Fluorescence Spectroscopy in Structural Studies of Plant Cell Walls

Abstract

Plant cell walls represent the most abundant, renewable and biodegradable composite on Earth. Its highly complex structure consists mainly of three organic compounds: cellulose, hemicelluloses, and lignin. Cell walls have wide applications in different industries, especially for biofuels and biomaterials. Fluorescence spectroscopy is the method allowing investigation of cell wall structure thought monitoring of lignin autoflorescence and thus interactions of lignin with the other cell wall constituents. Deconvolution of fluorescence spectra reveals the number and location of spectral component peaks by calculation of the approximation of the probability density (APD) of component positions. A characteristic of complex CW fluorescence is that the emission spectrum contains multiple log–normal components originating from different fluorophores, shorter wavelengths corresponding to phenolic structures and longer wavelengths to conjugated structures in lignin. Fluorescence spectroscopy has been used for fast screening of the cell wall properties from plants of different origin (hardwood, softwood and herbaceous plant), that may be important for selection of plants for possible applications. Fluorescence spectroscopy may be applicable in the investigation of the effect of stress on the cell wall. Lignin fluorescence emission spectra, peak intensities and shifts in the positions of the long-wavelength spectral components may be indicators of changes in cell wall structure during the stress. There is an increasing application of quantum dots (QDs) in plant science, as fluorescent markers. The isolated cell wall is an appropriate object for study of the interactions with nanoparticles. The results of different physico-chemical techniques including fluorescence spectroscopy combined with spectral deconvolution, show that in the cell walls, CdSe QDs predominantly bind to cellulose, via OH groups, and to lignin, via the conjugated C=C/C–C chains. Variability of bond types in lignin is related to the involvement of this polymer in plant response to various types of stress, by introducing local structural modifications in the cell wall. Different lignin model compounds have been used in order to reveal spectroscopic properties of lignin. Lignin model polymers were synthesized from three monomers, coniferyl alcohol, ferulic acid and p-coumaric acid mixed in various ratios, simulating lignin synthesis in the real cell walls. Further, by using fluorescence spectroscopy and appropriate mathematical methods, it is possible to get deeper insight into the structural characteristics of the molecule. Future investigations will be based on synthetic cell walls and on variation in a portion of all three main components: cellulose, hemicelluloses and lignin, also having in mind results of fine structural modifications in lignin model compounds.