79 



remarkable. The one is that serotonin is a constituent of normal 

 tissues, has a strong biological activity, and various pathological 

 conditions such as hypertension or schizophrenia have been con- 

 nected with serotonin (Woolley and Shaw). The other circum- 

 stance is that serotonin is an indole derivative and is closely related 

 to plant hormones, the ones affecting growth. The derivatives of 

 indole seem to be among the most powerful instruments of living 

 nature and so their action mechanism is one of the most important 

 problems of biochemistry. The discoloration of riboflavine sug- 

 gests that they can promote the direct transition from the ground 

 state into the triplet. Serotonin seems to be able to do this to some 

 extent even in an unfrozen watery solution, in the absence of 

 structures; 10~^ AI serotonin added to a solution of riboflavine 

 induces a brown tint in its color. The spectroscope indicates that 

 this change is due to the appearance of a shoulder in the absorp- 

 tion spectrum of riboflavine which according to its position cor- 

 responds to the triplet. 



Serotonin is not the only substance which can produce such 

 changes. Lysergic acid acts similarly, though more weakly; 1- 

 benzyl-2,5-dimethylbufotenine and l-benzyl-2,5-dimethylreotonine 

 act likewise.- For kindly supplying the last two I am deeply obliged 

 to Dr. D. W. Wooley. These substances seem to increase the 

 probability of a direct transition from the ground state into the 

 triplet and also to increase the stability of this triplet, as shown by 

 the quenching of the phosphorescent light emission induced by 

 O2 in frozen riboflavine. This emission, in a 10~^ At riboflavine 

 solution is completely quenched by 2.10"^ M serotonin or lysergic 

 acid. How far the actions on £* are limited to riboflavine or how 

 far they represent a general ability to alter the probabilities of 

 singlet triplet transitions in favor of the triplet, remains to be 

 shown by more detailed studies which these substances certainly 

 deserve. 



^ The similarity of action was surprising since Woolley and Shaw found 

 these substances to be antagonists of serotonine. Lately (1955), however, 

 Shaw and Woolley found also similarities in biological activity. 



