Basic studies of the cross-linking reaction of collagen with oxazolidine and plant polyphenols

To elucidate the reaction between collagen, condensed tannins and oxazolidine, two prodeiphinidin tannins have been studied: extracts from pecan nutshell pith and myrica esculenta bark. The tannin chemistry study showed pecan tannins are copolymers of catechin and gallocatechin with a ratio 1:6 and myrica tannins are prodeiphinidins with 40% of the structural units gallated. The average molecular weight was measured by GPC and the number average molecular weight of tannins was found to be 25 00-3000. The molecular structure of oxazolidine, 1 -aza-5-ethyl- 1,3-dioxacyc1o[3 .3.0] octane, was confirmed by IR, GC-MS and ‘H-NMR spectra. From NMR, the ratio between cis and trans conformations is nearly 1:1. Complete hydrolysis of oxazolidine is slow under weakly acidic or neutral conditions. The reaction of polyphenols with oxazolidine was modelled using simple phenols phioroglucinol, pyrogallol and resorcinol; the process was followed by chromatographic and NMR methods. The results show the reaction is second order: phloroglucinol has the highest reactivity. By using catechin and gallocatechin, it was shown that the cross-linking reaction of gallocatechins can happen between an A- ring and a B-ring, while for catechin, the reaction is only at the A-ring. Polyphenol polymer studies also support this result; more oxazolidine can be reacted with prodeiphinidins. The reaction of collagen with oxazolidine was studied; the reaction sites were determined using modified collagen. Few cross-linking reactions were found, which is supported by hydrothermal isometric tension (HIT) results. The reaction between polyphenol, collagen and oxazolidine was studied by HIT and it was found that the cross-linking is between collagen and tannins, which is responsible for the high stability of treated collagen. Based on this, organic combination tanning can be explained as an effect of covalent bonding, hydrogen bonding and hydrophobic bonding together, but the high shrinkage temperature is mainly due to the covalent bonding forming a complex matrix structure around the collagen fibres