6 



DIFFERENTIATION AND SPECIFICITY OF STARCHES. 



Starch is given the molecular formula ?i(C(jHio05)/i, and were it known, or had we 

 good reason for believing, that the molecule is a complex polymeride of preformed groups 

 such as CeHjoOs, or of larger or smaller groups, as is commonly stated, the number of pos- 

 sible stereoisomers could readily be determined were the molecular weight known. Assum- 

 ing that the latter is as high as 25,000, as set forth upon the basis of apparent osmotic 

 pressure, the number of stereoisomers would be a matter of hundreds rather than of thou- 

 sands. But, as stated in the preface, there is abundant evidence to be convincing that 

 starch is not a uniform substance and that it may exist in countless different forms, and 

 hence that the word starch should be used to express not only an individual, but also a 

 group of substances which have certain fundamental typical characteristics in conmion, 

 but which differ from each other in individual ways. 



The existence of starch in a vast number of different chemical forms can be accounted 

 for: (1) in forms of isomers having structural formulse which differ so little that all have 

 the typical characteristics of a given prototype; (2) in forms of stereoisomers; and (3) in 

 mechanical mixtures of two or more isomers or stereoisomers. Starch, like cellulose, is 

 undoubtedly a complex or aggregate of molecular units of great molecular weight, high 

 carbon content, and extreme complexity of molecular configuration. According to the 

 very recent experiments of Fouard (page 83) with a non-colloidal solution of starch, the 

 lowest possible molecular weight is 15,000. Figiues by former observers ranged usually 

 between 20,000 and 30,000. Knaffe prepared a body from glycogen ("arnmal starch") 

 by the agency of chlorine which had a molecular weight of 23,630, from which glycogen 

 was regenerated having a molecular weight of 15,350. Geinsbergen recorded for cellulose 

 5,508, Bumcke and Wolfenstein 1,944, and Nastukoff 6,480. 



The very striking relationships between starch, ceUulose, and glycogen in their element- 

 ary composition, molecular formula, color reactions with iodine, decomposition products in 

 the form of dextrins and sugars, and in a number of other ways that need not be specific- 

 ally stated, are very familiar to every biologist. Hence, a knowledge of the chemical 

 peculiarities of one substance becomes of direct value in explaining or indicating peculiar- 

 ities of another. Our information of the chemistry of the starch molecule is very limited, 

 but the common conception that it is a complex polymeride of molecidar units which 

 are in the nature of preformed hexose or other similar groups is not only apparently with- 

 out justification, but is opposed by the logical conception, and by modern literature, 

 that it is an extremely complex labile aggregate of unknown groups of ionic units. The 

 investigations of Cross and Bevan with cellulose, extending during a period of over ten 

 years, have brought to Ught many important facts from which deductions may be made 

 in respect to starch. For instance, these authors have shown that there is not a single 

 cellulose, but a group of celluloses that is divisible into subgroups in accordance with 

 their chemical peculiarities. Their classification is based on: (1) the degrees of resistance 

 to hydi'olytic and oxidative agents ; (2) the per cent yield of fm"f m'ol when decomposed by 

 boUing HCl.Aq; and (3) the elementary composition in respect to the ratio C : 0. The 

 most important findings of a fundamental character may be tabulated as follows: 



