MISCELLANEOUS COMPOUNDS 281 



Chemical degradation of lignin yields phenolic substances of 

 the following types: 



HO 

 (1) 



H3CO 

 Guaiacyl or vanillyl group (typical of gymnosperms). 



H3CO 



(2) HO-^ 



HsCC 

 Syringyl group (together with (1), typical of angiosperms). 



(3) HO-Y"^Vr 



p-OH phenyl group (together with 1 + 2, typical of some monocots, 

 e.g. certain grasses). 



It is generally regarded that the precursors in lignin synthesis 

 consist of C6 — C3 units such as the above wherein R represents allyl 

 alcohol (CH2=CHCH20H). Thus, the possible precursor of gymno- 

 sperm lignin would be coniferyl alcohol. The scheme shown in Fig. 11-2, 

 based on results of C^* labelling experiments from several laboratories, 

 summarizes current information. Important contributions are those 

 of Brown and Neish (1955), Brown et al. (1959), and McCalla and 

 Neish (1959), who established the probable routes of interconversion 

 of phenylpropane derivatives leading to such compounds as coniferyl 

 and syringyl alcohols, and Reznik and Urban (1956) who demonstrated 

 a very efficient incorporation of C^^ coniferin (glucoside of coniferyl 

 alcohol) into spruce lignin. 



Less is known about the linkage of monomers in the lignin 

 itself although Freudenberg (1959b) has obtained dimers, such as 

 those illustrated by Fig. 14-2, and higher polymers, using an enzyme 

 from the mushroom Psalliota campestris and coniferyl alcohol. Since 

 more than one type of linkage occurred in Freudenberg's synthetic 

 lignin, this may also be true of natural lignin. Furthermore, at least 

 three basic building units are presently thought to be involved, and 

 perhaps more occur. The number and sequence of monomers included 

 in the lignin molecule may vary, cross linkages between lignin mole- 

 cules probably occur, and it is likely that lignin is bound to carbo- 

 hydrate constituents of the cell wall. The extent and nature of its 



