The mechanism of chrome tanning

For over half a century, the mechanism by which chromium(III) salts can stabilise collagen, converting it into leather, has been accepted as based on the notion of crosslinking. This classical view involves chemical bonding directly between carboxyl groups on adjacent triple helices, whereby the highly structured protein is supported in the way stitching strengthens a tear in material, so that it can resist wet heat up to 120ºC or even higher, when the transition from helix to random coil occurs, observed as macroscopic shrinking. However, that view is showed here to be erroneous; the currently assumed mechanism is fatally flawed. In its stead, it is postulated that the reaction is better explained by a simpler mechanism, named 'Link­ Lock'. All tanning processes are either single- or multi-component. All single component reactions confer the same degree of moderate hydrothermal stability increase, regardless of the chemistry, because the bound species merely interfere with the shrinking transition: this is termed 'linking' because the reagent only links to the collagen structure, but even if the reagent is capable of creating crosslinks, the outcome is always the same. Multi­ component processes may additionally involve reaction between a primary tanning agent and a secondary reagent, to create a stable matrix within the collagen structure: by crosslinking the primary reagent, 'locking' the molecules together, analogous to scaffolding around a building, a new chemical matrix is formed. This synergistic interaction means higher temperature is required to cause the breakdown of the supramolecular matrix and the collapse of the helical structure, so this is the origin of high hydrothermal stability. Chromium(III) molecular ions are merely linking agents, so to achieve high shrinkage temperature, a locking agent must be present; several chemical species can perform this function, but the commonest is sulfate ion. Crosslinking in the classical sense may occur, but the evidence is that its occurrence is rare enough to be unimportant, so the tanning mechanism does not depend on it. The role of masking, changing the ligand field of the chromium(III) complex, is to affect the way further complexation can occur with collagen. Here, it is shown that the traditional view is incorrect: however, the reality is that the tanner can benefit, but not in the way currently assumed.