In addition to the galacturonan segments shown in Fig. 1,
neutral sugars are also present. Rhamnose (Rha) is a minor
component of the pectin backbone and introduces a kink into the
straight chain (Fig. 2) and other neutral sugars such as arabinose,
galactose and xylose occur in the side chains (Oakenful, 1991). A
chain of several hundred á-(1-4)-bonded GalA units with a varied
DE is a typical fragment.
The X-ray fibre diffraction studies showed that the galacturonan
segments in sodium pectate form helixes with three subunits per turn
and an identity period of 1.31 nm. The conformation of GalA units
as determined by NMR spectroscopy is 4C1 (Rees & Wright, 1971).
Calculations indicate that the helix is probably right-handed (Rees
& Wright 1971; Walkinshaw & Arnott, 1981a). Walkinshaw &
Arnott (1981a,b) indicated that X-ray fibre diffraction patterns of
sodium and calcium pectates, pectic acids, and pectinic acids show
the same helix structure, but the ways in which these helixes were
arranged relative to each other in the crystals seemed to differ. They
suggested that helical pectinic acid molecules pack in a parallel
arrangement, whereas the pectates pack as corrugated sheets of
antiparallel helixes.
Fig.