Proteomic strategies for the identification of proteinaceous binders in paintings

The identification of proteinaceous components in paintings remains a challenging task for several reasons. In addition to the minute amount of sample available, complex and variable chemical composition of the paints themselves, possible simultaneous presence of several binders and contaminants, and degradation of the original materials due to aging and pollution are complicating factors. We proposed proteomic strategies for the identification of proteins in binders of paintings that can be adapted to overcome the requirements and difficulties presented by specific samples. In particular, we worked on (1) the development of a minimally invasive method based on the direct tryptic cleavage of the sample without protein extraction; (2) the use of microwave to enhance the enzymatic digestion yield, followed by the analysis of the peptide mixtures by nanoLC-MS/MS with electrospray ionization (ESI). Moreover, as an additional tool to tackle the problem of contaminating proteins, we exploited the possibility of generating an exclusion list of the mass signals that in a first run had been fragmented and that the mass spectrometer had to ignore for fragmentation in a subsequent run. The methods, tested on model samples, allowed the identification of milk proteins in a sample from paintings attributed to Cimabue and Giotto, thirteenth-century Italian masters, decorating the vaults of the upper church in the Basilica of St. Francis in Assisi, Italy.