Assessment of Chemical Composition of High Molecular Fraction from Medicinal Plant Onosma sericea Willd. by Fourier Transform Infrared Spectroscopy and Nuclear Magnetic Resonance Technique

Abstract

Background: In our previous studies we investigated water-soluble mucilaginous high molecular fractions (HMF) of medicinal plants Symphytum asperum, S.caucasicum, S.grandiflorum, S.officinale, Anchusa italica, Cynoglossum officinale, Borago officinalis and Paracynoglossum imeretinum (Boraginaceae family). The water extracts of aforenamed plants were fractionated by ultrafiltration on membrane filters with cut values of 1000 kDa or 500 kDa. This fractionation procedure allowed us to remove most polysaccharides and to obtain water-soluble HMFs. The main chemical constituent of HMFs of plants described above was found to be biologically active unique caffeic acid derived biomacromolecule polyObjectives: Within our ongoing search for biologically active biopolymers in plant species belonging to different genera of the Boraginaceae family, the present study aimed to isolate and investigate a water-soluble high-molecular mucilage fraction (Mr>500 kDa) of Onosma sericea roots (HMF-OR) in order to study main chemical constituents of HMF-OR and carry out their structure elucidation. Methods: HMF-OR was isolated from water mucilage extract of O.sericea roots utilizing ultrafiltration with a membrane filter with a cut-off value of 500 kDa, as described in some of earlier publications. Analyses of HMF-OR were carried out by nondestructive physicochemical instrumental methods Fourier Transform Infrared Spectroscopy (FTIR), which is considered one of the most effective methods to identify the functional groups and Nuclear Magnetic Resonance (NMR) techniques which is a powerful tool for structure elucidation of chemical compounds. Results: According to data of FTIR and NMR spectroscopies, one of the main chemical constituents of HMF-OR was found to be novel p-coumaric acid-derived biopolymer, namely poly[oxy-1-carboxy-2-(4-hydroxyphenyl)ethylene], that is poly[3-(4-hydroxyphenyl)glyceric acid] (PHPGA). Besides, the data also reveals in HMF the presence of complex pectin type polysaccharide. Conclusions: PHPGA is a regular polymer with a residue of 3-(4-dihydroxyphenyl)glyceric acid as the repeating unit. The polyoxyethylene (polyethylene glycol) (PEG) chain is the backbone of PHPGA. 4-Hydroxyphenyl and carboxyl groups are regular substituents at two carbon atoms in the chain. The complex pectin type polysaccharide has consisted of a disaccharide repeating unit [→α-D-GalpA-1,2-α-L-Rhap-1,4→] backbone, with side chains contained highly branched α-(1→5)-linked arabinan and short linear β-(1→4)-linked galactan, attached to O-4 of the rhamnosyl residues.