Determining the degree of methylesterification of pectin by ATR/FT-IR: Methodology optimisation and comparison with theoretical calculations

The application of FT-IR to the study of the structure and interactions of the major plant-cell wall polysaccharide pectin has been reported for many decades. Nevertheless, here we show that the generally reported methodology for one of its most commonly utilised applications, the measurement of the degree of methylesterification (DM), requires careful interpretation and sample handling; including consideration of the moisture content and ionisation state. We propose instead a different methodology based on the assessment of the magnitude of C–H stretches in the methyl groups relative to those in the backbone and demonstrate experimentally the advantage of this method. In addition, we add a theoretical dimension to our work, performing full quantum chemical (DFT) calculations of monomeric-, dimeric-, and trimeric-pectic compounds, in various states of partial methylesterification. These extensive calculations not only confirm the identity of the proposed methyl-band and illustrate its scaling with DM; but also demonstrate the success of the theoretical approach. Thus, DFT calculations are expected to be a valuable tool in the interpretation of IR spectra obtained from more complex systems such as polysaccharide conjugates.