308 RIBOFLAVIN 



is exposed to sunlight, more than half of the riboflavin is destroyed within 

 2 hours.29 • ^0 The rate of destruction by light becomes higher with increas- 

 ing temperature and pH.^^ Alkali decomposes riboflavin rapidly. 



Riboflavin is stable against acids, air, and the common oxidizing agents 

 (except chromic acid, KMn04, and potassium persulfate), bromine and 

 nitrous acid. The stability of riboflavin has been used for the purification 

 of the crude synthetic product; in acid solution impurities are oxidized at 

 a temperature below 100° with use of CI., HoO, , HNO3 , or HCIO3 .=*^ But 

 the vitamin is destroyed by hydrogen peroxide in the presence of ferrous 

 ions. 



Reducing agents such as sodium dithionite (Na2S204), zinc in acid solu- 

 tion, catalytically activated hydrogen, and titanous chloride transform 

 riboflavin in alkaline, neutral, or acetic acid solutions directly into a color- 

 less dihydroflavin, which is reoxidized on shaking with air. The potential 

 of an equimolecular mixture of riboflavin and its leuco compound at pH 

 7.0 is —0.185 volt (—0.146 volt at pH 5.9), pretty much on the negative 

 side. Combination with the enzyme protein has been shown to raise the 

 redox potential from —0.19 volt for D-riboflavin-S'-phosphate to — O.OG 

 volt for the "old yellow enzyme. "^- 



By the action of zinc, tin, or sodium amalgam in strong HCl (pH < 1), 

 a red reduction interm.ediate form.s which is a semiquinone radical.^-^' ^^ 

 This behavior of riboflavin might be useful for its detection. 



Riboflavin gives a red-violet color with concentrated H2SO4 , which 

 changes to yellow on dilution. When heated with 50 % NaOH solution, 

 riboflavin produces a green color, changing to red on dilution.^^ 



Riboflavin shows a heightened affinity for Fe++.^^'* It has been found 

 associated with iron in a protein (conalbumin) which occurs in the a^'ian 

 blood stream and in egg white.^^*^ 



Bacteriostatic effects of riboflavins have been observed only in the light. 

 These may be explained possibly by the formation of toxic products and 

 in part by destruction of needed nutrients. Tt has been demonstrated that 



29 W. J. Peterson, F. M. Haig, and A. O. Shaw, /. .l//(. Chcin. Sac. 66, 662 (1944). 



30 J. A. Ziegler, J. Am. Chem. Soc. 66, 1039 (1944). 



31 R. Posternack and E. V. Brown (to Cliarles Pfizer and Co.), U. S. Pat. 2,324,800 

 (July 20, 1944) [C.A. 38, 221 (1944)]. 



32 R. Kuhn and P. Boulanger, Ber. 69, 1557 (1936). 



33 R. Kuhn and T. Wagner-Jauregg, Ber. 67, 361 (1934). See also K. G. Stern, Bio- 

 Chem J. 28, 949 (1934). 



34 L. Michaelis, M. P. Sohul)ert , and C. V. Smythe, ■/. Biol. Chcm. 116. 587 (1936). 



35 M. Z. Barakat and N. liadran, ./. Phnrm. ami Pharmacol. 3, 501 (1951). 



35'^ A. All)ert, Biochem. J. 54, 646 (1953); other metal chelates of riboflavin were de- 

 scribed recently by W. (). Faye and W. E. Lange, /. Am. Chem. Soc. 76. 2199 (1954). 

 35b J. Bain and H. Deutsch, ./. Bio]. Chem. 172. 547 (1948). 



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