Identification of proteinaceous binders in paintings: A targeted proteomic approach for cultural heritage

Non-Destructive Prediction of the Mixed Mineral Pigment Content of Ancient Chinese Wall Paintings Based on Multiple Spectroscopic Techniques

This study first developed non-destructive and accurate methods to predict the relative contents of mixed mineral pigments in ancient Chinese wall paintings using multiple spectroscopic techniques. The colorimetry, attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR), ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, and Raman spectroscopy were employed. Analyses were conducted including color difference, spectral reflection, ATR FT-IR spectra, and Raman mapping for simulated samples (malachite-lazurite mixed with rabbit glue samples) before and after aging. Models were then established for predicting the relative pigment contents of samples using UV-Vis-NIR and ATR FT-IR spectral data with Beer-Lambert law, and mathematical methods comprising principal component analysis (PCA) and nonlinear curve fitting. In particular, PCA and empty modeling methods combined with non-negative partial least squares were developed to predict the relative pigment contents based on Raman mapping data. The results demonstrated that approaches comprising PCA, mathematical model, and empty modeling based on the spectral data were effective at predicting the relative pigment contents. The predicted results obtained using the mathematical model based on UV-Vis-NIR spectra had an error of about 2%, and the best prediction based on ATR FT-IR spectra had an error of <3.6% at 1041 cm-1. The errors for the predictions using PCA and empty modeling based on Raman mapping data were 0.01-9.30% and 0.28-7.15%, respectively. However, the predicted relative pigment contents obtained based on ATR FT-IR data combined with the Beer-Lambert law had higher errors. The findings of this study confirm the strong feasibility of using spectroscopic techniques for quantitatively analyzing mixed mineral pigments.

Proteomic Analysis of Collagen: a Mass Spectrometry Approach to Material Identification of Shadow Puppet Cultural Relics

Shadow puppets are a popular art form in various regions, including China, Indonesia, and Turkey, and are rich in cultural significance. However, there is a considerable lack of research on the materials, diseases and conservation techniques related to shadow puppet relics. Material identification is the basis for understanding the production process of ancient shadow puppet relics and evaluating their deterioration degree. The microscopic morphology and infrared spectroscopy results in our experiments showed that the traditional methods of ancient skin identification were not effective in the shadow puppet samples. In order to achieve accurate identification, we used biological mass-spectrometry in proteomics to examine two puppet relics and commercially available modern shadow puppets. The results showed that the above samples could be detected by mass spectrometry with abundant peptides, including peptides specific for bovine skin. These peptides cannot be found in other commonly used materials for making shadow puppets, including the skins of pig, sheep, deer and horse. It is worth mentioning that we have found the peptides specific to yellow cowhide in two ancient shadow puppet relics samples. Therefore, the proteomic evidence shows that the raw materials of the two shadow puppet relics samples are yellow cowhide. Four modern samples also confirmed the reliability of material identification using proteomics. The proteomic evidence shows that the biological mass spectrometry will contribute to the scientific research of shadow puppet relics and other skin and leather cultural relics.

Molecular Evidence of Structural Changes in Silk Using Unlimited Degradation Mass Spectrometry

Proteomics has important uses in archeological science because it can distinguish species, reveal the evolution of paleontology, and provide biological evidence of historical events. However, this technique still has full potential in the study of silk aging mechanisms. In this work, we propose a strategy combining unlimited degradation with mass-spectrometry-based proteomics techniques, which interpret protein fragmentation propensity and secondary structure changes by detecting content changes of specific peptide groups in complex proteomes. This approach was employed to study the conformational changes in silk microscopic crystals after heat treatment. Combining conventional mechanics and crystallographic characterization, a thermal aging degradation mechanism model was proposed. At the same time, it explained the interesting problem that the crystallinity remained unchanged, but the mechanical properties decreased significantly. Focusing on the unlimited degradation process, this method will be widely applicable to the study of silk and wool aging processes and regenerated silk fibroin.

Review of recent advances on the use of mass spectrometry techniques for the study of organic materials in painted artworks

The study of painted artworks using scientific methods is fundamental for understanding the techniques used in their creation and their appropriate conservation. The ethical constraints involved in the handling of, and sampling from, these objects has steered recent developments in the field of Heritage science towards a range of new non-invasive/non-destructive spectroscopic techniques capable of providing important insights into their elemental or bulk chemical compositions. Due to the inherent complexities of heritage artefacts, however, their organic components are especially difficult to study in this way and their identification and degradation pathways are thus often best investigated using mass spectrometric (MS) techniques. The versatility, sensitivity and specificity of MS techniques are constantly increasing, with technological advances pushing the boundaries of their use in this field. The progress in the past ten years in the use of MS techniques for the analysis of paint media are described in the present review. While some historical context is included, the body of the review is structured around the five most widely used or emerging capabilities offered by MS. The first pertains to the use of spatially resolved MS to obtain chemical maps of components in cross-sections, which may yield information on both inorganic and organic materials, while the second area describes the development of novel sample preparation approaches for gas chromatography (GC)-MS to allow simultaneous analysis of a variety of components. The third focuses on thermally assisted analysis (either with direct MS or coupled with GC-MS), a powerful tool for studying macromolecules requiring zero (or minimal) sample pre-treatment. Subsequently, the use of soft ionisation techniques often combined with high-resolution MS for the study of peptides (proteomics) and other macromolecules (such as oligosaccharides and triglycerides) is outlined. The fifth area covers the advances in radiocarbon dating of painting components with accelerator MS (AMS). Lastly, future applications of other MS techniques to the study of paintings are mentioned; such as direct analysis in real time MS (DART-MS) and stable isotope ratio MS (IRMS). The latter, having proven its efficiency for the study of lipids in archaeological artefacts, is envisioned to become a valuable tool for this area, whereas DART-MS is already being utilised to study the surface composition of various museum objects. Rapid technological advances, resulting in increased sensitivity and selectivity of MS techniques, are opening up new approaches for paintings analysis, overcoming the fundamental hurdle of sample size available for destructive analysis. Importantly, while the last decade has seen proteomics applications come to the fore, this review aims to emphasise the wider potential of advanced MS techniques for the study of painting materials and their conservation.

