Differentiation of phenylpropanoid acids cyclobutane- and dehydrodimers isomers in barley leaf cell walls with LC/MS/MS system

Abstract Dimers of phenylpropanoid acids play an important role in cross-linking of polysaccharide chains present in the cell wall. These compounds are responsible not only for mechanical properties of cell walls, but also for plant tolerance to both biotic and abiotic stresses. Due to conjugated double bonds system these molecules are efficient radicals scavengers and UV light shield. Moreover, the presence of these natural products in many plant-derived foods provides a variety of health benefits and lowers disease risk. Therefore development of efficient methods of phenylpropanoid cyclobutane dimers and dehydrodimers analysis is of the highest interest. Liquid chromatography coupled to mass spectrometry with electrospray ionization is an analytical technique of choice for identification and – within certain limits – structural characterization of low molecular weight secondary metabolites in samples of plant origin. Utilization of high resolution analyzers in mass spectrometers enables accurate mass measurements, which allows to determine the elemental composition of these compounds with at least 5 ppm accuracy. The ions produced with electrospray ionization have low internal energy and their fragmentation can be generated by collision induced dissociation (CID MS/MS). LC/MS/MS system was used to profile phenylpropanoid dimers in barley leave cell wall extracts. Fifteen natural products were recognized after structural analysis of recorded CID MS/MS spectra, some of them were isomeric. We were able to detect: four isomers of two phenylpropanoid acids, three cyclobutane ferulic acid dimers, six isomeric ferulic acid dehydrodimers, one mixed dehydrodimer of sinapic and coumaric acid and one decarboxylated dehydrodimer of ferulic acid. Differentiation of isomeric compounds belonging to both classes of dimers, and their further quantitative analysis, will have influence on research directed to biological role of the dimers in plant responses to environmental biotic and abiotic factors.