Arabinan-rich architectures in pectin Rhamnogalacturonan I domain unveiled by termite digestion: In situ structural insights from solid-state NMR

Abstract

Pectic polymers, ubiquitous in plant cell walls and widely used in industry, exhibit a wide range of functional properties. However, the sophisticated architecture of their higher-order structures in its native environment, particularly for Rhamnogalacturonan I (RG-I) domain, remains largely elusive. In this study, we innovatively combined solid-state nuclear magnetic resonance (ssNMR) and selective degradation of termites to investigate the molecular architecture of pectin within pine cell wall in situ. Quantitative analysis revealed prominent accumulation of arabinans, particularly those with more α-1,3-branching linkages. Using paramagnetic relaxation enhancement (PRE) ions as molecular probes, we uncovered a correlation between reduced water mobility in the local environment and the increased recalcitrance to degradation of these sugar units. Additionally, molecular dynamics characterization showed that these enriched sugar units experienced minimal changes during digestion. Collectively, our findings reveal a higher-order structural feature within the RG-I region—a compact, arabinan-rich architecture that is resistant to enzymatic degradation. Furthermore, this organization potentially functions as critical structural junctions, akin to the egg-box model, reinforcing the pectic network and maintain cell wall integrity.