Lawrence J. Porter

The conversion of procyanidins and prodelphinidins to cyanidin and delphinidin

The hydrolysis of proanthocyanidins to anthocyanidins in n-BuOH-HCl (95:5) has been shown to be an autoxidation, the yield of anthocyanidin being critically dependent on trace metal-ion impurities. Reproducible yields of anthocyanidin may be achieved if iron (III) salts are added to the reaction medium, and a standard method of analysis of proanthocyanidins based on use of an n-BuOH-HCl-FeIII mixture is given. The ratio of absorbance maxima of the cyanidin (550 nm) produced to that near 280 nm for the original procyanidin polymer solution was ∼ 3.5.

Polymeric proanthocyanidins. Stereochemistry, structural units, and molecular weight

Homogeneous polymeric proanthocyanidins have been isolated from 22 plant sources and all are based on a C(4)–C(8)[or C(6)] linked polyflavan-3-ol structure. 13C N.m.r, spectroscopy in 2[H6]acetone–water is used to calculate the ratio of procyanidin to prodelphinidin monomer units, the average heterocyclic ring stereochemistry of the monomers, and the ratio of monomers to chain-terminating units. The majority of polymers isolated in this study possess monomers with predominantly a 2,3-cis stereochemistry [the same configuration as (–)-epicatechin]. The number-average molecular weight, calculated from monomer to terminal unit ratios, of the polymers is 1 500–5 000. The structure of the chain-terminating group is established by thiolysis degradation and g.l.c. analysis of the products.

The phytochemistry of proanthocyanidin polymers

Abstract The structures of 38 proanthocyanidin polymers (condensed tannins) from 14 widely distributed families of plants are described. The polymers have been isolated from a wide variety of tissues including fruit (ripe and unripe), leaves, bark and phloem. They are all based on a common 4-8 (or 6) linked polyflavan-3-ol structure, analogous to B-type proanthocyanidin dimers.

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.

Haemanalysis: The relative astringency of proanthocyanidin polymers

Abstract Effects of MW, stereochemistry of monomer units, and B-ring oxidation pattern on relative astringency were studied. Efficiency of protein precipitation is primarily a function of proanthocyanidin polymer (condensed tannin) size and whereas oligomeric proanthocyanidins have a relative astringency less than tannic acid, polymeric proanthocyanidins of sufficiently high average MW ( M n ∼2500) are equally as efficient at precipitating haemoglobin.

Compositional changes in lower molecular weight flavans during grape maturation

Abstract Grapes of the Gamay Beaujolais and Siebel varieties have been sampled during the period from just before the onset of colouration to harvest. Extraction has yielded data on the composition and quantities of lower molecular weight and polymeric flavanoids present. The structure of the major flavans of these grapes has been established and the variation in their relative amounts monitored during the growth season. The basic structural unit of the grape proanthocyanidin polymer is a monomer with (−)-epicatechin stereochemistry.

Plant proanthocyanidins. Part 3. Conformational and configurational studies of natural procyanidins

Means to generate the C-4 carbocations (3) and (4) corresponding to (+)-catechin (1) and (–)-epicatechin (2), respectively, are outlined, and the use of these intermediates for the synthesis of model procyanidins and for the biogenetically patterned synthesis of natural procyanidins is discussed. 13C N.m.r. data for model flavan systems and natural procyanidins are reported and analysed and the information is used to assign the 4R-configuration to four natural procyanidin dimers. The phenomenon of conformational isomerism is demonstrated for the natural procyanidin dimers, and two different forms of restricted rotation about the interflavan bond are proposed. The information is used to clarify many earlier structural anomalies, to predict preferred conformations, and to specify a C(4)–C(8) link for the four principal dimers (B-1–4). The properties of some procyanidin polymers are noted, and structures of opposite helicity are proposed for two of the major types found in nature.

Proanthocyanidins of barley and sorghum; composition as a function of maturity of barley ears

Abstract Sorghum vulgare seeds contain a proanthocyanidin polymer consisting largely of 2,3- cis procyanidin units with M n , 2500. Hordeum vulgare ears contain low levels of proanthocyanidin oligomers containing 2–4 units, and composed largely of 2,3- trans procyanidin and prodelphinidin units with catechin as the terminal unit. The concentration of the oligomers in barley ears was virtually constant throughout the 33 day growth and ripening period.

Proanthocyanidin polymers of fodder legumes

Abstract The structure of the condensed tannins of the most common fodder legumes is described. The number- ( M n ) and weight-average ( M w ) MW of the polymers have been determined and most legume tannins have an approximately normal distribution of MWs with M n values of 2000–4000.

Plant proanthocyanidins. Part 6. Chiroptical studies. Part 95. Circular dichroism of procyanidins

Circular dichroism spectra of 26 procyanidins and their derivatives have been measured. All exhibit a strong positive or negative couplet at 200–220 nm and this has been correlated with the absolute stereochemistry at C-4 on the interflavan linkage.