Four isomeric monophosphates and five isomeric bisphosphates of riboflavin were isolated from commercial FMN or were prepared by acid-catalyzed isomerization. The reaction of riboflavin monophosphates in aqueous solution was studied in the pH range between 0 and 9 under various conditions. The predominant reaction at pH values below 2 is the acid-catalyzed migration of the phosphate group. A detailed kinetic study of this reaction was performed by high pressure liquid chromatography. Experimental data were fitted with computer-generated curves based on an algorithm for a network of first-order reactions. Rate constants and activation parameters were obtained for the temperature range of 50-80 degrees C at pH 1. At thermodynamic equilibrium, the reaction mixture contains about 66% 5'-phosphate, 11% 4'-phosphate, 8% 3'-phosphate, and 15% 2'-phosphate (pH 1.0, 50 degrees C). In the pH range between 3 and 7, the hydrolysis of FMN is the prevailing reaction with a rate maximum at about pH 4. The same experimental approach was used in a subsequent kinetic study on the isomerization of riboflavin bisphosphates. The formation of five out of six possible isomers was studied quantitatively and rate constants for each partial reaction were obtained.
