Rotational isomeric state analysis unperturbed dimensions

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. 

Determination of the unperturbed dimensions of the polymeric pro-cyanidins presents special challenges. Well-characterized samples of high molecular weight are not readily available, and the solubility characteristics of the high polymers are incompatible with the application of the classical techniques for the measurement of the dimensions of a macromolecule. An alternative route to the unperturbed dimensions exploits structural determinations in the solid state (8, 9), spectroscopic studies of well-characterized oligomers in dilute solution (iO, ii), molecular mechanics (MM2 software) (i2) calculations (i3, 14), and rotational isomeric state analysis (15-17) to provide a realistic description of the dimensions of the high polymers. A vital piece of information comes from the time-resolved fluorescence of the monomers and oligomers of well-defined covalent structure. The fluorescence measurements also show promise for the characterization of the com-... 

The description of the emission in the constrained dimer by equation 1, and the successful correlation of the populations deduced for the peracetylated dimer from 400-MHz proton NMR, with the preexponential factors in equation 2 show that the heterogeneity of the emission in the unconstrained dimers arises from the population of two rotational isomers at the interflavan bond. The populations of these rotational isomers in the free phenol forms can be deduced from the preexponential factors on the third and fourth lines of Table 1. This assignment of the populations provides the necessary ingredient for a rotational isomeric state analysis of the unperturbed dimensions of the polymers. 

Rotational Isomeric State Analysis of the Unperturbed Dimensions... 

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See also in sourсe #XX -- [ ]

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