Micro-DSC, FTIR-ATR and NMR MOUSE study of the dose-dependent effects of gamma irradiation on vegetable-tanned leather: The influence of leather thermal stability

Highlights: • Effect of gamma irradiation on vegetable-tanned leather depends on dose. • Effect of gamma irradiation on leather depends on collagen thermal stability. • Thermally destabilized leather undergoes de-tanning at 10 kGy irradiation dose. • De-tanned collagen partially converts to gelatin at (50–100) kGy irradiation dose. • Highly stable leather withstand damage up to 100 kGy irradiation dose. Leather artefacts, archaeological, historical or modern, are prone to microbiological attack which could lead to irreversible degradation. Previous studies performed on new leather mock-ups indicated good resistance of leather at doses up to (25–50) kGy. The aim of this research is to improve our understanding about the changes in the thermal stability and structural order of collagen within vegetable-tanned leather exposed to gamma irradiation. Variously vegetable-tanned leather samples were therefore exposed to irradiation with Co-60 gamma-rays with increasing doses (10–100 kGy) and the dose-dependent effect on collagen-tannin matrix was investigated using micro-DSC, the best technique for determining proteins' thermal stability, coupled with FTIR-ATR and NMR MOUSE spectroscopy, to enhance the analytical coverage of the calorimetric profile. FTIR-ATR technique revealed the very subtle changes in collagen at the molecular (lower order) level by, which are reflected in the properties of the structures with a higher level of order, namely the hydrothermal stability of fibrils (measured by micro-DSC) and mobility of macromolecular chains (determined by NMR MOUSE). It was proved that a 10 kGy dose causes significant variation on collagen molecular structure, fibrils hydrothermal stability, and macromolecular chain's mobility when the collagen-tannin matrix is already thermally destabilized and present a wide distribution of collagen populations with distinct thermal stability. On the other hand, leathers characterized by high thermal stability and homogeneous distribution of collagen populations better withstood radiation up to either 25 kGy or 50 kGy, depending on the collagen-tannin interactions' strength. This protocol, with high analytical and diagnostic sensitivity, can thus be applied to characterize a broader range of collagen materials sterilized/crosslinked with gamma radiation.