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)-β-d-GlcA-(1→4)-α-l-Rha 3-sulfate-(1→] (symbolised as A3s) and contained disaccharides composed of [→4)-β-d-Xyl-(1→4)-α-l-Rha 3-sulfate-(1→] and [→4)-β-d-Xyl 2-sulfate-(1→4)-α-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 α-l-Rha 3-sulfate by a terminal β-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.