Effect of Proton Donors on the Absorption Spectrum of Flavin Compounds in Apolar Media

Abstract

For the comprehensive characterization of the diversiform spectral data of flavoproteins, an investigation was made concerning the effect of proton donors such as trihalogenoacetic acids and phenols on the absorption spectrum of riboflavin derivative in apolar media. Spectral changes due to the interaction between the isoalloxazine nucleus of riboflavin 2', 3', 4', 5'-tetrabutyrate and these proton donors in carbon tetrachloride were characterized by the marked red shift of 340mμ absorption band (from 340 to ca. 370mμ) and the broadening of 445mμ band with its slight red shift. These changes which occur with proton donors in a moderate concentration range are accompanied by the appearance of isosbestic points, indicating that equilibrium processes are involved in the reactions. Equilibrium constants of these processes were evaluated by spectrophotometry, and the presence of hydrogen bonding in the complexing between flavin and phenols was proved by near-infrared spectrophotometry. Based on these findings, some postulations as to the interactions between flavin coenzyme and apoenzyme of some typical flavoproteins are presented.