Interflavanyl Bond Research Articles

Proanthocyanidin Tetramers and Pentamers from Cinnamomum verum Bark and Their Dentin Biomodification Bioactivities.

To enable the further exploration of structure-activity relationships (SARs) of proanthocyanidins (PACs) with dentin biomodification abilities, Cinnamomum verum was selected for scaled-up purification of mixed A-/B-type, medium-size PAC oligomers. Sequential purification by centrifugal partition chromatography (CPC), Sephadex LH-20, and semiprep HPLC chromatography yielded four underivatized tetrameric (5-8) and two pentameric (9-10) PACs. Their unambiguous structural characterization involved extensive spectral and chemical degradation approaches to show that epicatechin units are connected by plant-specific combinations of doubly linked A- and singly linked B-type interflavanyl bonds. The biomechanical properties (via dynamic mechanical analysis) and physicochemical structure (via infrared spectroscopy) were assessed to evaluate the biomodification potency of PAC-treated collagen in a preclinical dentin model. This study revealed that (4→8) versus (4→6) bonds in PAC interflavan linkages have limited influence on biomechanical outcomes of dentin. By exhibiting a 25-fold increase in the complex modulus of treated dentin compared to control, aesculitannin E (5) was found to be the most potent PAC known to date for enhancing the mechanical properties of dentin in this preclinical model.

Analysis of Commercial Proanthocyanidins. Part 6: Sulfitation of Flavan-3-Ols Catechin and Epicatechin, and Procyanidin B-3.

Proanthocyanidins (PACs) are natural plant-derived polymers consisting of flavan-3-ol monomers. Quebracho (Schinopsis lorentzii and balansae) heartwood and mimosa (Acacia mearnsii) bark extracts are the major industrial sources of PACs. These commercial extracts are often sulfited to reduce their viscosity and increase their solubility in water. The chemical process of sulfitation is still poorly understood regarding stereochemical influences during the reaction and during the cleavage of the interflavanyl bond of oligomers. To gain a better understanding of sulfitation, two diastereomeric flavan-3-ol monomers were sulfited under industrial conditions, and procyanidin B-3 (catechin-4α→8-catechin) were sulfited to investigate interflavanyl bond cleavage with sulfitation at C-4. Treatment of diastereomeric flavan-3-ols 2R,3S-catechin and 2R,3R-epicatechin with NaHSO3 at 100 °C in aqueous medium afforded the enantiomeric (1R,2S)- and (1S,2R)-1-(3,4-dihydroxyphenyl)-2-hydroxy-3-(2,4,6-trihydroxyphenyl)propane-1-sulfonic acid, respectively. Utilizing computational NMR PD4 calculations it was determined that the direction of stereoselective nucleophilic attack is controlled by the C-3 configuration of the flavan-3-ols catechin and epicatechin. Sulfitation of the catechin-4α→8-catechin dimer 7 (procyanidin B-3) under the same conditions led to the cleavage of the interflavanyl bond yielding the C-4 sulfonic acid substituted catechin momomer. From the heterocyclic ring coupling constants it was determined that nucleophilic attack occurs from the β-face of the dimer leading to the 2,3-trans-3,4-cis isomer as product.

Analysis of commercial proanthocyanidins. Part 5: A high resolution mass spectrometry investigation of the chemical composition of sulfited wattle (Acacia mearnsii De Wild.) bark extract

Wattle (Acacia mearnsii De Wild., Leguminosae) bark extract is used commercially to tan leather and manufacture adhesives. The extract is treated with sodium hydrogen sulfite (sulfited) to improve its tanning properties. These include reduced viscosity, improved solubility, and better raw skin penetration. High resolution ESIMS allows unambiguous assignment of sulphur-containing monomeric and oligomeric products from sulfitation. It reveals that during sulfitation the constituent flavan-3-ol building blocks are sulfited at both C-2 and C-4 by a sulfite ion. A sulfonic acid moiety is introduced at C-2 to open the pyran ring, and C-4 to cleave the interflavanyl bond and reduce the degree of polymerization, respectively, explaining the improved tanning properties. MS2 fragmentation spectra and comparison with unsulfited extract support the interpretation of peaks and composition of sufited wattle bark extract. It also supports our published work that mimosa bark extract consists of a catechin or gallocatechin starter unit and fisetinidol or robinetinidol extender units.

Lepisanthes alata (Malay cherry) leaves are potent inhibitors of starch hydrolases due to proanthocyanidins with high degree of polymerization

Type-II diabetes has become a health threat globally. One major challenge for diabetes patients is to control their post-prandial hyperglycaemia, which could be achieved through the inhibition of α-amylase and α-glucosidase. Lepisanthes alata (Malay cherry) was discovered as a promising edible plant with high starch hydrolase inhibition activities for corn starch hydrolysis with IC50 of its aqueous extracts at 0.77 ± 0.09 µg/mL (for porcine α-amylase) and 10.83 ± 0.67 µg/mL (for rat intestinal α-glucosidase). Proanthocyanidins were identified to be responsible as active compounds. The proanthocyanidin exhibited high degree (mDP at 27) of polymerization and was composed of (epi)catechins and (epi)gallocatechins linked through B-type 4–8 interflavanyl bonds. Inhibition kinetic study revealed that the proanthocyanidins were a mixed noncompetitive inhibitor against α-amylase but competitive inhibitor against α-glucosidase. Our discovery highlighted the potential of the leaves of L. alata for use in prevention of post prandial hyperglycaemia.

