40 SECTIONAL ADDRESSES 
starch, shows a much greater molecular size as compared with that of 
amylose (or B-amylose). In spite of this, however, the end group method 
of determination yields invariably one and the same value for all specimens. 
Moreover, it is possible by keeping amylose for some time to observe its 
molecular aggregation to amylopectin, exhibiting enhanced viscosities 
both for the acetates and for the methylated derivatives. Recently we 
have been able to bring these factors into line and have succeeded in 
preparing, after surface etching of the starch grains with ethyl alcohol 
containing small quantities of hydrogen chloride, a disaggregated starch 
which, in the form of its acetate and its methylated derivatives, furnishes 
the same value for molecular weight both by viscosity methods and by 
the gravimetric assay of the end group. This simplified disaggregated 
variety of starch is not degraded. We believe that it represents the 
chemical unit of starch, and by keeping it for a short time it reverts by 
re-aggregation to physical assemblages of increasingly high viscosity 
corresponding to the original amylose or amylopectin of the starch 
grains. We shall have occasion to inquire into the factors controlling 
this change. In my view, however, there can be no doubt that the chemical 
unit of starch is of limited size, having an average molecular weight of 
about 5,000 and that these units undergo aggregation to physical units 
of much larger dimensions. 
In Birmingham we have now prepared a considerable number of 
break-down products of starch representing the starch dextrins of varying 
chain-length. ‘The study of these products has yielded results of interest 
and value from many points of view. ‘Their gravimetric assay has fur- 
nished progressively different values for the end group, and this graded 
diminution in value corresponds exactly to their properties, such as solubility 
of the dextrin and its acetates and the capacity to undergo re-aggregation. 
This convinced us that we were dealing with a terminated chain of glucose 
units and not a closed loop. If the latter model were adopted and a closed 
loop of glucose units, represented as a flat ring, were assumed to be the 
picture of starch, then, in interpreting results of the end-group assay 
method, we should have to envisage side chains of glucose extended at 
regular or irregular intervals from different parts of this loop and to con- 
sider that these side chains were responsible for the tetramethyl glucose 
isolated in the end-group assay of methylated starch. In such an event 
we should expect to find, in dealing with the break-down products of 
starch, no very regular and progressive value for the ‘ end-group,’ and 
indeed it would be possible to isolate a starch dextrin which gave the 
same ‘ end-group’” value as the undegraded methylated starch. This 
has never been found to be the case. Starch dextrins corresponding to 
17, 12,9, 7, and 5 glucose units have been prepared. The «-amylodextrin 
obtained by the action of barley diastase on starch shows an end-group 
value corresponding to 17 glucose units. This product is significant in 
that it contains in its diminished chain-length almost all the phosphorus 
which was originally present in starch. Whether for this or other reasons 
this dextrin exhibits a remarkable capacity towards molecular aggregation 
and in this respect differs markedly from samples of glycogen which have 
similarly been examined. The factors which underlie this tendency 
