Determination of the weight-average molecular weight and monomer composition of sugar beet pulp pectin using NMR...

Sugar beet pulp is rich in rhamnogalacturonan-I (RG-I) dominant pectin and promises functional properties. In this work, an integrated nuclear magnetic resonance (NMR) strategy combining diffusion ordered spectroscopy (DOSY) with qualitative and quantitative NMR was established. Then the molecular hydrodynamic properties and monosaccharide profile of an RG-I containing pectin from sugar beet pulp was characterized. DOSY was applied to measure translational diffusion coefficients, enabling calculation of the hydrodynamic radius (R_H = 6.99 nm) and weight-average molecular weight (M_w = 4.66 × 10⁴ Da) without chromatographic separation. After mild acid hydrolysis (2 M TFA, 105 °C, 4 h), one-dimensional ¹H and ¹³C NMR together with two-dimensiona ¹H¹H COSY and ¹H¹³C HSQC NMR were employed for qualitative identification of monosaccharide residues. Quantitative ¹H NMR (zg30, D1 = 30 s) with external calibration using mixed monosaccharide standards was then used to determine monosaccharide molar ratios. The polysaccharide was composed of D-Galacturonic acid (D-GalA), d-Galactopyranose (D-Galp), L-Arabinofuranose (L-Araf) and L-Rhamnopyranose (L-Rhap) in a molar ratio of 72.84:9.19:14.05:3.93. The high proportions of L-Rhap, L-Araf and D-Galp confirmed the typical highly branched RG-I structural feature. This derivatization free quantitative NMR methodology exhibited excellent precision and repeatability (RSD < 3.4%), providing a reliable alternative to conventional chromatographic methods for RG-I type pectin characterization.