The ability of 32
purified and characterized hydrolyzable tannins
to form insoluble complexes with model protein bovine serum albumin
was investigated with a turbidimetric 96-well plate reader method.
The results showed a clear relationship between the hydrolyzable tannin
structure and the intensity of haze that formed during the tannin–protein
complexation. In addition to molecular weight, structural features
such as number of galloyl groups, degree of oxidative coupling between
the galloyls, positional isomerism, and cyclic vs acyclic glucose
core were the major structural features that affected the ability
of the monomeric hydrolyzable tannins to form insoluble complexes
with bovine serum albumin. While oligomers were superior to monomers
in their capability to precipitate the model protein, their activity
depended less on the functional groups, but mostly on their size and
overall flexibility. These results allowed us to construct an equation
that predicted the protein precipitation capacity of the studied hydrolyzable
tannins with high accuracy.
The energies of the preferred conformations of four 7a-methyl octa(or hexa)hydrocyclopenta[d][1,3]oxazines, five 8a-methyl octa(or hexa)hydro[3,1]benzoxazines, and 8a-methyl hexahydro[1,3]benzoxazinone, all cis-fused, were investigated by DFT methods. Following geometry optimization at the B3LYP/6-31G(d,p) level, both the proton chemical shifts and the vicinal coupling constants between H-4a and the H-4 and H-5 protons were calculated at the B3LYP/cc-pVTZ level and compared to the previously experimentally measured values. The agreement between the calculated and the experimental chemical shifts was found to be good. Similarly, the agreement between the calculated and the experimental vicinal coupling constants was also found to be good, thus providing a methodology for determining the conformational equilibria of such systems that is comparable in many respects to experimental approaches such as variable-temperature NMR or to the use of model coupling constant values, when available, from analogous compounds.
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