A1 Refereed original research article in a scientific journal
The structural and conformational analyses and antioxidant activities of chebulinic acid and its thrice-hydrolyzed derivative, 2,4-chebuloyl-beta-D-glucopyranoside, isolated from the fruit of Terminalia chebula
Authors: Klika KD, Saleem A, Sinkkonen J, Kahkonen M, Loponen J, Tahtinen P, Pihlaja K
Publisher: ARKAT USA INC
Publication year: 2004
Journal name in source: ARKIVOC
Journal acronym: ARKIVOC
First page : 83
Last page: 105
Number of pages: 23
ISSN: 1551-7004
DOI: https://doi.org/10.3998/ark.5550190.0005.708
Abstract
1,3,6-Tri-O-galloyl-2,4-chebuloyl-beta-D-glucopyranoside ( chebulinic acid, 1) and its novel thrice-hydrolyzed derivative, 2,4-chebuloyl-beta-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 C-1(4) 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 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 (S-5(1), S-1(5), S-1(3), and S-2(O)) 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.
1,3,6-Tri-O-galloyl-2,4-chebuloyl-beta-D-glucopyranoside ( chebulinic acid, 1) and its novel thrice-hydrolyzed derivative, 2,4-chebuloyl-beta-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 C-1(4) 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 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 (S-5(1), S-1(5), S-1(3), and S-2(O)) 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.
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