50
The second well-characterized component of pectin is the ‘hairy’ regions also
known as rhamnogalacturonan I (RGI) regions. RGI consists of a backbone composed of a repeating disaccharide of GalA and rhamnose (Rha) residues [−4)-α-DGalA – (1,2)-α-L-Rha-(1-] n (Ho et al. 2016). They are highly branched structures
with neutral sugar side chains of varying degrees of polymerization attached to O-4
or O-3 position on the α-L- rhamnose residues (Toivonen et al. 2014) (Fig. 4.8).
The side chains consist mainly of α-L-arabinose and/or β-D-galactose residues.
The major types of side-chain present are:
(i) Arabinan (Ara) which is composed of (1 → 5)-α-L-Ara units and often ramified with short (1 → 3)-α-L-Ara or single α-L-Ara nonreducing units at O-2,
O-3 or O-5 positions
(ii) Galactan (Gal) which has linear, type I (1 → 4)-β-D-Gal or branched, type II
(1 → 3,6)- β-D-Gal, depending on the plant source.
O
HO
OH
OH
O
C
C
CH 3
CH 3
O
Methyl esters
O-acetyl esters
O
O
COOO
O
O
HO
HO
OH
OH
OH
OH
O
O
O
O
O
O
C
H 3 C
H 3 C
O
O
O
O
O
O
O
O
O
C
HO
COOCOO -
COOCOO- O
Fig. 4.7 Homogalacturonans (HG)
T. A. Oyedepo and A. A. A. Kayode