I CARBOHYDRATES, CHITIN AND CUTIN 519 



trum reverts to a B spectrum in the so-called retrogradation of paste, 

 in which process the quantity of bound water plays a certain part. 

 Thus the following conversions may be observed in wheat starch 

 which is pasted up and then retrogresses: A ^ V ^ B. 



Several investigators (Sponsler, 1923; v. Naray-Szabo, 1928) have 

 attempted to deduce the size of the elementary cell of crystallized 

 starch. Bear and French (1941) find for B starch an orthorhombic 

 cell with a volume of 930 A^ and for A starch a triclinic cell with 

 843 A^ volume. This is much more than the cellulose cell which 

 occupies only 670 A^. This proves that, besides glucose residues, 

 water molecules are enclosed in the cell. But these results are doubtful, 

 as starch produces only powder diagrams, i.e., Debye-Scherrer 

 rings. Recently Kreger (1946, 195 1) has succeeded in irradiating only 

 part of the large starch grains oi Phajus grandifolius by a special micro- 

 method. In this way he gets a fibre pattern, which enables him to 

 calculate the cell of B starch more exactly. Rundle, Daasch and 

 French (1944) were able to prepare artificial amylose threads, which 

 yielded a fibre period of 10.6 A, whereas that of cellulose is only 

 10.3 A. They think that the two glucose residues of the glucosan chain 

 is somewhat stretched in crystallized B-starch, whereas Kreger 

 (195 1) places three helically arranged a-glucose rings into the distance 

 of 10.6 A. When the results of the investigators mentioned are 

 combined, the following orthorhombic unit cell is found for crystal- 

 lized starch (B-diagram) : 



a : b : c = 9.0 : 10.6 : 1 5.6 A. 



Of these periods a : c show the ratio i : -\/5, indicating a hexagonal 

 symmetry. This is in accordance with a threefold screw axis along the 

 chains suggested above. The hexagonal unit cell has the periods 

 a : b = 18 : 10.6 A and contains 18 chains, i.e. 54 glucose residues and 

 54 water molecules. The density of starch under water of 1.60-1,63 

 is in agreement with this unit cell which is illustrated by Fig. 158 

 showing two possible arrangements of the starch chains (Kreger, 

 1951). 



Senti and Witnauer (1946) have shown that in the A spectrum 

 of starch there is also a fibre period of 10.6 A. From this it follows 

 that in the starch grain of either A or B type the amylose chains are 

 expanding, forming a spiral pitch of 10.6 A with three glucose 



