The D-Lysine GTA can combine with collagen and improve the hydrothermal stability of the tanned leather. The amino groups of collagen side chains after combining with the D-Lysine GTA become unavailable for hydrogen ions. This indicates the mechanism by which D-Lysine GTA combines with collagen (Schemes 2 and 3).
Its reaction with collagen is commonly assumed to involve the formation of Schiff bases between the two carbonyl ends of GTA and positively charged amino groups on the surface of the collagen and offered D-Lysine.
Research has indicated the best candidate to be the -amino group of lysine, hydroxylysine and arginine side chains of collagen in the protein molecules creating a structure more stable than other proteins.
In process, GTA easily polymerises by condensation, giving rise to mixtures of elongated species that can in turn also cross-link intra and intermolecular lysine of collagen in a nonspecific manner.
The additional and surprising ring formation on the arginine side with GTA could arise from the formation of a further Schiff base between one of the nearest-aldehyde groups of the GTA monomer/oligomer and the second amino group of arginine.
Revisiting GTA cross-linking: the case of the arginineelysine intermolecular doublet has been studied.
The previously exposed amino groups facilitate in increasing the stability of tanned leather.
This explains the increase in uptake of D-Lysine when GTA is offered prior to GTA tanning.
Interestingly, the D-Lysine uptake was found to be more when GTA was offered. Amino groups of both lysine and hydroxyproline are involved in the GTA tanning reaction.
Proximity of the amino groups to the GTA is much essential for GTA tanning and improves the properties. It may be possible that D-Lysine might have improved the hydrothermal stability of tanned leather.