314 



RIBOFLAVIN 



By analysis of the titration curves, Michaelis and Schwarzenbach^'^ 

 showed that in solution, at low concentration including the physiological 

 concentration range, there is an intermediate form of reduction which is 

 entirely represented by a free radical. The maximum ratio of this to the 

 total dye is 0.10 at pH 4.62, and 0.14 at pH 6.92, at 30°. In higher concen- 

 trations, a partial dimerization of the radical to a bimolecular compound 

 takes place. No other molecular species on an oxidation level between 

 flavin and dihydroflavin could be detected in solution. 



On reduction of the yellow enzyme (riboflavin phosphate attached to a 

 specific protein) at 0° by the reduced triphosphopyridine nucleotide (co- 

 enzyme), under anaerobic conditions, a red intermediate is obtained which 

 possesses the same absorption spectrum as rhodoflavin. Thus the red reduc- 

 tion form of riboflavin can be produced under nearly neutral conditions. 



D. SYNTHESIS 



1. Chemical Methods 



In 1891, O. Kiihling synthesized alloxazines by condensation of o-phenyl- 

 enediamine hydrochloride ^^dth alloxan. 



NHo 



NH, 



NH 



OC 



+ I 



OC 



CO 



I 



NH 



N NH 



CO 



N 



/ 

 CO 



NH 



Using the same principle, R. Kuhn and P. Karrer worked out methods 

 for the synthesis of flavins, based on o-xylene, D-ribose, and alloxan as 



CHoOH 



CH2OH 



" L. Michaelis and G. Schwarzenbach, J. Biol. Chem. 123, 527 (1938). 



