Comparison of structure of gum exudate polysaccharides from the trunk and fruit of the peach tree (Prunus persica)

The peach tree (Prunus persica) produces gum exudates on its trunk and fruit. The former was extracted with water to give polysaccharide (PPN) and residual material was extracted with alkali giving structurally similar PPNA (NMR examination), which was formed in a slightly higher yield (42% compared with 37%). PPNA consisted of Ara, Xyl, Man, Gal, and uronic acids in a 36:7:2:42:13 molar ratio and was homogeneous on HSPEC with Mw 5.61(±0.22) × 106 g mol−1. Methylation analysis showed mainly nonreducing end- (20%), and 3-O- (6%) and 4-O-substituted Arap and/or 5-O-substituted Araf units (14%) and nonreducing end-units of Xylp (13%). The core Galp units were mainly 3,6-di-O- (19%) and 3,4,6-tri-O-substituted (14%). The 13C NMR spectrum of PPNA confirmed its complexity with 5 C-1 signals from δ 107.5–109.5 from α-l-Araf and a main one at δ 103.2 from the β-d-Galp units. A controlled Smith degradation of PPNA gave 19% polymer (PPNAS), which was almost completely degraded, in a very low yield, to an inner core of PPNAS2 after a second cycle. This contained mainly β-d-Galp units, that were nonreducing end- (25%), 3-O- (34%), and 2,3-di-O-substituted units (21%), with no evidence of 6-O-substitution. Partial hydrolysis of PPNAS with 0.1 M TFA at 100 °C removed Araf units, to give polymeric PPNAS60 (20% yield), which had Gal as its major component (85%). Methylation analysis showed a branched structure mainly nonreducing end- (18%), 3-O- (15%), 6-O- (45%), and 3,6-di-O-substituted Galp units (9%): its 13C NMR spectrum had a C-1 main signal at δ 103.8 from β-d-linked units. Under stronger hydrolysis conditions β-d-GlcpA-(1 → 6)-αβ-d-Gal and β-d-GlcpA-(1 → 2)-αβ-d-Man were formed. The fruit gum polysaccharide (PPNF) was homogeneous with Mw 6.43(±0.64) × 106 g mol−1 and according to 13C NMR and HMQC, and TOCSY H-1 correlations, showing few structural differences. These were not apparent on monosaccharide composition and methylation analyses, which only revealed quantitative variations.