Abstract
We describe an integrated and straightforward new analytical protocol that identifies plant gums from various sample sources including cultural heritage. Our approach is based on the identification of saccharidic fingerprints using mass spectrometry following controlled enzymatic hydrolysis. We developed an enzyme cocktail suitable for plant gums of unknown composition. Distinctive MS profiles of gums such as arabic, cherry and locust-bean gums were successfully identified. A wide range of oligosaccharidic combinations of pentose, hexose, deoxyhexose and hexuronic acid were accurately identified in gum arabic whereas cherry and locust bean gums showed respectively PentxHexy and Hexn profiles. Optimized for low sample quantities, the analytical protocol was successfully applied to contemporary and historic samples including ‘Colour Box Charles Roberson & Co’ dating 1870s and drawings from the American painter Arthur Dove (1880–1946). This is the first time that a gum is accurately identified in a cultural heritage sample using structural information. Furthermore, this methodology is applicable to other domains (food, cosmetic, pharmaceutical, biomedical).
Highlights
We describe an integrated and straightforward new analytical protocol that identifies plant gums from various sample sources including cultural heritage
While a method, called peptide mass fingerprint, is widely known for the straightforward identification of proteins in the biomedical field[39] and it has been successfully adapted to the study of proteins in samples from cultural heritage[40], the identification of plant gums is still challenging and this paper aims to exceed these obstacles by adapting metabolomics-like strategies[41]
A new method based on the partial enzymatic digestion of plant gum polysaccharides coupled to matrix-assisted laser desorption/ionization (MALDI)-TOF MS and MS/MS analysis was successfully developed to the identification of arabic, cherry and locust bean gums in various samples including samples from cultural heritage
Summary
We describe an integrated and straightforward new analytical protocol that identifies plant gums from various sample sources including cultural heritage. To elucidate its structural features, methylation-gas-chromatography mass-spectrometry (GC-MS) associated with 2D-NMR are employed These techniques recently showed that the polysaccharide backbone of gum arabic (Acacia senegal) consists of 1,3-linked β-D-Galp residues, substituted at O-2/O-6/O-4 positions[12], with residues of 2,3,6-β-D-Galp1, 3,4-Galp1, 3,4,6-Galp[1] present in side chains[12], in addition to 1,6-β-D-Galp side chains to which many Ara, UA and Rha residues are linked[12,18] (see saccharide nomenclature/abbreviations in Supplementary Table 1). An application to highly branched arabinogalactans (AGs) from Arabidopsis thaliana leaf, unveiled some structural features of the polysaccharide using a set of enzymes[31] (e.g. β-(1-3)-galactan backbone with β-(1-6)-galactan side chains substituted with L-Araf residues, 4-O-methyl-GlcA, L-Fuc) These mass spectrometric strategies have mainly been applied to the study of natural polymers (raw products) and they are still under development for a better understanding of the gums’ molecular structures and, their function in a specific environment (e.g. application to food/pharmaceutical industries). Combination of scanning-electron microscopy with energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy was used to study the surface of watercolors paints and the identification of gum arabic was suggested[38]
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