Structures, Derivatives And Bioactivities Of The Proanthocyanidins From Peanut Skin

The skin of the seed of Arachis hypogea or peanut skin (called HongYi in Traditional Chinese Medicine) is rich in A-type proanthocyanidins, in which the subunits are connected by a4â†'8or4â†'6carbon bond and a2â†'Oâ†'7ether bond. The proanthocyanidin structures are complex, and their isolation and purification are usually difficult, leaving lots of research work to do on pure proanthocyanidins. In the light of the above, the urgent need for further studies into its constituent proanthocyanidins and other bioactive phytochemicals cannot be overemphasized. This work was primarily aimed at isolating and chemically characterizing proanthocyanidins and related phytochemical constituents of peanut skin, followed by the evaluation of the extracts, fractions, isolates and the derivatives for possible important biological activities including intestinal a-glucosidase (or disaccharidase) inhibitory and free radical scavenging activities. Because peanut is often consumed cooked, the effect of different cooking methods on the concentration of proanthocyanidin contents of peanut skin was intended to be evaluated with a view to providing a platform for the suggestion of the best way of consuming un-skinned peanuts for therapeutic purposes. The research resulted in the following contributions to knowledge.1. One phenolic acid and ten proanthocyanidins were isolated and chemically characterized. Chemical characterization of known compounds was done largely by analysis of spectroscopic (UV, MS and NMR) data and comparison with literature. The phenolic acid was identified as protocatechuic acid (1). The proanthocyanidins comprised two monomers, catechin (2) and epicatechin (3), four dimers, procyanidin A1(4), procyanidin A2(5), epicatechin-(2βâ†'Oâ†'7,4βâ†'8)-ent-epicatechin (6), and epicatechin-(2βâ†'Oâ†'7,4βâ†'6)-catechin (7), three trimers, cinnamtannin D-1(8), epicatechin-(2βâ†'Oâ†'7,4βâ†'8)-[catechin-(6â†'4β)]-epicatechin (9), and epicatechin-(4βâ†'8)-epicatechin-(2βâ†'Oâ†'7,4βâ†'8)-catechin (10), and a tetramer, epicatechin-(2βâ†'Oâ†'7,4βâ†'8)-epicatechin-(4βâ†'6)-epicatechin-(2βâ†'Oâ†'7,4βâ†'8)- catechin (11). Among these, compound10is reported from peanut skin for the first time, compound9is a novel trimeric A-type proanthocyanidin with new unit-connetivity, and compound11is a novel A-type tetrameric proanthocyanidin.2. Peanut skin proanthocyanidins were separated according to their polimerization degrees by size exclusion chromatography on Sephadex LH20. Retro-Diels-Alder (RDA) and interflavan bond cleavage were found to be typical fragmentation patterns in the MS spectra of oligomeric proanthocyanidins.3. Chemical characterization of polymeric proanthocyanidins, involving the determination of the terminal and extender monomeric units and their linkage orders, was achieved by degradation analysis using L-cysteine which is safer and more environment-friendly than the conventionally used degradation reagent, benzyl mercaptan. We isolated and identified an A-type proanthocyanidin-cystein conjugate (14) for the first time from the cystein degradation product of peanut proanthocyanidin polymers.4. Crude extracts of peanut skin were subjected to size exclusion chromatography on Sephadex LH20eluting with ethanol and methanol in succession to arrive at ethanol (E8) and methanol (M) polymeric fractions, each of which was subjected to L-cysteine degradation and LC-MS analysis structural investigation. It was found that both of the two polymers contained epicatechin and proanthocyanidin A2as the main extending units, and proanthocyanidinds Al as the dominant terminal unit. The molecular weight of M is larger than E8, and M contains more proanthocyanidin A2as extending unit.5. Most fractions, isolates and the A-type proanthocyanidin derivatives showed impressive antioxidant and α-glucosidase inhibitory activities. Proanthocyanidin A1-acetone conjugate emanating from this work is the first polymeric proanthocyanidin-acetone conjugate to be reported. The proanthocyanidin-acetone conjugate showed strong antioxidant and much enhanced α-glucosidase inhibitory activities. The bioactive proanthocyanidin derivatives have mitigated hydrophylicity and would cross biological membranes more readily. They would thus encounter less pharmacokinetic and bioavailability problems if used as templates in the discovery of new therapeutic agents.6. An UPLC-MS method was established for simultaneous quantification of10major constituents in peanut skin. Using UV and UPLC-MS analysis and DPPH radical scavenging assay to examine cooked peanut with skin, we found that the proanthocyanidin content was correlated with the antioxidant activity. The two a-glucosidase inhibitory compounds (9and10) were found to be in higher level in peanut in sauce, suggesting it is the best form of consumption of cooked un-skinned peanut for maximum therapeutic purposes to the diabetic and obese communities.