Analysis of protein-based media commonly found in paintings using synchronous fluorescence spectroscopy combined with multivariate statistical analysis

Palaeoproteomics guidelines to identify proteinaceous binders in artworks following the study of a 15th-century painting by Sandro Botticelli's workshop

Undertaking the conservation of artworks informed by the results of molecular analyses has gained growing importance over the last decades, and today it can take advantage of state-of-the-art analytical techniques, such as mass spectrometry-based proteomics. Protein-based binders are among the most common organic materials used in artworks, having been used in their production for centuries. However, the applications of proteomics to these materials are still limited. In this work, a palaeoproteomic workflow was successfully tested on paint reconstructions, and subsequently applied to micro-samples from a 15th-century panel painting, attributed to the workshop of Sandro Botticelli. This method allowed the confident identification of the protein-based binders and their biological origin, as well as the discrimination of the binder used in the ground and paint layers of the painting. These results show that the approach is accurate, highly sensitive, and broadly applicable in the cultural heritage field, due to the limited amount of starting material required. Accordingly, a set of guidelines are suggested, covering the main steps of the data analysis and interpretation of protein sequencing results, optimised for artworks.

Synchrotron Deep-UV Photoluminescence Imaging for the Submicrometer Analysis of Chemically Altered Zinc White Oil Paints

Zinc oxide (ZnO) is a II-VI semiconductor that has been used for the last 150 years as an artists' pigment under the name of zinc white. Oil paints containing zinc white are known to be prone to the formation of zinc carboxylates, which can cause protrusions and mechanical failure. In this article, it is demonstrated how a multispectral synchrotron-based deep-UV photoluminescence microimaging technique can be used to show the distribution of zinc soaps on the submicrometer scale and how this information is used to further the understanding of zinc white degradation processes in oil paint. The technique is based on the luminescence of zinc soaps in the near-UV (∼3.65 eV) upon excitation in the deep-UV (4.51 eV), involving transitions that are argued to subsequently involve ligand-to-metal and metal-to-ligand charge transfer with intermediate structural reconfiguration. Because the primary emission peak lies at a higher energy than the band gap of ZnO (3.3 eV), the signal can easily be isolated from the pigment's very intense band gap and trap state emission by employing a multispectral acquisition approach. Moreover, analysis at such short wavelengths, in combination with a UV-transparent optical setup, allows for lateral resolution on the order of 200 nm to be obtained. The unprecedented capabilities of the microimaging technique are illustrated by showing its application to the study of a historical cross section from an early 20th century painting by Piet Mondrian. Revealing the submicrometer distribution of crystalline zinc soaps in this cross section provides new insights that suggest that microfissures, the starting points of paint delamination, are the result of an overall expansion of a heavily saponified zinc white layer.

Probing the toxic mechanism of bisphenol A with acid phosphatase at the molecular level

As an endocrine-disrupting chemical, bisphenol A (BPA), can affect normal endocrine function of hormone. This paper studied the toxic effect of BPA on acid phosphatase at the molecular level by multi-spectroscopic measurements, molecular docking, and enzyme activity experiment. BPA could enhance the fluorescence intensity, change the structure, and cause an increased hydrophobicity of acid phosphatase. Hydrogen bond interaction and van der Waals forces were the main forces to generate the BPA-acid phosphatase complex on account of the negative ΔH (- 36.92 kJ mol^-1) and ΔS (- 50.78 J mol^-1 K^-1). BPA led to the loosening and unfolding of protein structure and extending the peptide strands, as revealed by UV-vis absorption and CD spectra. Based on the enzyme activity experiment, BPA could decrease the activity of the acid phosphatase by entering the active site of the enzyme. The molecular docking model showed that BPA could bind into the cavity of acid phosphatase and interact with Tyr A252 and a hydrogen bond (1.47 Å) was formed in the binding process. This work suggested the structures and functions of acid phosphatase were both affected by BPA.

Probing the toxic mechanism of Ag⁺ with lysozyme

Silver (Ag) is widely used in human activities, which provides possibilities to distribute in organisms and tissues, resulting in harmful effects on human health. In this work, lysozyme was chosen as the target molecule to study the mechanism of toxic interactions between Ag(+) and protein using fluorescence emission spectra, synchronous fluorescence spectra, UV-vis absorption spectra, circular dichroism (CD) spectra, isothermal titration calorimetry (ITC), and enzyme activity assay. The results of fluorescence emission and synchronous fluorescence showed that there were interactions between Ag(+) and lysozyme by eliminating the inner filter effect (IFE). Data from UV-vis spectra indicated that the frame structure of lysozyme became looser with Ag(+) existent, while the micro-environment of aromatic amino acid residues did not show any significant alteration. CD results suggested that the secondary structure of lysozyme presented a decrease in α-helix contents with the increasing amount of Ag(+). ITC results showed Ag(+) can spontaneously bind with lysozyme through hydrogen bonding and van der Waals forces with one binding site (Ka=1.93×10(6)). The lysozyme activity was inhibited by Ag(+) according to the enzyme activity assay, revealing that Ag(+) bound to lysozyme at the active site which resulted in inhibition of lysozyme activity. This work showed that Ag(+) can cause damages to the structure and function of lysozyme.

Synchrotron DUV luminescence micro-imaging to identify and map historical organic coatings on wood

Collagen-based materials in historical coatings were characterised and imaged at the sub-micrometer scale using synchrotron DUV luminescence microspectroscopy and spectro-imaging.

Application of the Kubelka-Munk correction for self-absorption of fluorescence emission in carmine lake paint layers

The variations of the fluorescence emission of carmine lake travelling through an absorbing and scattering medium, such as a paint layer, were investigated by ultraviolet (UV)-visible absorption, fluorescence spectroscopy, and imaging techniques. Samples of the lake were studied in dilute and saturated solutions, on a reference test panel and a real case study. Relevant spectral modifications have been observed as a function of the lake concentration mainly consisting of a fluorescence quenching, red shift of emission maxima, and deformation of emission band. The application of a correction factor based on the Kubelka-Munk model allowed fluorescence spectra obtained in solution and on painted samples of known composition to be compared and correlated, highlighting that the fluorescence of the lake within paint layers is affected by both self-absorption and aggregation phenomena. This approach has been successfully applied on a painting by G. Vasari for the noninvasive identification of carmine lake. The results reported here emphasize the necessity of taking physical phenomena into account in the interpretation of the fluorescence spectra for a proper and reliable characterization and identification of painting materials in works of art.