Location: Food Quality LaboratoryTitle: Two new antioxidant malonated caffeoylquinic acid isomers in fruits of wild eggplant relatives Author
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2011
Publication Date: 7/29/2011
Citation: Ma, C., Dastmalchi, K., Whitaker, B.D., Kennelly, E. 2011. Two new antioxidant malonated caffeoylquinic acid isomers in fruits of wild eggplant relatives. Journal of Agricultural and Food Chemistry. 59:9645-9651. Interpretive Summary: Fruit of eggplant and its wild relatives are rich in antioxidant chemicals that are potentially beneficial to human health. In most eggplant lines, these so-called nutraceuticals are made up of a small number of known compounds, but in a few wild relatives there is an abundance of rare and/or unknown compounds that may have novel beneficial biological activity. Two such previously unidentified antioxidant compounds were isolated from fruit of two wild eggplant relatives and their structures were elucidated. This will enable future testing of these natural chemicals as chemopreventive agents that block or delay the advent of chronic diseases. The long-range goal of this work is to develop new commercial eggplant lines bearing fruit with a broader array of nutraceuticals that combat various cancers and cardiovascular disease.
Technical Abstract: Fruits of both the cultivated eggplant species Solanum melongena and its wild relative Solanum incanum have a high content of hydroxycinnamic acid conjugates, which along with other phenylpropanoids are implicated in the human health benefits of various fruits and vegetables. Monocaffeoylquinic acid esters, and in particular 5-O-(E)-caffeoylquinic acid, are usually predominant in solanaceous fruits and tubers. Two closely related caffeoylquinic acid derivatives with longer C18-HPLC retention times than those of monocaffeoylquinic acids are minor constituents in cultivated eggplant fruit. In a prior study, these two compounds were tentatively identified as 3-O-acetyl- and 4-O-acetyl-5-O-(E)-caffeoylquinic acids, and together composed less than 2% of the total hydroxycinnamic acid conjugates in fruit of most S. melongena accessions. Recently we found that these two caffeoylquinic acid derivatives can compose up to 25% of the total hydroxycinnamic acid conjugates in fruit of S. incanum and 17% in fruit of wild S. melongena. This enabled C18-HPLC isolation of sufficient quantities for structural elucidation using 1H- and 13C-NMR techniques and HR-ToF-MS. Isomeric compounds 1 and 2, respectively, were identified as 3-O-malonyl-5-O-(E)-caffeoylquinic acid (1ß,4ß-dihydroxy-3ß-carboxyacetoxy-5a-[(E)-3-(3,4-dihydroxyphenyl)acryloyloxy]cyclohexanecarboxylic acid ), and 4-O-(E)-caffeoyl-5-O-malonylquinic acid (1ß,3ß-dihydroxy-5a-carboxyacetoxy-4ß-[(E)-3-(3,4-dihydroxyphenyl)acryloyloxy]cyclohexanecarboxylic acid). Both 1 and 2 exhibited free radical scavenging activity, albeit about fourfold lower than that of the flavonol quercetin. By contrast, the iron chelation activities of 1 and 2 were about three- and six-fold greater, respectively, than that of quercetin. Whereas malonation of glycosyl moieties on anthocyanins and other phenylpropanoids is common, reports of malonylhydroxycinnamoylquinic acids are rare, and only a few of these compounds have been structurally elucidated using both NMR and MS techniques. To our knowledge, the malonylcaffeoylquinic acid isomers 1 and 2 have not previously been reported.