CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 27 



after repeated purification. Pine lignin which contains one phenolic 

 group is about 20 per cent higher in electrophoretic mobility than 

 oak lignin, which contains three such groups. The negative charge 

 on lignins must be due to dissociable phenolic or enolic hydroxyl 

 groups. Electrophoretic measurements at pH 10 would allow some 

 dissociation of these weak acidic groups. Phenols in general have 

 pK values of 10-12, hence may be appreciably ionized at pH 10. 

 The substituents on the benzene rings of lignin may further modify 

 the pK. 



Physical analysis by X-ray diffraction and polarized light 

 demonstrates that isolated lignins lack the highly ordered crystal- 

 line structure found in many polysaccharides. The data agree 

 with viscosity measurements which indicate a spherical form, and 

 suggest a molecule with structural members arrayed so as to form 

 a three-dimensional polymer, perhaps a reticulum. Although 

 isolated lignins are optically symmetrical, they can exhibit form 

 double refraction when carefully divested of the polysaccharide 

 framework of the wall. 



As we have already noted, the constitution of the lignin molecule 

 has been studied by rather vigorous means which yield an imposing 

 list of degradation products. Before we examine these substances, 

 it would be well to examine the overall constitution of the lignins. 

 It will be recalled that lignin may constitute one-half, or even more 

 of the cell wall substance in older tissues, or may be indetectable (or 

 absent) in youthful tissues. Commonly, it makes up 10-30 per cent 

 of the weight of the cell wall. When this wall fraction is isolated, 

 it commonly analyzes approximately 60 per cent carbon, 6 per 

 cent hydrogen, and 1 5-20 per cent methyoxyl. The mode of isolation 

 affects the constitution but a variety of methods may nevertheless 

 yield similar data. In the case of spruce wood, "native" lignin 

 isolated in small amounts with ethanol, and lignins isolated by the 

 use of fuming HC1, 72 per cent sulfuric acid or cuprammonium 

 solution yield, in the aggregate, the range C 62.4-63.6 per cent, 

 H 5.3-6.4 per cent, OCH 3 14.6-16.0 per cent. Solvolytic agents 

 which introduce alkoxy groups or modify the molecule otherwise will 

 of course alter the constitution, but often in a predictable manner 

 amenable to correction when actual percentages are calculated. 



