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Revealing Molecular Level Indicators of Collagen Stability: Minimizing Chrome Usage in Leather Processing

Zhang, Yi, Mansel, Bradley William, Naffa, Rafea, Cheong, Soshan, Yao, Yin, Holmes, Geoff, Chen, Hsin-Lung, Prabakar, Sujay
ACS sustainable chemistry & engineering 2018 v.6 no.5 pp. 7096-7104
amino acids, chemical bonding, chromium, collagen, cross-linking reagents, crosslinking, differential scanning calorimetry, leather, leather industry, pollution, small-angle X-ray scattering, strength (mechanics), sulfates, tanning agents
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.