Fluorescence of Proanthocyanidin Polymers (Rotational Isomer).

Abstract

The lifetimes and quantum yields of proanthocyanidins, monomers and dimers of catechin and epicatechin, were measured. The fluorescence quantum yield and lifetime of the monomers was three times greater in dioxane than in water. The fluorescence decay curves of catechin and epicatechin were found to be fit best by a monoexponential function. The fluorescence decay of dimers of epicatechin and catechin were best fit to a sum of two exponentials. The fluorescence decay curve of a bridged dimer, Procyaninidin A-2, in dioxane was best fit to a monoexponential function. The 400 MHz proton NMR spectra of Procyanidin B-1 decaacetate in nitrobenzene and dioxane shows two rotational isomers. The rotamer's relative population was determined by the peak integrals. The ratio of the pre-exponential factors from the fluorescence lifetime measurements of the acetylated dimer in dioxane were similar to the relative populations for the two rotational isomers determined by proton NMR in dioxane. These results suggest that the heterogeneity of the fluorescence decay for the dimers is due to rotational isomerism. The combination of molecular mechanics calculations and fluorescence permits assignment of a particular rotational isomer to a particular lifetime in the dimers studied. From the molecular mechanics calculations for the dimers of epicatechin and catechin, the bond lengths, bond angles and dihedral angles were used to construct a rotational isomeric state (RIS) model for the homopolymers of catechin and epicatechin which have an inerflavan linkage between C(4) of one monomer and C(6) of its neighbor. The nature of the helices and unperturbed dimensions for these hompolymers wre found to be very sensitive to the fractional contribution of a particular rotational isomer. A combination of the time-resolved fluorescence and RIS predicts a random coil for the polymer ((,0)/nl('2) = 5.8). The fluorescence of catechin and proanthocyanidin dimer is enhanced upon complexation with poly(vinylpyrrolidone) in water. The intrinsic viscosity shows a dramatic collapse of the poly(vinylpyrrolidone) chain in the presence of catechin. Quenching of the fluorescence is observed when proanthocyanidins complex poly(L-proline) in water. It is found that emission by the fluorophores is useful for discrimination between local right- and left-handed helices in the poly(L-proline) chain.