Richard W. Hemingway

Molecular weight profiles of proanthocyanidin polymers

Abstract The MW profiles of proanthocyanidin polymers (condensed tannins) from 32 samples representing a wide range of plant tissues of many different species have been obtained by gel permeation chromatography of the peracetate derivatives. The tannins vary widely in MW, with Mn values for the peracetates in the range 1600–5500. The MW profiles vary greatly from those with narrow, rather smooth distributions, to those which are discontinuous.

Flavonoid biocides: wood preservatives based on condensed tannins

Summary The condensed tannins are natural wood preservatives found in high concentrations in the bark and wood of some tree species. Condensed tannin-containing bark extracts from loblolly pine (Pinus taeda) were evaluated as wood preservatives using standard methods. Bark extracts by themselves did not cause any reduction in weight loss of pressure-treated wood blocks at the retentions tested. However, they do have efficacy as wood preservatives when complexed with copper (II) ions. The best experimental wood preservative formulation was a dual treatment using a sulphited bark extract first, followed by a Cu~ treatment. At some retentions, this method yielded wood blocks with greater resistance to decay by Corio/us versicolor than pentachlorophenol. A single stage treatment of extract plus copper using an aqueous ammoniacal solvent was also successful but not as effective as the dual treatment.

Kinetics of acid-catalyzed cleavage of procyanidins

Abstract Comparisons of the rates of cleavage of isomeric procyanidin dimers in the presence of excess phenylmethane thiol and acetic acid showed that compounds with a C(4)–C(8) interflavanoid bond were cleaved more rapidly than their C(4)–C(6) linked isomers, that 2,3-cis isomers with an axial flavan substituent were cleaved more rapidly than a 2,3-trans isomer with an equatorial substituent, and that cleavage rate was independent of the stereochemistry in the terminal unit. Cleavage rate constants followed Arrhenius temperature dependence. The C(4)–C(8) interflavanoid bond in 2,3-cls procyanidins should be labile to cleavage at 20°C over a pH range of 3.6 to 5.4. Reaction of loblolly pine bark tannins in the presence of excess (+)-catechin with acetic acid rapidly produced oligomeric procyanidins. The ratio of the procyanidin Bl to B7 decreased from 2.9:1 after 4 hours to 1.3:1 after 50–100 hours at 90°C. The structure and reactions of condensed tannins are interpreted in light of these results.

Heterogeneity of interflavanoid bond Location in loblolly pine bark procyanidins

Abstract Procyanidins B-1, B-3 and B-7 were obtained from Pinus taeda phloem in yields of 0.076, 0.021 and 0.034% of unextracted dry wt. Procyanidins B-1 and B-7 were produced in relative yields of 2.4:1 by biosynthetically patterned synthesis from catechin and loblolly pine tannins. Partial acid-catalysed thiolytic cleavage of loblolly pine phloem tannins produced (2R,3S,4S)-2,3,-cis-3,4-trans-3,3′,4′,5,7-pentahydroxy-4-phenylthioflavan and both (2R,3R,4R)-2,3-cis-3,4-trans-3,3′,4′,5,7-pentahydroxy-4-[(2R,3S,4S)-2,3-cis-3,4-trans-3,3′,4′,5,7-pentahydroxy-4-phenylthioflavan-8-yl]flavan and (2R,3R,4R)-2,3-cis-3,4-trans-3,3′,4′,5,7-4-[(2R,3S,4S)-2,3-cis-3,4-trans-3,3′,4′,5,7-pentahydroxy-4-phenylthioflavan-6-yl]flavan in ratios of 3:1 demonstrating regio-isomerism of the interflavanoid linkage in the polymeric procyanidins of loblolly pine bark.

Conformation and Complexation of Tannins: NMR Spectra and Molecular Search Modeling of Flavan-3-ols

Studies of flavan‐3‐ols in their biologically significant phenolic form show that both H‐6 and C‐6 resonances are downfield from H‐8 and C‐8. Therefore, assignments for the H atoms of the A‐ring are inverse to those commonly reported. By contrast, in the methyl ether and methyl ether acetate derivatives, both H‐8 and C‐8 are downfield from H‐6 and C‐6 and assignments commonly reported for C‐6 and C‐8 are incorrect. The assignments commonly reported for the peracetate derivatives are correct. In contrast to results observed for dimeric flavans, solvent effects on chemical shifts and coupling constants are small in monomeric flavan derivatives. The small heterocyclic ring J2,3 coupling constants of 2,3‐cis‐flavans can be defined by lineshape analysis of H‐3. These results provide the first evidence for the conformations of the common 2,3‐cis‐flavans in solution. With the exception of compounds carrying a bulky acetate at C‐3, a GMMX global search protocol provides reasonable predictions of heterocyclic ring coupling constants.

