Peanut skins are a rich source of oligomeric and polymeric procyanidins. The oligomeric fractions are dominated by dimers, trimers, and tetramers. A multistep chromatographic fractionation led to the isolation of four new A-type procyanidins of tri- and tetrameric structures. The structures of the new trimers were defined by NMR, electronic circular dichroism, and MS data as epicatechin-(4β→8,2β→O→7)-epicatechin-(4β→8,2β→O→7)-catechin, peanut procyanidin B (3), and epicatechin-(4β→8,2β→O→7)-epicatechin-(4β→6)-catechin, peanut procyanidin C (4). The new tetramers were defined as epicatechin-(4β→8,2β→O→7)-epicatechin-(4β→6)-epicatechin-(4β→8,2β→O→7)-catechin, peanut procyanidin E (1), and epicatechin-(4β→8,2β→O→7)-epicatechin-(4β→6)-epicatechin-(4β→8,2β→O→7)-epicatechin, peanut procyanidin F (2). In addition, both A-type dimers A1, epicatechin-(4β→8,2β→O→7)-catechin, and A2, epicatechin-(4β→8,2β→O→7)-epicatechin, as well as two known peanut trimers, ent-epicatechin-(4β→6)-epicatechin-(4β→8,2β→O→7)-catechin, peanut procyanidin A (5), and epicatechin-(4β→8)-epicatechin-(4β→8,2β→O→7)-catechin, peanut procyanidin D (6), were also isolated. Dimer A1, the four trimers, and two tetramers were evaluated for anti-inflammatory activity in an in vitro assay, in which LPS-stimulated macrophages were responding with secretion of TNF-α, a pro-inflammatory cytokine. Tetramer F (2) was the most potent, suppressing TNF-α secretion to 82% at 8.7 μM (10 μg/mL), while tetramer E (1) at the same concentrations caused a 4% suppression. The results of the TNF-α secretion inhibition indicate that small structural differences, as in peanut procyanidin tetramers E and F, can be strongly differentiated in biological systems.
A comparative study of amine and silver carboxylate adducts [R1COOAg-2(R2NH2)] (R1 = 1, 7, 11; R2 = 8, 12) as a key intermediate in NPs synthesis is carried out via differential scanning calorimetry, solid-state FT-infrared spectroscopy, 13C CP MAS NMR, powder X-ray diffraction and X-ray photoelectron spectroscopy, and various solution NMR spectroscopies (1H and 13C NMR, pulsed field gradient spin-echo NMR, and ROESY). It is proposed that carboxyl moieties in the presence of amine ligands are bound to silver ions via chelating bidentate type of coordination as opposed to bridging bidentate coordination of pure silver carboxylates resulting from the formation of dimeric units. All complexes are packed as lamellar bilayer structures. Silver carboxylate/amine complexes show one first-order melting transition. The evidence presented in this study shows that phase behavior of monovalent metal carboxylates are controlled, mainly, by head group bonding. In solution, insoluble silver salt is stabilized by amine molecules which exist in dynamic equilibrium. Using (bis)amine-silver carboxylate complex as precursor, silver nanoparticles were fabricated. During high-temperature thermolysis, the (bis)amine-carboxylate adduct decomposes to produce silver nanoparticles of small size. NPs are stabilized by strongly interacting carboxylate and trace amounts of amine derived from the silver precursor interacting with carboxylic acid. A corresponding aliphatic amide obtained from silver precursor at high-temperature reaction conditions is not taking part in the stabilization. Combining NMR techniques with FTIR, it was possible to follow an original stabilization mechanism. Graphical abstractThe synthesis of a series (bis)alkylamine silver(I) carboxylate complexes in nonpolar solvents were carried out and fully characterized both in the solid and solution. Carboxyl moieties in the presence of amine ligands are bound to silver ions via chelating bidentate type of coordination. The complexes form layered structures which thermally decompose forming nanoparticles stabilized only by aliphatic carboxylates.
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