CARBOHYDRATES, CHITIN AND CUTIN 



513 



Molecules with branched chains produce red (amylopectin) or even 

 brown colouring (glycogen) with iodine. 



The branching is due to glucosidic bonds from the aldehyde group 

 of one amylose chain to another chain (Fig. 154). Such bifurcations 



CH.OH r 



-0, ' 



CHfiH 



non-reducing 

 end groups 



branching 

 member 



additional aldehyde 

 branching end groups 



Fig. 154. End branching of amylopectin (from Frey-Wyssling, 1948 c). 



are frequently repeated and it can be shown that most of them corre- 

 spond to 1-6 bonds (Myrback, 1938; Gibbons and Boissonnas, 

 1950). When branched chains are methylated and then hydrolyzed 

 (Irvine, 1932), considerable quantities of dimethyl glucose (from 

 branching junctions) and tetramethyl glucose (from the end members 

 of the side chains) are formed in addition to trimethyl glucose. For 

 amylopectin the amount of 2, 3 -dimethyl glucose formed is similar to 

 that of 2,3,4, 6-tetramethyl glucose (5-5%). This means that the 

 number of end members /// is about the same as the number of bifur- 

 cations b. A dichotomous branching would satisfy this relation, 

 because it yields h ^ m — i (Fig. 155). 



aldehydic end member 



1 bifurcation 



2 bifurcations 



16 end members 



64 end members 

 (b) 



Fig. 155. Dichotomous branching of amylopectin, a) in a plane, b) in space (from Frey- 

 Wyssling, 1948 c). 



