Abstract

The nature of the essentially weak, reversible interaction between the polyphenolic constituents of natural vegetable extracts and collagen and cellulose has been examined further in chromatographic experiments in which the mobility/affinity pattern of the constituents on collagen (acetone dehydrated sheepskin) and paper strips has been compared. General qualitative similarity in the mobility variation on the two substrates over a wide range of aqueous composition and solvent type serves to confirm the basic similarity in the underlying interaction mechanism, considered to involve hydrogen bond formation between proton-donor, phenolic groups on the tannin molecules and accessible electronegative atoms in the two substrates without restriction to specific functional groups. Quantitatively, the mobility of constituents on collagen was consistently lower than on cellulose, attributable to the stronger hydrogen bonding activity of the predominant carbonyl oxygen atoms in the former.

Systematic study of solvent structural and functional factors indicates that in the case of both collagen and cellulose, solvent effects on tannin affinity may be rationalised in terms of hydrophobic “shielding” and polarity both of which influence competitive hydrogen bonding inter actions between tannin and environment. The dynamic nature of binding implicit in the hydrogen bond mechanism, gives rise to a gradation of affinity with increasing molecular size in natural vegetable extracts, consistent with the continuous nature of experimental elution profiles obtained for both “water solubles” and “fixed tans” in extraction of tanned hide powder.

Comparative chromatography of “hydrolysable” tannins at acid and neutral pH confirmed the presence of a degree of p1—I-dependent, electrostatic binding on collagen contributing to the distinctive reactivity and affinity characteristics of this class of tannins in comparison with the “condensed” tannins. No evidence could be found for the presence of covalent bonding in the association of “condensed” tannins with collagen.

The experimental observations support the view that interaction between collagen and natural vegetable extracts is mediated predominantly through hydrogen bonding and electrostatic forces depending on func tionality, affinity gradation among extract constituents giving rise in the limit to small amounts of “irreversibly’ bound, high molecular weight fractions which cannot readily be displaced by competitive solvation by less structured, solvent molecules.