Rigidity, conformation, and solvation of native and oxidized tanninmacromolecules in water-ethanol solution

摘要

We studied by light scattering and small angle x-rays scattering (SAXS) conformations andsolvation of plant tannins (oligomers and polymers) in mixed water-ethanol solutions. Theirstructures are not simple linear chains but contain about 6% of branching. Ab initio reconstructionreveals that monomers within a branch are closely bound pairwise. The chains are rather rigid, withthe Kuhn length b=13 ± 3 nm, corresponding to about 35 linearly bound monomers. Contributionof solvation layer to SAXS intensity varies in a nonmonotonic way with ethanol content OA, whichis an indication of amphipathic nature of tannin molecules. Best solvent composition ?_A~Bis adecreasing function of polymerization degree N, in agreement with increasing water solubility oftannins with N. Polymers longer than b present a power-law behavior I — Q~(-d)in the SAXS profileat high momentum transfer Q. The monotonic decrease in d with increasing ?_A(from 2.4 in waterto 1.9 in ethanol) points that the tannins are more compact in water than in ethanol, presumably dueto attractive intramolecular interactions in water. Tannins were then oxidized in controlledconditions similar to real biological or food systems. Oxidation does not produce any intermolecularcondensation, but generates additional intramolecular links. Some oxidation products are insolublein water rich solvent. For that reason, we identify these species as a fraction of natural tannins called"T1" in the notation of Zanchi et al. [Langmuir 23, 9949 (2007)]. Within the fraction left solubleafter oxidation, conformations of polymeric tannins, despite their higher rigidity, remain sensitive tosolvent composition.