ASCORBIC ACID — INTERMEDIATE OR CATALYST? 271 



the stroma is blackened and the grana stand out colorless after treatment with silver 

 nitrate, and interpreted this as an indication that the (water-soluble) ascorbic acid is 

 accumulated in the hydrophyhc stroma rather than in the more lipophyUc chlorophyll 



grana. 



However, in interpreting these results, two points must be kept in mind. In the 

 first place, the structure revealed by the Mohsch test is a post mortem effect, since 

 silver nitrate kills the cells; in the second place, the granular structure is not the only 

 one observed in silver nitrate experiments — in some plants, silver patterns of a different 

 kind are observed (c/. Weber 1937S and Liebaldt 1938). If one insists on ascribing 

 the Molisch reaction to ascorbid acid, and at the same time believes Neish's results 

 shomng an approximately uniform distribution of this acid between the chloroplasts 

 and the rest of the leaves, one has to assume that some imknown factors preclude 

 the reduction of silver nitrate by ascorbic acid in the cytoplasm and favor the same 

 reaction in the chloroplasts. 



Gauteret (1934, 1935) observed that the Molisch reaction occurs only in light; 

 according to Giroud (1938) it is true that the reaction starts more rapidly in hght; but it 

 can proceed in the dark as well. Perhaps, then, the preferential reduction of silver 

 nitrate by the chloroplasts is a photographic development process, with the nuclei 

 being provided by a photochemical reaction, and chlorophyll playing the part of sensi- 

 tizer. Rackshit (1938) found an argument in favor of an association between chloro- 

 phyll and ascorbic acid in the observation that ascorbic acid is protected against autoxi- 

 cation by 2 X 10-« mole per liter of colloidal chlorophyll. Ascorbic acid also has a 

 tendency of associating itself with proteins (compare von Euler 1937 and Reedman 

 and McHenry 1938). 



2. Ascorbic Acid — An Intermediate or Catalyst? 



Ascorbic acid contains a six-membered carbon chain and a five- 

 membered lactone ring (CeHgOe; molecular weight, 176; L = 0.33) shown 

 in the following formula 10. 1. The close relation of ascorbic acid to 



*H0 OH* O O 



HOCH.CHOHCH C=0 HOCH,CHOH-CH C=0 



o o 



Formula lO.I Formula 10.11 



Ascorbic acid Dehydroascorbic acid 



hexoses is indicated. The compound can be synthesized in vitro from 

 glucose, mannose, and other sugars, and it is probable that plants produce 

 it in the same way (cf. Ray 1934). Gaha and Ghosh (1935), Bukatsch 

 (1940), Reid (1938), and Moldtmann (1939) found that the concentration 

 of ascorbic acid in plants can be increased by supply of glucose. Con- 

 ditions which indirectly increase the production of sugar, as an ample 

 supply of carbon dioxide and good illumination, also tend to increase 

 the concentration of ascorbic acid. 



Ascorbic acid is characterized by its acidity and its capacity for reversible oxidation. 

 The two H atoms marked by asterisks in the formula dissociate as H+ ions with a first 



