CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 29 



of lignin structure including the nature of the aromatic groups and 

 their hydroxylation and methoxylation patterns; the nature of 

 side-chains and their oxygen functions; and the nature of the 

 linkage between monomers. 



Cross and Bevan, and others, in past years, sought to account 

 for the results of degradation studies by invoking a structure in 

 which these degradation products are derived from a uniquely 

 constituted molecule. As more complete analytical data were 

 accumulated, both the inadequacies of such unique structures 

 and the utility of the "Baustein" concept became apparent. Al- 

 though structure among the lignins is far from being resolved in 

 detail, a representative picture can now be put forth. The idealized 

 structure of a conifer lignin which is illustrated below does not 

 represent any one polymer segment, but rather attempts to incorpo- 

 rate in one structure most of the salient features of these substances. 



For the guaiacylpropane heptamer depicted we may calculate 

 a molecular weight of 1272, and the composition, C 65.1 per cent, 

 H 5.9 per cent, OCH 3 17.1 per cent. 



The molecular formula C 6 9H 75 023 when normalized to the 

 phenylpropane building stone would yield C 10 H 10 . 8 O3. 3 . By com- 

 parison, the formula for coniferylalcohol, one of the important 

 precursors of lignin is CioH 12 3 . Thus, the theoretical polymer 

 segment corresponds to a multiple of a slightly oxidized coniferyl- 

 alcohol unit. Such a structure does not account for all known 

 degradation products, but can explain the majority of them. 

 Notable for its absence is the pyrogallol dimethyl ether (syringyl) 

 groups found in angiosperms. 



The chemical groups which join the benzene rings in this model 

 structure are: (a) the prevalent glyceryl-^-phenyl ether linkage at 

 rings l->2, 3->4, 6->7; (b) the phenylcoumaran, or hydrofuran, 

 linkage which occurs at rings 2->3; (c) the pinoresinol or lignane 

 linkage, which occurs at rings 4->5; and (d) the biphenyl linkage 

 which occurs at rings 5-»6. Thus, both ether bridges and carbon- 

 to-carbon bonds are present in side chain-to-nucleus, side chain- 

 to-side chain, and nucleus-to-nucleus condensation. The common 

 hydroxylation-methoxylation pattern is that of guaiacol, which 

 accounts for the vanillin formed by oxidation. Primary and second- 



