DSC on ovalbumin-hematite "tempera" paints: the role of water and pigment on protein stability

The role of water and hematite (Fe2O3) on the stability of ovalbumin-based model paint layers was investigated by means of DSC and FTIR. The aim of this research is to improve our understanding on the stability of paint layers based on proteinaceous media, assessing the water content and the pigment presence effects. Pigments may play a fundamental role in determining the structure of proteins in paint layers, thus affecting the possible interactions among proteins and the external environment, including humidity. Previous studies revealed that hematite affects the secondary structure of OVA in paint layers, although no experimental evidence of hematite/OVA covalent bonds has been reported in the literature. In this paper we investigate the synergic effect of water and hematite on OVA structure and stability. DSC analyses coupled with FTIR measures on protein hydration revealed that below 30 % of humidity the amount of water strongly influence the protein structure and stability: the less the water content, the higher the protein stability. Furthermore, our results suggest that a water phase separation occurs in the presence of hematite for which, in water-limiting condition, the hematite's hydration shell becomes almost negligible if compared to the bulk water available for the protein hydration because of the high protein-water affinity. Accordingly, the protein phase humidity is higher than the sample's nominal value. Paints at the same overall humidity exhibit different protein hydration state following the pigment/binder ratio, and in turn different resistance to damages throughout aging.