The structural and conformational analyses and antioxidant activities of chebulinic acid and its thrice-hydrolyzed derivative, 2,4-chebuloyl-β-D-glucopyranoside, isolated from the fruit of Terminalia chebula

Karel D Klika,Jyrki Loponen,Ammar Saleem,Marja Kähkönen,Petri Tähtinen,Jari Sinkkonen,Kalevi Pihlaja,Kenneth Turnbull

doi:10.3998/ark.5550190.0005.708

Abstract

Tri-O-galloyl-2,4-chebuloyl-β-D-glucopyranoside (chebulinic acid, 1) and its novel thrice- hydrolyzed derivative, 2,4-chebuloyl-β-D-glucopyranoside (galloyl-free chebulinic acid, 2), together with ellagic (3) and gallic acids (4), ethyl gallate (5), and luteolin (6), were isolated from the dried fruit of Terminalia chebula by bioactivity-guided fractionation of the extract. The compounds were variously identified on the basis of UV, MS, and NMR data. The latter proved that the glucose unit in 1 adopts a well defined 1 C4 chair conformation with all substituents in axial positions. However, the absolute configurations of the three stereocenters in the chebuloyl group have been reassigned. For 2, a solvent-dependent mixture of both open-chain and cyclic pyranose forms was observed, the latter with a β configuration for the anomeric C-1 sugar carbon being heavily preferred in aqueous solution. In contrast to 1, a dynamic equilibrium consisting of several possible skew conformers ( 5 S1, 1 S5, 1 S3, and 2 SO) was designated for the sugar ring of 2. Antioxidant activities of the isolated compounds were assessed by measuring their ability to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals and to inhibit the autoxidation of methyl linoleate in vitro. Of the isolated compounds, 1 showed the highest radical scavenging activity in the DPPH assay. For the methyl linoleate assay, 2 and 4−6 all exhibited strong antioxidant activities whereas the activities of 1 and 3 were only moderate. The fruit extract itself was highly effective in both tests.

Highlights

The hydrolyzable tannins1 of dicotyledonous plants are useful compounds that find wide application as tanning agents, but they may be important components of traditional medical concoctions.2,3. They incorporate the basic components of a glucose moiety with various numbers of galloyl ester substituents, but despite this basic template the group comprises of a vast array of interesting structures incorporating all possible permutations; for the ellagitannins alone there are some 500-odd representatives that have been structurally characterized
Characterization, and antioxidant activities of T. chebula compounds The extraction and isolation procedures for the compounds 1−6 have been described in detail previously,18 but it was the water partition of a dried aqueous methanol extract of T. chebula fruit originally showing promising antioxidant17and cancer-cell growth inhibition18 properties that furnished, after SEC fractionation and consequent HPLC, chebuloyl-free chebulinic acid [2]
The possible presence of chebulagic acid was noted in the extract by HPLC-ESI-MS, its low concentration in these particular samplings precluded the isolation of sufficient quantities for further investigative work

Summary

Introduction

The hydrolyzable tannins1 of dicotyledonous plants are useful compounds that find wide application as tanning agents, but they may be important components of traditional medical concoctions.2,3 Structurally they incorporate the basic components of a glucose moiety with various numbers of galloyl ester substituents, but despite this basic template the group comprises of a vast array of interesting structures incorporating all possible permutations; for the ellagitannins alone there are some 500-odd representatives that have been structurally characterized.3,4 For these reasons, they are an enticing class of compounds attracting as much attention recently for their bioactive properties2,3 as they did originally for their tanning properties.3 the excitement that this class of compounds is potentially able to generate is tempered by the confusion and structural inconsistencies that cloud some of its members.

Results

Conclusion