Galacturonic acid chains

Kauss (39) has shown that the esterification of the carboxyl groups in the D-galacturonic acid chain takes place by a transfer of the methyl groups from S-adenosyl-L-methionine, analogous to the case in which the 4-methyl ether groups are transferred to D-glucuronic acid of hemi-cellulose (40). 

The length of the arabinan side chains present in subunit II may vary between 30 and 50 arabinose residues and several of these chains might be connected to the rhamnogalacturonan backbone which consists of only 8-10 sugar residues (ratio of rhamnose to galacturonic acid is 0.2 to 0.4). The arabinans might be connected directly to a rhamnose residue in the backbone or via one or more galactose residues [41]. 

The galacturonic acids of a plant cell wall mainly belong to smooth chains of homopolygalacturonic acid (PGA) and to hairy regions of rhamnogalacturonan I (RGI). In green plants, other uronic acids can be found in hemicelluloses. Provided they are not methylesterified, all these carboxylic acids deprotonate at the more or less acidic pH of wall water. The electrostatic charges of these polyanions are then compensated by cations ultimately derived from the environment. 

The ratios between linear and branched residues are presented as well. All values obtained without prior reduction (NS residues only), are in the range between one and two. From this it can be assumed, that side chains contain only slightly more linear than branched residues. After prior reduction (NS and UA residues) much more linear than branched residues are found. This surplus of linear residues originates from the 1,4-linked galacturonic acid backbone, which is also known as smooth region. 

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