26 THE PLANT CELL WALL 



have been obtained. Various diffusion and sedimentation measure- 

 ments have provided MW values of 800-12,000. 



Recent molecular weight determinations carried out on lignin 

 liberated enzymatically from pine, oak, and sugar cane yielded 

 values of 695-890. 



From the molecular weights of degradation (ethanolysis) 

 products derived from synthetic lignin (from eugenol), minimum 

 values of 1300-1500 were calculated. 



Chemical molecular weight determinations based upon intro- 

 duction of bromine, O-methyl groups and other substituents give 

 values of 800-900, hence agree with many of the physical methods. 



Clearly, we do not have certain knowledge about the molecular 

 weight of the native polymers. In all probability, the lignins are 

 quite polydispeise, hence give a variety of weights rather than one 

 or a few. The native lignins probably exist in a wide range of mo- 

 lecular weights and sizes. It is difficult to distinguish between basic 

 polymer units containing primary linkages and aggregates of 

 these involving secondary forces. Viscosity and other physical 

 measurements suggest more or less spherical molecules. The lower 

 molecular units indicated by weights on the order of 1000 are 

 unbranched, and have a DP == 5-6. The lignins exhibit a high 

 index of refraction and strong aromatic absorption. Commonly, 

 ultraviolet maxima center about 280 m\i, and range from 

 274-285 mfx. Rarely, lignins have strong absorption bands beyond 

 300 m[i. Extinction values (as Ej |£ cent ) of about 100-200 have 

 been found for pine, oak, birch, and maple lignins, and for synthetic 

 lignin formed from eugenol in pea and celery tissue. 



Lignins also possess identifiable infrared absorption bands. 

 A band at 3400 cm ' (about 2.9[jl) is due to bonded OH groups. 

 In the 1660- 1700 cm 'region, aldehydic or ketonic C=0 appears; 

 absorption bands at 1600 and 1510 cm ' have been assigned to 

 the phenyl skeleton whereas a 1430 cm ' band is associated with 

 aliphatic groups. 



The electrophoretic behavior of the lignins has been little 

 studied. The information now available suggests that some lignins 

 (oak, for example) are quite homogeneous, whereas others (Scots 

 pine, for example) give evidence of a multicomponent nature even 



