Sequencing of ancient protein residues from the ground layer of Danish Golden Age paintings by tandem mass spectrometry

Fabiana Di Gianvincenzo§, Meaghan Mackie§, Cecil Krarup Andersen‡, Sibilla Orsini†, Ilaria Bonaduce†, David Peggie¤, Jesper V. Olsenʭ, Enrico Cappellini§

§ Natural History Museum of Denmark; ‡ The Royal Danish Academy of Fine Arts, Schools of Architecture, Design and Conservation, Copenhagen (Denmark); † Dipartimento di Chimica e Chimica Industriale, University of Pisa, Pisa (Italy); ¤ National Gallery, London (United Kingdom); ʭ Novo Nordisk Foundation Center for Protein Research, University of Copenhagen, Copenhagen (Denmark)

The recent application of proteomics to cultural heritage objects has proven to have a potential in the characterisation of damage and the identification of the biological species of origin of ancient protein residues in artistic materials. A protein sequencing protocol, based on tandem mass spectrometry analysis, was tested on a series of mock-up paint samples, containing proteinaceous and non-proteinaceous binding media and different types of pigments. Protein extraction was performed on the solid samples in one step, with a guanidine hydrochloride solution, followed by enzymatic protein digestion. The same protocol was subsequently applied to characterise the proteinaceous residues from the ground layer of a series of easel paintings, dating back to the Danish Golden Age. Five samples have been collected from the tacking edges of five 19th-century artworks, currently preserved in the National Gallery of Denmark and the Royal Danish Academy of Fine Arts, by Christen Schiellerup Købke and Christoffer Wilhelm Eckersberg. The identification of the biological source of the material, as well as a partial characterisation of the damage of the proteins, have been achieved. Animal glue, the most common protein material in preparation layers, from Bos species has been confidently identified. The obtained results show the efficacy of proteomics to investigate the history of artworks, leading to a better comprehension of artistic techniques and traditions. The characterisation of protein residues in paint and preparation layers can provide precious information to guide conservation and restoration treatments.