Relative reactivities in the O-methylation of glucomannans: the influence of stereochemistry at C-2 and the solvent effect
2015 (English)In: Carbohydrate Research, ISSN 0008-6215, E-ISSN 1873-426X, Vol. 402, 172-179 p.Article in journal (Refereed) Published
The main hemicellulose in softwood, glucomannan (GM), structurally resembles cellulose but has quite different physical and chemical properties. In addition to branching and original acetylation, the only other difference between these two beta-1,4-linked glycans is the configuration at C-2 in approximately 80% of the sugar residues. In contrast to glucose, the 2-OH in mannose has an axial orientation. The influence of this stereochemistry on the relative reactivities of glucosyl compared to mannosyl units in methylation reactions are studied in this work. Glucomannan isolated from spruce (SGM) and commercially available konjac glucomannan (KGM) was methylated in DMSO/Li-dimsyl/MeI and water/NaOH/MeI system, respectively. In the early stage of the reaction, the glucose part of the SGM achieved slightly higher DS values than the mannose residues, but the overall relative rate constants were close to 1:1. The order of reactivities in glucose was k(2) > k(3) > k(6) and k(3) > k(2) > k(6) for mannose (in DMSO/Li-dimsyl/MeI). The rate constants did not remain constant, but k(3) decreased when k(2) increased for both epimeric sugars. In water/NaOH/MeI, the methylation of the primary 6-OH was much more pronounced with an order of reactivity of O-6 > O-2 > O-3 for mannose and O-2 > O-6 > O-3 for glucose. The results are discussed with respect to the OH-acidity and the stereoelectronic, sterical, and solvent effects.
Place, publisher, year, edition, pages
2015. Vol. 402, 172-179 p.
Spruce glucomannan, Relative reactivities, Partial methylation, Solvent effect
Biochemistry and Molecular Biology
IdentifiersURN: urn:nbn:se:kth:diva-159350DOI: 10.1016/j.carres.2014.06.005ISI: 000346825800022ScopusID: 2-s2.0-84919691742OAI: oai:DiVA.org:kth-159350DiVA: diva2:785113
QC 201502022015-02-022015-01-292015-02-02Bibliographically approved