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Specific association phenomena of riboflavin-2’’,3’’,4’’,5’’,-tetraacetate and salicylic acid derivatives, such as salicylic acid, aspirin and salicylamide have been measured by infrared and fluorescence spectroscopy. Salicylic acid and riboflavin tetraacetate form the 1:1 cyclic hydrogen bonded dimer through the imino group and the 2-C carbonyl group of isoalloxazine ring of the latter, and the carbonyl group and carboxylic hydroxyl group of the former. Asprin and riboflavin tetraacetate form the 1:1 cyclic hydrogen bonded dimer by the same mode. Salicylamide forms the 1:1 cyclic hydrogen bonded dimer with riboflavin tetraacetate by using its amide group and carbonyl group. Salicylic acid derivatives are effective quenchers of the fluorescence of riboflavin tetraacetate. It is appeared that salicylamide is the strongest quencher among them. The quenching effect is attributed to the formation of association dimer.
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