Collagen and elastin are the most abundant structural proteins in animals and play an integral biological and structural role in the extracellular matrix. The biosynthesis and maturation of collagen and...
Chromium(III)
sulfate is extensively used in leather processing
to stabilize the collagen molecules in hides and skins. Although its
excess usage causes severe environmental pollution and health concerns,
the role of chromium in stabilizing collagen still remains poorly
understood. For the first time, by integrating a number of techniques,
including real-time small-angle X-ray scattering, differential scanning
calorimetry and natural cross-link analysis, we reveal crucial molecular-level
indicators of collagen stability. The results indicate that collagen
molecules achieve maximum molecular stability at concentrations as
low as 1.8 wt % even if excess chromium (>3.7%) is introduced into
the collagen matrix. At low concentrations (1.8% to 3.7%), the active
amino acid residues are saturated via covalent bonding with chromium.
Any excess chromium interacts purely non-covalently with the collagen
molecule and, we propose, can be substituted by environment-friendly
alternatives. Further, important natural cross-links, which are crucial
in imparting mechanical strength, were observed to decrease with increasing
chromium concentration, highlighting the adverse impact of chromium(III)
sulfate on collagen matrix and the importance of identifying alternative
cross-linking agents. Our findings provide tools which will enable
the evaluation of greener tanning agents to facilitate a more sustainable
future for the leather industry.
The collagen structure in skins is
significantly influenced by
the cross-linking chemistry adopted during leather processing. We
have developed an in situ technique to measure real-time collagen
structure changes using synchrotron-based small-angle X-ray scattering
(SAXS). Three common mineral tanning systems, basic chromium sulfate
(BCS), zirconium sulfate (ZIR) and an aluminosilicate-based reagent
(ALS) were used to stabilize collagen in ovine skin. Studying the
molecular changes by in situ SAXS revealed a range of tanning mechanisms:
a complex combination of covalent cross-linking, electrostatic interactions
and hydrogen bonding by BCS, hydrogen bonding interactions by ZIR,
and the formation of colloidal aggregates by ALS. These results unravel
the mechanisms of producing leathers with different properties, explaining
why ZIR produces denser leathers while ALS produces softer leathers
compared to conventional BCS leathers. ZIR and ALS are environment-friendly
alternatives to BCS, and understanding their mechanisms is important
for a more sustainable future for the leather industry.
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