Critical reevaluation of the 4-(dimethylamino)cinnamaldehyde assay: Cranberry proanthocyanidin standard is superior to procyanidin A2 dimer for accurate quantification of proanthocyanidins in cranberry products

The 4-(dimethylamino)cinnamaldehyde (DMAC) assay is currently used to quantify proanthocyanidin (PAC) content in cranberry products. In a multi-operator/multi-day study design, a cranberry proanthocyanidin (c-PAC) standard was compared to procyanidin A2 (ProA2) dimer for accurate quantification of PAC in commercial cranberry juices, lab generated cranberry blends and cranberry powders. The c-PAC standard reflects the structural heterogeneity of cranberry PAC degree of polymerization, hydroxylation pattern and ratios of ‘A-type’ to ‘B-type’ interflavanyl bonds. Use of the c-PAC standard to quantify PAC content in cranberry samples resulted in values that were 3.6 times higher than those determined by ProA2. Overall, there was no effect (P > 0.05) of operator or day on estimation of PAC concentration. The adoption of c-PAC standard should be considered as an improvement over the use of ProA2 for accurate quantification of cranberry PAC. Improved standardization of bioactive PAC components in functional cranberry foods will aid in establishment of dosage guidelines.

Analysis of commercial proanthocyanidins. Part 3: The chemical composition of wattle (Acacia mearnsii) bark extract

Wattle (Acacia mearnsii) bark extract is an important renewable industrial source of natural polymers for leather tanning and adhesive manufacturing. The wattle bark proanthocyanidin oligomers have 5-deoxy extender units that render the interflavanyl bonds resistant to acid catalysed hydrolysis and their composition cannot be determined via conventional thiolysis. We combined established phyto- and synthetic chemistry perspectives with an electrospray mass spectrometry investigation to establish that the flavan-3-ol based oligomers consist of a starter unit which is either catechin or gallocatechin, angularly bonded to fisetinidol or predominantly robinetinidol extender units.

Analysis of commercial proanthocyanidins. Part 2: An electrospray mass spectrometry investigation into the chemical composition of sulfited quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood extract

Proanthocyanidins (PACs) are natural plant-derived polymers used in leather tanning, wood adhesives, water purification, and mud additives for oil drilling. Quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood and mimosa (Acacia mearnsii) bark extracts are the major industrial sources of PACs. These commercial extracts are often sulfited via treatment with sodium hydrogen sulfite to reduce their viscosity and increase their solubility in water. An ESI-MS investigation into the molecular composition of sulfited (cold-water-soluble) quebracho heartwood extract indicates that sulfitation of the PACs occurs via SN2 attack of a sulfite ion at both C-2 and C-4 of the constituent flavan-3-ol monomer extender units. Attack at C-2 leads to the opening of the pyran ring. This releases an additional electron-donating phenolic hydroxy group on the A-ring and renders the extract more nucleophilic and suitable for the manufacturing of adhesives. Attack at C-4 leads to interflavanyl bond fission and decrease of the PAC oligomer chain length. The introduction of sulfonic acid moieties at C-2 or C-4 increases the polarity and water solubility of the hot water soluble (unsulfited) extract and transforms it into a cold-water-soluble extract.

ChemInform Abstract: Towards the Synthesis of Proanthocyanidins: Half a Century of Innovation

AbstractReview: 63 refs.

Analysis of commercial proanthocyanidins. Part 1: The chemical composition of quebracho ( Schinopsis lorentzii and Schinopsis balansae) heartwood extract

Quebracho ( Schinopsis lorentzii and Schinopsis balansae) extract is an important source of natural polymers for leather tanning and adhesive manufacturing. We combined established phyto- and synthetic chemistry perspectives with electrospray mass spectrometry experiments to prove that quebracho proanthocyanidin polymers consist of an homologous series of flavan-3-ol based oligomers. The starter unit is always catechin which is angularly bonded to fisetinidol extender units. By comparison of the MS 2 fragmentation spectra of the oligomer with product ion scans of authentic catechin and robinetinidol samples, we proved that quebracho extract contains no robinetinidol, as is often reported. Quebracho proanthocyanidins have acid resistant interflavanyl bonds, due to the absence of 5-OH groups in fisetinidol, and the aDP cannot be determined via conventional thiolysis and phloroglucinolysis. We used the MS data to estimate a conservative (minimum value) aDP of 3.1.

