Binder-pigment interaction by proteomic approaches

The appearance of a painting is the result of the materials used in terms of the pigments present (which determine the color) and the organic binder (which determines the painting technique). However, paintings are not stable or immovable and reactions take place over time, which strongly influence the final look of a painting. Once applied, the paint undergoes chemical reactions resulting in the formation of a dry film. Further chemical changes occur during ageing, such as oxidation and cross-linking of the organic media, reactions between the paint binder and the pigments as well as reactions of the pigments with the atmosphere. Although modifications arising from binders-pigments interactions may change drastically the appearance and physico-chemical stability of a painting, very little has been published about these changes and research has mainly focused on oily media. In particular, very little is known about the nature of the interactions between the inorganic pigments and the proteinaceous binders, and their impact on paint aging under variable environmental conditions. As an initial approach to this problem, we studied the interaction of casein (proteinaceous binder used in the tempera technique) with four inorganic pigments that are commonly employed in paintings: azurite (Cu3(CO3)2(OH)2), Saint John’s white (CaCO3), cinnabar (HgS) and red lead (Pb3O4). Paint reconstructions were prepared by applying pigments mixed with water solutions/dispersions of casein onto glass slides. The resulting paint films had been artificially aged and then scratched to obtain fine powders. The chemical modifications undergone by proteins as an effect of ageing, and depending on the pigment, were investigated by proteomics approaches based on Nano LC-MS/MS. Moreover, a LiP-MS-based approach, new to the cultural heritage field, was used to explore how the pigments can influence the 3D structures of proteinaceous binders. This technique enabled the identification of specific protein regions possibly affected by protein binder-pigment interaction.