An easily transferable protocol for in-situ quasi-non-invasive analysis of protein binders in works of art

Proteomic approaches based on mass spectrometry have become increasingly popular for protein binder's identification in works of art. The identification of the binder employed may offer key information on paintings and other polychrome objects and contribute to assess their historical and technical context, also providing useful hints for a proper restoration and/or conservation treatment. Usually, the protocols employed to this purpose are invasive and at least micro sampling is required. Here, we present a simple transferable method for a quasi-non-invasive analysis of binders in artworks based on the use of a very small poly (2-hydroxyethyl methacrylate)/poly (vinylpyrrolidone) hydrogel (3 mm × 3 mm) previously loaded with trypsin for the in-situ digestion of proteins and applied onto the objects' surface. Upon extraction of digested peptides from the hydrogel, they were examined by MALDI-TOF-MS and/or LC-ESI-MS/MS. The method was validated on fresh and aged model pictorial layers; optical microscope images, and spectrophotocolorimetry confirmed that neither damage nor color alteration of the painting layer occurred, and no hydrogel residue was left. X-ray photoelectron spectroscopy carried out on paint models confirmed that the treatment with trypsin-loaded gels did not modify the pigment composition, even on aged samples. The protocol was successfully applied to a painting on wood mockup aged thirty years, a statue dated XV century exposed in San Lorenzo church (Bisceglie, Bari, Apulia), and a liturgical scroll Benedictio ignis et fontis (Benedizionale) of the Museo Diocesano of Bari dated eleventh century; in all these objects the proteinaceous binder was readily and successfully identified.