Pectic acid

Pectic substances comprise a group of plant polysaccarides in which D-galacturonic acid is the principal unit. The basic structure in pectin is (→4)-α-D-GalA-(1→) (pectic acid), with interruptions of (1→2)-α-L-rhamose units inserted in the chain (pectin). The substitution is typically around 10%. The backbone may be partially acetylated and may be further substituted with terminal xylose. The galacturonans can be methylated i.e. methyl esterified [7]. The degree of methylation (DM) is often used for destinction of commercial grades of pectin. The rheological properties of pectin is further controlled by the degree of esterification (DE) (methyl esters) [8].

A part of the rhamnose units are branching points for neutral sugar side chains of L-arabinose and D-galactose [7]. The galactans are mainly linear chains of (→4)-α-D-Gal-(1→), whereas the arabinans are principally chains of (1→5)-linked-α-L-arabinofuranosyl residues with substituents at C(3) of additional units of the same sugar or of D-galactopyranose. In a few cases, well-defined L-arabino-D-galactans have been isolated [1]. The rhamnosyl substitution is thought to cluster in "hairy" regions, leaving "smooth" galacturonan sequences as the backbone [7].

X-ray difrraction patterns from polycrystalline fibers of sodium pectate show that the molecule forms 3-fold helices that pack in a rectangular unit cell of dimensions a= 8.39 Å, b= 14.27Å and c= 13.36 Å [5].