Spectroscopic characterization of mucilage (Chia seed) and polygalacturonic acid

Abstract

Polygalacturonic acid (PGA) has frequently been suggested and used as a model substance for studying mucilage properties and effects in soil. While PGA has a defined chemical structure, the composition of mucilage as natural product can vary in space and time depending on the plant and soil conditions. However, it is still unclear if PGA can be used as surrogate for original mucilage when considering soil–mucilage interactions in the rhizosphere. Here the organic matter (OM) composition of PGA was compared with that of Chia seed mucilage and small-scale spatial distribution of OM composition in mucilage droplets was analysed using Fourier transform mid infrared spectroscopy in KBr-transmission technique (FTIR). Selected regions of dried Chia seed mucilage droplets were analysed using micro- Fourier transform mid infrared spectroscopy in transflection technique (micro-FTIR). For PGA, the FTIR spectra revealed lower C–H/C=O and higher C=O/C–O–C ratios as compared to Chia seed mucilage, indicating a relatively lower potential hydrophobicity and higher sorption capacity of the OM in PGA than OM in mucilage. The micro-FTIR spectra revealed that the potential hydrophobicity of a single freeze-dried mucilage droplet was higher at the tip as compared to regions located above the tip. The results suggest that the use of PGA as model substance for mucilage is limited especially when trying to imitate the sorption and wettability properties of the Chia seed mucilage OM. The spatial heterogeneity in OM composition as well as shifts in maxima of C=O and O–H bands in micro FTIR spectra of the cross sectioned mucilage droplet suggest that the composition of mucilage is changing with time. These findings may help initiating future studies on the dynamics and variability of OM composition of mucilage.