A Strategy for the Study of the Interactions between Metal-Dyes and Proteins with QM/MM Approaches: the Case of Iron-Gall Dye

Historical textiles dyed with tannins usually show more extended degradation than fabrics dyed with other coloring materials. In order to shed light on, this phenomenon we investigated the molecular interactions between tannin dyes and protein based textiles using quantum mechanical tools. In particular, we focused on the iron-gall, complex with, a fragment of alpha-helix-wool keratin. We developed a step by step protocol which moves from the simplest ternary complexes With free amino acids (all treated quantum. mechanically) to the more realistic system Of. the polypeptide fragment (treated at QM/MM level), passing through an intermediate model of interacting sites to evaluate the local environmental effects The analysis of the interactions between the iron-gall complexes and free amino acids allowed us to identify possible coordination modes as well as determining their relative,geometries However, we also showed that Only with the addition of the proteic environment a detailed picture Of the interaction sites and binding Modes can be achieved An important role is in fact played by the microenvironment which can favor specific coordinations with respect to others due to both structural and electronic changes in the possible interaction sites.