Paleoproteomics

Paleoproteomics, the study of ancient proteins, is a rapidly growing field at the intersection of molecular biology, paleontology, archaeology, paleoecology, and history. Paleoproteomics research leverages the longevity and diversity of proteins to explore fundamental questions about the past. While its origins predate the characterization of DNA, it was only with the advent of soft ionization mass spectrometry that the study of ancient proteins became truly feasible. Technological gains over the past 20 years have allowed increasing opportunities to better understand preservation, degradation, and recovery of the rich bioarchive of ancient proteins found in the archaeological and paleontological records. Growing from a handful of studies in the 1990s on individual highly abundant ancient proteins, paleoproteomics today is an expanding field with diverse applications ranging from the taxonomic identification of highly fragmented bones and shells and the phylogenetic resolution of extinct species to the exploration of past cuisines from dental calculus and pottery food crusts and the characterization of past diseases. More broadly, these studies have opened new doors in understanding past human-animal interactions, the reconstruction of past environments and environmental changes, the expansion of the hominin fossil record through large scale screening of nondiagnostic bone fragments, and the phylogenetic resolution of the vertebrate fossil record. Even with these advances, much of the ancient proteomic record still remains unexplored. Here we provide an overview of the history of the field, a summary of the major methods and applications currently in use, and a critical evaluation of current challenges. We conclude by looking to the future, for which innovative solutions and emerging technology will play an important role in enabling us to access the still unexplored "dark" proteome, allowing for a fuller understanding of the role ancient proteins can play in the interpretation of the past.

Mass spectrometry in art conservation-With focus on paintings

Conservation of historic artifacts has been a multidisciplinary field from its very beginning. Traditionally, it has been and still is associated with the history of art. It applies knowledge from technical and basic sciences, adapting their solutions to its goals. At present, however, a new tendency is clearly emerging-scientific research is starting to play an increasingly important role not only as a service, but also by proposing new solutions both in the traditional conservation areas and in new areas of conservation activities. The above trend opens up new perspectives for the field of preservation of our heritage but may also create new threats. Therefore, the conservators' caution in introducing new technologies should always be justified; after all, they are responsible for the effects of any activities on the historic objects. This, quite selective review, discusses application of mass spectrometry techniques for the detection of various components that are important to the conservators of our heritage with particular focus on paintings. The text also contains some basic knowledge of technical details to introduce the methodology to a broader group of professionals.

In Situ Hydrogel Extraction with Dual-Enzyme Digestion of Proteinaceous Binders: the Key for Reliable Mass Spectrometry Investigations of Artworks

A novel strategy based on in situ dual-enzyme digestion of paint layer proteinaceous binders is introduced for faster and more confident identification, resulting in a bottom-up proteomics approach by MALDI-TOF mass spectrometry (MS). In situ sampling/extraction of proteinaceous binders using small pieces of a hydrophilic gel, previously loaded with trypsin and chymotrypsin proteolytic enzymes, was successfully exploited. Along with minimal invasiveness, the synergy of both enzymes was very useful to increase the number of annotated peptide peaks with their corresponding amino acid sequence by database search and subsequent MALDI-TOF/TOF analysis. The protocol was initially aimed at enhancing the identification of egg-based binders and then validated on fresh and aged model pictorial layers; an increased protein coverage was significantly attained regardless of the used painting binders. Optical microscope images and spectrophotocolorimetry analysis evidenced that the painting layers were not damaged or altered because of contact/sampling without leaving hydrogel residues. The proposed protocol was successfully applied on a painted altarpiece "Assumption of the Virgin" dated to the XVI century and on an angel statue of the Nativity crib dated to the XII century, both from Altamura's Cathedral (Apulia, Italy). The occurrence of various protein binders of animal origin was easily and reliably ascertained.

Nontargeted Pattern Recognition in the Search for Pyrolysis Gas Chromatography/Mass Spectrometry Resin Markers in Historic Lacquered Objects

A differential expression analysis technology developed for linear modeling of gene expression data was used in combination with thermally assisted hydrolysis and methylation gas chromatography/mass spectrometry (THM-GC/MS) to support the analysis of lacquers and varnishes on historical objects. Exudates from tropical trees, such as Manila copal, sandarac, South American copal, and Congo copal, which were frequently used in finishing layers on decorative objects up to the early 20th century, were compared through this approach. Highly discriminating features indicate biomarkers that can help to identify copals in resinous lacquers. The approach allows new, more systematic ways for finding biomarkers in the analysis of lacquered objects of art and varnishes.