A CRITICAL-ASSESSMENT OF A ONE-TUBE PROCEDURE FOR THE LINKAGE ANALYSIS OF POLYSACCHARIDES AS PARTIALLY METHYLATED ALDITOL ACETATES

Methylation analysis data for the exudate gum from Actinidia deliciosa , obtained by a one-tube procedure, were compared with published data. Several discrepancies were apparent, which were subsequently attribut ed to differences in the partially methylated alditol acetate (PMAA) d erivatisations used. These deficiencies were also noted during methyla tion analyses of pectic polysaccharides from potato, xyloglucan from t amarind seed, and amylase-resistant starches from maize and pea. Under estimation of terminal galactosyl and glucosyl groups was traced to de gradation of their partially methylated alditol derivatives in the per chloric acid-catalysed acetylation procedure used as the final stage o f PMAA preparation. In addition, extended acid-catalysed reactions res ulted in low values for terminal arabinosyl and fucosyl groups. Triflu oromethanesulfonic (triflic) acid, adopted in these reactions as a saf er, non-oxidative alternative to perchloric acid, gave the same losses although it catalysed acetylation of non-terminal derivatives success fully. Underestimation of terminal arabinosyl groups was traced to ina dequate extraction of the corresponding PMAAs into dichloromethane. Su bsequently, other highly methylated sugar derivatives, such as those o btained from the alditol end groups of reduced oligosaccharides, prove d highly susceptible to this loss; in addition, their tendency to evap orative loss, particularly during the early stages of their derivatisa tion (i.e., after hydrolysis and reduction rather than after acetylati on), was demonstrated. A method which minimises these losses is descri bed. Long-term instability of samples for gas chromatography was preve nted by washing them with water prior to storage to remove residual ac etic acid.