Methodological developments based on mass spectrometry for the analysis of glycoproteins and polysaccharides of plant gums: an application to cultural heritage samples

Abstract

Plant gums are naturally occurring compounds coming from several plant species and their application as binding media and adhesives in the field of cultural heritage traces back to the third millennium B.C. On the contrary of other organic materials, information on gums are fairly limited since their complex nature which consists of a mixture of polysaccharide and polypeptide chains. The analytical study of these materials is important to shed light on artists’ techniques, on the methods of manufacture of ancient artefacts, and to inform appropriate strategies for their preservation. In the cultural heritage field, characterization of plant gums is usually based on the monosaccharide composition as determined by gas chromatography mass spectrometry (GCMS) after complete acid hydrolysis of the gum. However, inorganic and/or organic compounds simultaneously present in the sample were demonstrated to alter the gum chromatographic profile thus preventing the correct identification. The present research aims to evaluate a proper analytical approach for plant gums identification in samples from works of art based on their chromatographic, electrophoretic and mass spectrometric profile. Analytical parameters have been optimized for the analysis of arabic, tragacanth, karaya, guar, ghatti, locust bean and fruit tree gums raw samples. In the first part of the thesis, size exclusion chromatography (SEC) was used to highlight the proteinaceous component of plant gums thus revealing the presence of glycoproteins with a molecular weight of 1-2 million Da. Complementary information were obtained by higher resolution techniques such as polyacrylamide gel electrophoresis (PAGE) that revealed to be a promising method for discriminating plant gums according to their protein electrophoretic profile. In the second part of the research, a novel method involving partial enzymatic digestion of the plant gums polysaccharide component, followed by analysis of the released oligosaccharides by mass spectrometry was developed. Due to the different polysaccharide structure of the gums, the obtained MALDI mass spectra represented the unique profile or “saccharide mass fingerprint” of each gum, thus allowing their discrimination. The efficacy of the strategy was also successfully tested on an aged gum arabic sample and on fresh and old watercolors.