Studies on O-acetyl-glucomannans from Amorphophallus species: Comparison of fine structure

The present study mainly elucidated fine structural information of four O-acetyl-glucomannans (AcGMs) from Amorphophallus rivirei (konjac), A. albus, and two local types of A. bulbifer (wild type M and wild type B), which were named as KGM, AGM, MGM, and BGM, respectively. Distributions of sugar units and acetyl groups along the main chain were examined by NMR spectroscopy and MALDI-TOF-MS methods in combination with partial degradations. Results of 2D NMR suggested that the basic building blocks of backbones were almost the same for all fractions regardless of their sources. Mono-acetylation at O-2 and O-3 of mannoses were the dominant substitution forms of the four samples. Data from HMBC and NOESY spectra showed that the partially acetylated mannosyl residues linked to both non-acetylated sugar residues and acetylated mannosyl residues. The high relative percentage of oligosaccharides with lower molecular weights in MALDI-TOF-MS spectra of KGM indicated that it had more M4-M4 disaccharide units in the polymer chain compared with AGM, MGM, and BGM. In addition, KGM had O-2,3 di-acetylated mannosyl residues deduced from the trimers with five acetyl groups. Monosaccharide compositions of the products after partial acid hydrolysis suggested that non-random repeating units consisting of mannose and glucose existed in AGM. However, such regularity of distribution of sugar residues was not found in KGM, MGM, and BGM. Glucoses in these fractions presented at the periphery of polymer chain with different random patterns.