Abstract

A resumé of investigations into the function of the basic groups of collagen in its binding of vegetable tannins is given. The main fixation of vegetable tannins by collagen is generally agreed to be of the nature of hydrogen bonding of the polyphenolic tannins to the backbone polypeptide groups of collagen. Evidence is presented for the acceptor function of the oxygen atoms of these links, partial crosslinking resulting. However, a number of findings indicate the participation of the basic protein groups in the attachment and irreversible fixation of tannins by stronger valence forces than hydrogen bonds, particularly in the case of tannins belonging to the condensed class. The fortifying of the crosslinks by this supplemen tary fixation of condensed tannins, in some instances quantitatively comparable with the hydrogen bonding reaction, manifests itself in several respects, compared with the effect of hydrolysable tannins. Thus, the greater stability is evidenced by the hydrothermal stability, ATE, the greater resistance to proteinases (trypsin), and to hydrogen-bond breaking agents, such as urea, as well as to the detanning action of acid, alkali and degrading compounds generally. Dissimilarities in the reactions of the two classes of tannins with collagen are also discussed, and factors such as pH and temperature.

The nature of the reaction of vegetable tannins with the basic protein group is not known. Electrovalent binding appears only feasible for tannins carrying the carboxyl group, mainly ellagi-tannins, in a limited pH range. Constitutionally, the condensed tannins possess the qualifi cation of covalent binding of some of their constituents on the basic groups, quinone-like structures possibly being involved, particularly on ageing. Numerous findings are presented in support of the view that the fixation of condensed tannins. such as mimosa and quebracho, involves the formation of stable compounds of crosslinking function on the basic groups of collagen by other than a hydrogen bonding mechanism