CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 17 



are distinguished by their insolubility, and, in general, by a higher 

 molecular weight. To account for their unique properties it has 

 been proposed that protopectin is associated with cellulose or 

 polyvalent cations, or has an intrinsically greater molecular size. 

 One aspect of ion-pectin interaction of some interest involves the 

 insolubility of double salts with D-galacturonic acid; solubilities are 

 ranked: Na+>Ca + +>Ca++, Na + . Double salts which form the 

 most insoluble combinations could easily be provided in biological 

 systems. In essence, pectic acids are poly- (anhydro-a-D-galacturonic) 

 acids formed as is cellulose by 1,4-linkages. The presence of other 

 linkages is still a matter of uncertainty. From X-ray diffraction 

 studies, it has been established that the pectic acids differ in spatial 

 arrangement from other polysaccharides such as cellulose or alginic 

 acid. The examination of fibers prepared from sodium pectate 

 and pectic acid shows that a right angle is formed by the glycosidic 

 C — O bond and the plane of the galactopyranose ring, whereas 

 the comparable angle in cellulose is only 20°. The ring planes must, 

 therefore, be trans to one another, and the galacturonide chain 

 must contain a three-fold screw axis. The identity (fiber) period 

 is 13.1 A. This structure suggests a molecule with more rigidity 

 than is found in cellulose or alginic acid. 



In general the molecules are 730-5400 A in length, but regularly 

 10 A in thickness. Dimensional variations exist, according to 

 the biological origin and physical method employed. Thus, 

 pectinic acid (the partially esterified acid) from apple has dimen- 

 sions of 1200 Ax 10 A based upon sedimentation velocity, but 

 2630 Ax 7 A when measured by streaming birefringence (double 

 refraction). By viscometry, an intermediate length of 1700 A is 

 found. The macromolecule in lemon peel measures 1600 A 10 A 

 in the ultracentrifuge, but 3780 A x 7 A by streaming birefringence. 

 The removal of pectinic acid methyl ester groups to yield pectic 

 acid obviously alters the overall size of the molecule, principally 

 by allowing increased hydration, but leaves the general picture 

 of a comparatively elongated form unchanged. 



The length-diameter ratio given by the foregoing measurements 

 are 120-377 (apple) and 160-540 (lemon). From intrinsic viscosity 

 data, values of 53-165 have been obtained. 