Towards the Synthesis of Proanthocyanidins: Half a Century of Innovation

Results emanating from the synthesis of proanthocyanidins played a crucial role in defining the constitution, regiochemistry, and absolute configuration of this complex but fascinating group of plant secondary metabolites. The initial efforts, commencing in 1966, were focused on structure elucidation of, especially, the procyanidins, profisetinidins, and prorobinetinidins. However, over the past 12 years the emphasis has shifted to the synthesis of the bioactive procyanidins and some of their derivatives at a scale that would permit assessment of their pharmacological properties. With a few exceptions, the vast majority of these synthetic protocols involve the formation of the interflavanyl bond by acid/Lewis acid activation at C-4 of a flavan-3,4-diol or its equivalent, and subsequent trapping of the incipient C-4 carbocation by the nucleophilic centers of a flavan-3-ol (catechin). This review represents the first comprehensive chronicle depicting the development of the subject of proanthocyanidin synthesis.

Synthesis of Proanthocyanidins. Part 1. The First Oxidative Formation of the Interflavanyl Bond in Procyanidins

A novel and efficient method for the oxidative condensation of tetra-O-methyl-3-oxocatechin 4 with tetra-O-methylcatechin is described. Treatment of a solution of 3 (2 equiv) and 4 (1 equiv) with silver tetrafluoroborate readily affords the phenolic per-O-methyl ethers of 3-oxocatechin(4-8)-catechin 18 and 19. Subsequent metal hydride reduction provides access to procyanidin B-3 analogues with the 3,4-cis diastereomers predominating.

Heterogeneity of the Interflavanyl Bond in Proanthocyanidins from Natural Sources Lacking C‐4 (C‐Ring) Deoxy Flavonoid Nucleophiles.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structure and stereochemistry of triflavanoids containing both ether and carbon–carbon interflavanyl bonds

The first triflavanoids with both C–C and C–O–C interflavanyl bonds, epioritin-(4β→6)-epioritin-(4α→4)-epioritin-4β-ol, epioritin-(4β→3)-epioritin-(4β→6)-epioritin-4β-ol and epioritin-(4β→3)-epioritin-(4β→6)-epimesquitol-4α-ol, were identified in the heartwood of Acacia caffra. The ethereal interflavanyl bond is readily susceptible to reductive cleavage with sodium cyanoborohydride in trifluoroacetic acid/dichloromethane which hence permits the unequivocal assignment of the absolute configuration of constituent flavanyl units.

Oligomeric flavanoids. Part 27. Interflavanyl bond formation in procyanidins under neutral conditions

Dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) and silver tetrafluoroborate (AgBF 4) activate the C 4S bond in the 4-thioethers of flavan-3-ols toward carbon nucleophiles to permit formation of the interflavanyl bond in procyanidins under neutral conditions.

Oligomeric flavanoids. Part 24. Controlled biomimetic synthesis of profisetinidin triflavanoid related phlobatannins

The complex structures of the economically important group of profisetinidin triflavanoid related phlobatannins are synthetically accessable in a controlled biomimetic fashion via the repetitive formation of the interflavanyl bond and pyran ring rearrangement of the chain extender unit under mild alkaline conditions. The repetitive formation of the interflavanyl bond and pyran ring rearrangement of the chain extender unit permit controlled synthetic access to phlobatannins at the ‘trimeric’ level [Display omitted]

ChemInform Abstract: Oligomeric Flavanoids. Part 19. Reductive Cleavage of the Interflavanyl Bond in Proanthocyanidins.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Oligomeric flavanoids. Part 18. Dimeric prorobinetinidins from Robinia pseudacacia

The range of naturally occurring prorobinetinidins is extended by characterization of the novel robinetinidol-leucorobinetinidins 3, 5, 7, and 9, the robinetinidol-dihydrorobinetins 11, 13, and 15, a robinetinidol-robinetin 17, and a robinetinidol-flavone analogue 19. The diversity regarding the oxidation level of the chain terminating moieties suggests that the biflavanoids in Robinia pseudacacia may be interrelated via oxidation/reduction of these units. The conspicuous absence of the effects of dynamic rotational isomerism about the interflavanyl bond in the 1H NMR spectra of some of the derivatives is explained in terms of a preferred conformation of this bond rather than to ‘free rotation’.

Oligomeric flavanoids. Part 19. Reductive cleavage of the interflavanyl bond in proanthocyanidins

The interflavanyl bond in profisetinidins 1, 4 and 6, and methyl ethers 3, 5, 28 and 29 and procyanidins 24 and 26, and their methyl ethers 25 and 27 is readily subject to cleavage with sodium cyanoboranuide in trifluoroacetic acid at 0 °C. This method will contribute significantly to the structure elucidation of the 5-deoxy (A-ring) proanthocyanidins from important commercial sources. Boltzmann-averaged heterocyclic ring coupling constants as determined by a conformational global search routine (GMMX) and NOE difference spectroscopy were used to assign unequivocally the diastereotopic methylene protons in the 1H NMR spectra of flavan-3-ols, a prerequisite for corroboration of the cleavage mechanism.

ChemInform Abstract: Cleavage of the Interflavanyl Bond in 5‐Deoxy (A Ring) Proanthocyanidins.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Cleavage of the interflavanyl bond in 5-deoxy (A ring) proanthocyanidins

The fisetinidol-(4,8) and (4,6)-catechin profisetinidin biflavanoids 1,4 and 5 respectively, are subject to facile cleavage of the interflavanyl linkage with sodium cyanoborohydride in trifluoroacetic acid at 0 °C.