198 PHOTOSYNTHESIS 



This asymmetric influence and orienting effect has been observed in a 

 variety of reactions of compounds containing asymmetric carbon atoms. 

 A selective action of one of the components of a racemic mixture has 

 been observed to occur because of differences of the rate of reaction of 

 the two components, so that if the reaction is interrupted before it has 

 been completed, there will be a preponderance of one of the products, 

 which will be optically active/® 



Another means whereby an asymmetric compound may give rise to 

 other active compounds is through the action of asymmetric catalysts. 

 It has been observed that one member of a racemate has a higher rate 

 of reaction in the presence of an asymmetric catalyst than the other 

 member.'*^ This is of particular interest in relation to reactions taking 

 place in living organisms. Here the catalysts are chiefly enzymes which 

 are made up in part of optically active substances. Some of these 

 enzymes are capable of synthesizing optically active compounds from in- 

 active substances. '^'^ Thus, emulsin forms d-mandelic nitrile from benzal- 

 dehyde and hydrocyanic acid. Some of the reactions brought about by 

 enzymes can also l^e produced by pure chemicals and the rotatory power 

 of the product of the latter reaction is strongly influenced by the activity 

 of the catalyst. In the synthesis of mandelic nitrile, just mentioned, 

 the emulsin can be replaced by optically active alkaloids and in this case 

 1-quinine produces d-mandelic nitrile and d-quinine forms the 1-nitrile.^^ 



Some enzymes will act only upon one of two space isomers and it 

 is a very familiar fact that living organisms differentiate sharply between 

 compounds which differ but slightly in their space relationship. Thus, 

 while many of these compounds dift'er from each other in their chemical 

 structure but very slightly, their behavior in the living cell exhibits wide 

 variation. In fact, the interchange in spatial position of a single hydrogen 

 and hydroxyl group determines whether the compound can or cannot 

 be broken down by the living cell. So that while one of these com- 

 pounds may be completely oxidized to carbon dioxide and water, the 

 other may act as a chemically inert substance in the cell fluid. It was 

 a consideration of these facts which led Fischer to develop his conception 

 of lock and key to portray the relation of an enzyme to the substance 

 on which it acts. In order that an enzyme may act upon a certain sugar 

 the structure of the two must fit and as a simile of this Fischer suggested 

 the relation of the structure of a key to the lock to which it has been fitted. 



An indication of the asymmetric influences in plants is offered by the 

 series of investigations on phytochemical reductions by Neuberg and his 

 collaborators. They have shown that the yeast plant when supplied with 

 glucose is capable of reducing a variety of substances. Acetol, CH3.CO. 

 CHoOH, is reduced to propylene glycol, CH3.CHOH.CH0OH, the 



^'Markwald, Ber. chcm. Gcs.. 32, 2130 (1899). Fischer, ibid., 39, 530 (1906). 

 Erlenmeyer, 5/oc/if«;. Z^i/., 97, 261 (1919). ,,,^„, 



**Bredig and Faians, Ber. chcm. Gcs., 31, 783 (1698). 



- Rosenthaler, Biochem. Zeit., 14. 238 (1909); 17, 238 (1909); 19, 186 (1909). 



"Bredig and Fisk, Biochem. Zeit., 46, 7 (1912). 



