Rhamnogalacturonan-I entrapped in insoluble plant cell wall networks: Structural basis, confinement mechanisms, and implications for targeted release: A review

Rhamnogalacturonan-I (RG-I) is a highly branched pectic polysaccharide whose recovery from plant-derived residues is often treated solely as an extraction problem. While the existing literature frequently investigates the physical and chemical behavior of total pectin, this review explicitly isolates the structural entrapment of the RG-I domain. We examine a complementary explanation: the retention of RG-I within insoluble plant cell wall networks. Evidence from solid-state nuclear magnetic resonance, selective extraction, enzymatic digestion, adsorption models, and compositional analyses is synthesized to assess interactions between RG-I, and where necessary, carefully extrapolated data from bulk pectic polysaccharides, and cellulose- and hemicellulose-rich regions. Its heterogeneous arabinan and galactan side chains may promote multivalent association, molecular crowding, steric restriction, and topological interpenetration. Covalent linkages, hydrogen bonding, van der Waals interactions, competitive adsorption, and pore-size constraints may contribute to this retention, although their relative importance varies among wall systems. These interactions also complicate extraction data: an increased RG-I proportion after concentrated alkali treatment may reflect preferential homogalacturonan loss rather than selective release of intact RG-I. Extraction performance should therefore be evaluated using yield together with molecular-size distribution, linkage integrity, side-chain preservation, and mass balance between soluble and residual fractions. Distinguishing true release from apparent enrichment is essential for comparing strategies and preserving structurally meaningful RG-I domains during processing.