Submitted to: Journal of American Leather Chemists Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 2, 2005
Publication Date: October 1, 2005
Citation: Brown, E.M., Dudley, R.L. 2005. Approach to a tanning mechanism: study of the interaction of aluminum sulfate with collagen. Journal of American Leather Chemists Association. 100(10):401-409. Interpretive Summary: Tanning of animal hides or skins produces leather, a high value co-product of the meat industry. Tanning to stabilize the hide against putrefaction can be accomplished by treating the hide with plant extracts, organic crosslinking agents or minerals. Salts of the mineral chromium are the most commonly used tanning agents for the production of high quality leathers. Because of environmental considerations and customer preference, the tanning industry is interested in developing chrome-free tannages. Although the art of tanning is highly developed, the scientific rationale is poorly understood. The research reported here is a step in the elucidation of a general tanning mechanism that may incorporate different kinds of tanning agents. In this study we used a colorless aluminum salt as a model for mineral tannages. This model has spectroscopic advantages that allowed us to measure a wider variety of parameters than can be measured with chromium salts. The findings that aluminum salts partially destabilize the protein structure by forming complexes with acidic groups and that these complexes dissociate more readily than chromium-collagen complexes will contribute to a general mechanism for tanning and the development of chrome-free tannages.
Technical Abstract: The production of leather from animal hides or skins is accomplished through the use of tanning agents that stabilize the collagen matrix. The molecular characteristics of tanning agents are quite varied, and led to the expectation that leathers produced with different tanning agents would have fundamentally different structures. Current proposals suggest that the structural similarities between leathers produced with different tanning agents are greater than the differences. The goal of this work is to contribute to the elucidation of a general mechanism for tanning. Although complex salts of Cr(III) are currently the most effective tanning agents, salts of other metals, including aluminum, have been used either alone or in combination with vegetable tannins or other organic chemicals. In the present study, the interaction of aluminum sulfate with collagen is investigated. In a model system, using soluble collagen, C-13 NMR spectroscopy showed that Al(III) formed a complex with carboxyl groups on collagen. Al-27 NMR revealed the formation of a second sphere complex between collagen and aluminum. The effects of Al(III) binding on the thermal stability of collagen were studied by circular dichroism spectroscopy of soluble complexes and differential scanning calorimetry of insoluble complexes. Comparison of Al(III)-collagen interactions with Cr(III)-collagen interactions is expected to provide insight into a more generalized mechanism for tanning.