Conformation epicatechin

Fig. 9D.3 Representations of the preferred conformations of procyanidin dimers B2 (epicatechin-4,8-epicatechin) and B6 (epicatechin-4,6-epicatechin) (adapted from de Freitas et al. 1998)...

Polymers of + ) catechin and (-)-epicatechin have an intrinsic fluorescence because chromophores are an integral part of each monomer unit. The time-resolved emission from well-characterized dimers can be used to determine the relative populations of two rotational isomers at the interflavan bond between monomer units. When combined with the solid-state conformations, molecular mechanics calculations, and rotational isomeric state analysis, the interpretation of the time-resolved fluorescence leads to the unperturbed dimensions of the polymers. Significant population of both rotational isomers causes the chains to have unperturbed dimensions comparable with those in atactic polystyrene molecules of the same molecular weight. 

Two independent determinations of the structure of (-)-epicatechin in the crystalline state have been performed (8,18). The reports describe the same basic structure, but that of Fronczek et al. (8) provides higher resolution. The heterocyclic ring is approximately a half-chair with C-2 26.3 pm above and C-3 49.5 pm below the mean plane of the fused ring system (8). The dihydroxyphenyl substituent at C-2 adopts a pseudoequatorial position. Thus far, (+ )-catechin has resisted crystallization into a form amenable to a structure determination. Molecular mechanics calculations that reproduce the observed structure of (-)-epicatechin predict that the heterocyclic ring in (+ )-catechin has a similar conformation (9). 

Bergmann W R, Viswanadhan V N, Mattice W L 1987 Conformations of polymeric proantho-cyanidins composed of (+)-catechin or (-)-epicatechin joined by 4->6 inter flavan bonds. J Chem Soc Perkin Trans 11 45 — 51... 

Fronczek F R, Gannuch G, Mattice W L, Tobiason F L, Broeker J L, Hemingway R W 1984 Dipole moment, solution conformation and solid state structure of (-)-epicatechin, a monomer of procyanidin polymers. J Chem Soc Perkin Trans II 1611-1616... 

Mattice W L, Tobiason F L, Houglum K, Shanafelt A 1982 Conformational analysis and dipole moments of tetra-0-methyl-(+)-catechin and tetra-0-methyl-(-)-epicatechin. J Am Chem Soc 104 3359-3362... 

Viswanadhan V N, Mattice W L (in press) Preferred conformations of the sixteen (4- 6) and (4- 8) linked dimers of (-l-)-catechin and (-)-epicatechin with axial or equatorial dihydrox-yphenyl rings at C(2). J Chem Soc Perkin Trans II... 

Low temperature H NMR studies (111) showed that the E A ratio for catechin (14) and epicatechin (11) were 62 38 and 86 14 respectively. Acetylation of the 3-hydroxy group stabilised the A-conformation of catechin and changed the ratio to 48 52. 

Substitution at C-4 by a hydroxy or aryl substituent strongly favors the E-conformation (i.e. as in catechin-4 or epicatechin-4 units) due to minimization of 1,3-diaxial interactions and thep ewrfo-allylic or A(l, 3)-strain effect (111). It was also concluded that the favored E-conformers of catechin-4 and epicatechin-4 units would adopt C(2)-sofa and half-chair conformation, respectively as minimum energy species. 

The stereochemistry of proanthocyanidins is further complicated by atropiso-merism. Two such isomers occur for a proanthocyanidin dimer. For example in Figure 7.7.3, a dimer with an epicatechin-4 T unit will be 4)8 linked and adopt the two conformations 19 and 20, whereas a dimer with a catechin-4 T unit will be 4a linked and adopt the conformations 21 and 22. The atropisomers are manifested in H NMR spectra of dimers by many of the resonances occurring as doublets of unequal intensity (32, 38). The ratio of atropisomers may also be estimated from fluorescence decay experiments (12). 

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