpha-L-Rhap-(1--> were isolated from the hydrolysate by ion-exchange and gel-permeation. Glycosyl linkage composition analysis and 1H NMR studies indicated that the GlcA was attached to O-3 of a galacturonic acid (GalA) residue, as shown for the two pentamers beta-D-GlcpA-(1-->3)-alpha-D-GalpA-(1-->2)-alpha-L-Rhap-(1-->4)-alpha-D-GalpA-(1-->2)-L-Rhap and alpha-D-GalpA-(1-->2)-alpha-L-Rhap-(1-->4)-[beta-D-GlcpA-(1-->3)]-alpha-D-GalpA-(1-->2)-L-Rhap. Substitution by GlcA was estimated as occurring on one GalA residue out of 72 in the rhamnogalacturonan fraction of the backbone of beet pectins.

NMR spectroscopic characterisation of oligosaccharides from two Ulva rigida ulvan samples (Ulvales, Chlorophyta) degraded by a lyase

The chemical structure and the sequence of repeating units in ulvans of similar compositions from two different Ulva rigida samples collected in the Canary Islands and in Brittany were studied after ulvan-lyase degradation and NMR spectroscopic analysis of the reaction products. Both ulvans were composed of ulvanobiuronic acid 3-sulfate type A [-->4)-beta-D-GlcA-(1-->4)-alpha-L-Rha 3-sulfate-(1-->] (symbolised as A3s) and contained disaccharides composed of [-->4)-beta-D-Xyl-(1-->4)-alpha-L-Rha 3-sulfate-(1-->] and [-->4)-beta-D-Xyl 2-sulfate-(1-->4)-alpha-L-Rha 3-sulfate], respectively referred to as ulvanobiose 3-sulfate (U3s) and ulvanobiose 2',3-disulfate (U2's,3s). In the Canary Islands sample, these U3s and U2's,3s occurred dispersed among A3s sequences and as short blocks of two or three units. In contrast, in the Brittany samples, these units were dispersed among A3s structures and next to A3s units branched at O-2 of alpha-L-Rha 3-sulfate by a terminal beta-D-GlcA and symbolised as A2g,3s. However, more complex structures are likely to occur in the enzyme resistant fraction remaining from this ulvan. An average structure sequence of these two ulvans was proposed. The transposition of the 13C NMR data of the new identified structures to the parent polysaccharides was not possible, probably due to the different sequence distributions affecting the carbons chemical shifts.

Fine chemical structure analysis of oligosaccharides produced by an ulvan-lyase degradation of the water-soluble cell-wall polysaccharides from Ulva sp, (Ulvales, Chlorophyta)

A marine bacterium degrading the water-soluble cell wall polysaccharides from Ulva sp. (ulvan) has been isolated. The good correlation between ulvan degradation monitored by reducing-power, UV absorbance and viscosimetry, indicated that the crude enzymatic extract contains essentially an endo-ulvan lyase activity. This activity was rapidly inhibited by the reaction products which consisted of a series of ulvanobiouronic acid A 3-sulfate [-->4)-beta-D-GlcpA-(1-->4)-alpha-L-Rhap 3-sulfate-(1-->]n with 4-deoxy-L-threo-hex-4-enopyranosiduronic acid at the non-reducing end. Other deviant repeating structures with beta-D-Xylp or alpha-IdopA replacing beta-D-GlcpA in the repeating ulvanobiouronic acid disaccharide and the presence of two consecutive (1-->4) linked beta-D-Glc pA demonstrated the great variability and complexity of ulvan chemical structure.