Linkage isomerism in trimeric and polymeric 2,3-cis-procyanidins

Four procyanidin trimers have been isolated and their structures unequivocally established as: epicatechin-(4β→8)-epicatechin-(4β→8)-catechin (5); epicatechin-(4β→6)-epicatechin-(4β→8)-catechin (6); epicatechin-(4β→8)-epicatechin-(4β→6)-catechin (7); and epicatechin-(4β-8)-epicatechin-(4β→8)-epicatechin (13). The presence of the first three trimers in Pinus taeda phloem, and the isolation of both epicatechin-(4β→8)-4β-benzylthioepicatechin (9) and epicatechin-(4β→6)-4β-benzylthioepicatechin (10) from partial acid-catalyzed cleavage of the tannins from Pinus palustris phloem and Photinia glabrescens leaves, shows that linkage isomerism exists in natural oligomeric and polymeric procyanidins. A new system of nomenclature for proanthocyanidins is proposed.

Structural Variations in Proanthocyanidins and Their Derivatives

Structural complexity in the condensed tannins (polymeric proanthocyanidins) is centered principally on variations in hydroxylation patterns of the flavan chain extender units, the stereochemistry at the three chiral centers of the heterocyclic ring, the location and type of interflavanoid bond, and the structure of the terminal unit. Superimposed on these basic structural features are derivatizations such as O-methylation, C- and O-glycosylation, and O-galloylation. Because of the strong nucleophilicity of the resorcinolic or phloroglucinolic A-rings, some unusual conjugates linked through the C-6 or C-8 positions have also been found. A further level of structural diversity results from facile rearrangements of these compounds. Although great progress has been made in elucidation of the structure of oligomeric proanthocyanidins, our knowledge of the structure of higher polymers rests primarily on interpretations of their 13C-NMR spectra.

Size exclusion chromatographic analysis of polyphenol-serum albumin complexes.

Formation of water-soluble polyphenol-protein complexes was investigated by size-exclusion chromatography (SEC). The combination of (-)-epigallocatechin gallate (EGCG) and bovine serum albumin (BSA), which did not form a precipitate after the solutions were mixed, showed an SEC peak due to complex formation 2-24 h after mixing. Peak size of the complex varied with time, suggesting slow change of the conformation of the protein accompanied by complexation. Formation of the complex was substantiated by ultrafiltration of the mixture; the complex did not pass through a membrane with a 100,000 nominal molecular weight limit (NMWL). The SEC profile varied with the combination of compounds. The peaks due to the complexes showed that the apparent value of the number average molecular weight (M(n)) of the EGCG-BSA complex was 2.8x10(5), while that of a pentagalloylglucose (PGG)-BSA complex was 9.5x10(5) under the conditions used. Dimeric hydrolyzable tannins, oenothein B and cornusiin A, also caused changes in the SEC profile of BSA, although the combinations did not show peaks attributable to formation of such large complexes observed for EGCG and PGG. Procyanidin B3 and (+)-catechin did not cause changes in the SEC profile of BSA. With cytochrome c, EGCG did not show any chromatographic changes.

Oligomeric flavanoids. Part W. base-catalyzed pyran rearrangements of procyanidin B-2, and evidence for the oxidative transformation of B- to A-type procyanidins☆

Abstract Procyanidin B-2 3 is subject to facile C-ring isomerizations in 0.1M NaHCO3 solution to form a novel series of 3,4,9, 10-tetrahydro-2H, 8-pyrano[2,3-h]chromenes 7 , 9 , and 10 . The low percentage conversion of B- to A-type procyanidin 2 is rationalized in terms of an initial oxidative removal of hydride ion at C-2 (C-ring).

Condensed tannins: synthesis of the first ‘branched’ procyanidin trimer

Synthesis of the branched procyanidin trimer, epicatechin-(4β→8)-catechin-(6→4β)-epicatechin has been accomplished and in higher yield than liner analogues, suggesting that procyanidin polymers may be highly branched.