51
(iii) Arabinogalactan I (AGI) consisting of a basal chain of (1 → 4)-β-D-Gal substituted at O-3 with short (1  →  2)/(1  →  3)-α-L-Ara or single α-L-Ara nonreducing units.
(iv) type II arabinogalactan (AGII) which has a backbone of (1  →  3)-β-D-Gal
heavily substituted at position 6 by mono- and oligosaccharide Ara and Gal
side chains (Udani et al. 2009).
Studies have highlighted the potential importance of these neutral sugar chaincontaining regions (Jiang et al. 2016). Pectins are also highly variable depending on
source and extraction. Minor amounts of other sugars such as fucose, glucose, mannose, xylose, glucuronic acid, and methyl esterified glucuronic acid are sometimes
present in the side chains. In some cases, phenolics are also present in the side
chains (Dhingra et al. 2012).
α-L-Araf
T-α-L-Araf-(1→3)-α-L-Araf
T-α-L-Araf-(1→3)-α-L-Araf
T-α-L-Araf-(1→3)-α-L-Araf-(2←1)–R 2
→2)-α-L-Rhmp-(1→4)-α-D-GalpA-(1→2)-α-L-Rhmp-(1→4)-α-D-GalpA-(1→R 3
R 1
α-L-Araf
α-L-Araf
1
5
↓
1
5
↓
1
5
↓
1
1
1
i
m
RG-I
Arabinans
n
4
4
5
↓
↑
1
↓
Fig. 4.8 Rhamnogalacturonan I (RGI)
4 Bioactive Carbohydrates, Biological Activities, and Sources