Photophysics and conformations of procyanidin monomers, oligomers, and polymers

A variety of photophysical and conformational behaviors in dilute solutions is studied for naturally occurring procyanidin monomers, oligomers, and polymers using UV absorption, steady-state and time-resolved fluorescence, circular dichroism, and molecular modeling techniques. The absorption and emission spectra for all procyanidins used are measured. The correlations between fluorescence and the structures and conformations of procyanidins are examined. A new method for determining the number-average molecular weight of polydisperse procyanidin polymers is developed using the fluorescence quantum yield, {dollar}Q{dollar}, for a series of monodisperse oligomers of epicatechin with 4{dollar}betato8{dollar} interflavan bonds. The {dollar}Q{dollar}, the decay profile and the lifetime of the fluorescence are highly sensitive to different stereochemistries and characteristics in the procyanidin structures as well as the solvent environment. A homogeneous decay is observed for monomers and a bridged dimer. The heterogeneity in the decay seen in all unbridged dimers is most apparent in dimers with 4{dollar}alphato8{dollar} interflavan bonds. In view of the molecular origin of the heterogeneous decay in the presence of two rotational isomers at the interflavan bond, procyanidin polymers have conformational versatility ranging all the way from disordered compact random coils to highly extended rod-like chains with a locally helical structure. Anisotropy of the fluorescence for the serial procyanidins documents the presence of intramolecular energy migration from one monomer to another in the dimer and species of higher x{dollar}sb{lcub}rm n{rcub}{dollar}, number-average degree of polymerization. Analysis of the influence of the quencher on the emission intensity and the fluorescence decay shows that the quenching mechanism of procyanidin monomers by DMSO has both dynamic and static components. Only about 1/3 of the collisions with DMSO are effective in dynamic quenching. The trimer of epicatechin, which exhibits more enhanced emission than the monomer and dimer in the presence of poly(vinylpyrrolidone) (PVP), is found to be the smallest oligomer that has optimal binding in its initial complexation with PVP. Circular dichroism studies provide the chiroptical properties of the series of epicatechin oligomers, based on absolute stereochemistry, and solvent and molecular weight dependence. A simple model for intrachromophoric interaction provides good agreement between the experimental and calculated values of the circular dichroism.