NMR spectroscopic analysis of sulfated beta-1,3-xylan and sulfation stereochemistry

A novel sulfated beta-1,3-xylan product was synthesized from algal cell wall microfibril homoxylan by the N,N-dimethylformamide (DMF)-SO3 complex sulfation method. Antithrombin activity appeared in this product was 6.5 times higher than that of standard heparin. From the results of 1H- and 13C-NMR spectroscopic analyses by DQF-COSY and HMQC and an infrared spectroscopic analysis, it was revealed that the ordered structure of beta-1,3-xylan as a triple helix had decayed and the resulting conformational changes had been caused by the sulfation reaction. The sulfated positions on the C-4 hydroxyl groups of the xylose residues were determined from 13C-NMR chemical shifts, and it was found that regioselective sulfation had occurred predominantly with the C-4 secondary hydroxyl groups to produce a mono-substituent. Another type of sulfation of beta-1,4-xylan that showed no regioselectivity is considered to have been due to the different conformation of both xylans chains such as the triple helix in beta-1,3-xylan and the double straight chain like cellulose in beta-1,4-xylan. Therefore, the different type of regioselective sulfation of beta-1,3- and beta-1,4-xylan was caused by the difference in steric hindrance due to these conformations. These different types of regioselective sulfation with different linkage positions are also discussed for the secondary hydroxyl groups in beta-1,3- and beta-1,4-glucan after chemoselective sulfation of the C-6 primary hydroxyl groups.