INTRODUCTORY DISCUSSION. 13 



series. The difference in structure of these two sugars lies especially in the 

 space relation of the hydrogen and hydroxyl about the fourth and fifth 

 carbon atom in the chain from the carbonyl group. It appears, therefore, 

 that these reciprocal transformations involve only the first three carbon 

 atoms of the chain, and that there exists a gradient of reaction in the sugar 

 molecule, the highest being at the carbon atom adjacent to the carbonyl 

 group and decreasing with each carbon atom removed therefrom. It is 

 interesting to compare the proportion of the various sugars found in these 

 mixtures with the conditions as they exist in nature. The majority of the 

 32 theoretically possible aldo-, 2-keto-, and 3-keto-hexoses are now known 

 as synthetic products ; it is, however, a striking fact (which has puzzled 

 chemists for a long time) that but a very small number of all these sub- 

 stances is found in plants. D-glucose and d-fructose are by far the most 

 common hexoses and are found in greatest abundance, and the only other 

 hexoses at all common in plants are d-mannose, d-galactose, and 1-sorbose. 

 In these equilibrium sugar mixtures, Nef found aldoses and ketoses in 

 about equal amounts, but in the glucose series, of the aldoses there were 

 present only d-glucose and d-mannose and in the proportion of 5 parts of 

 the former to 1 of the latter. In the galactose series over 90 per cent of the 

 aldoses was d-galactose. Furthermore, no trace of d-allose nor d-latose 

 on the one hand, nor of 1-gulose or 1-idose on the other was ever formed. It 

 is most suggestive that the composition and proportion of the various sugars 

 in equilibrium found in these experiments should so closely approach the 

 conditions existing in nature. For the detailed chemical dynamics under- 

 lying these phenomena, reference must be made to the original elaborate 

 discussion of RTef. However, the physiologist has herein for the first time 

 the basis of a rational explanation of this most perplexing problem. 



In the presence of higher concentrations of alkali the sugar molecule is 

 affected in a more profound manner, and these reactions are of special 

 interest in relation to glycolysis and carbohydrate metabolism in general. 

 It should be stated that so far the study of plant juices and of body fluids 

 by physico-chemical methods has not revealed concentrations of OH ions 

 such as would be necessary to bring about reactions in simple solution like 

 those under discussion here. At the same time our knowledge of the con- 

 dition of plant protoplasm as to acidity or alkalinity must be considered as 

 most unsatisfactory, such determinations as have been attempted being of a 

 very crude nature. It is not implied that these sugar dissociations are 

 accomplished necessarily by alkaline hydroxides or OH ions in the organism, 



NOTE. Nef's investigations on the composition of formose are also of interest in 

 this connection. The formation of formose (the mixture of sugars formed 

 by the action of alkalies on formaldehyde) can be regarded as the keystone of 

 the formaldehyde hypothesis of photosynthesis as first suggested by Baeyer. 

 Nef found that formose consists of a mixture half pentose and half hexose, 

 and from the experiments of the action of weak alkalies on sugars he con- 

 cluded that the hexoses in formose consist of 24 members, i. e., 8 aldohexoses, 

 and 8 2-ketohexoses and 8 3-ketohexoses, and that the aldoses are primarily 

 dl-glucose and dl-galactose. 



