Solvent effects on thiamin-enzyme model interactions. I. Interactions with tryptophan

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Abstract
The solvent polarity dependence of the interaction between thiamin and tryptophan was studied by spectrophotometric methods. The UV data clearly indicate that the interaction is weakened when the complex is transferred from water to aqueous ethanol or aqueous dioxane. The interaction of thiamin and tryptophan could also be detected by fluorescence-quenching studies (excitation of tryptophan at 287 nm, maximum emission at 348 nm). Appropriate treatment of the quenching data allowed dissection into static and dynamic contributions. A pyrimidine derivative related to thiamin, in its neutral and protonated states, interacted with tryptophan by fluorescence techniques, but not by UV. A thiazolium model interacted with tryptophan by UV but was an inefficient quencher of the tryptophan fluorescence. Theoretical models are presented to explain the solvent dielectric constant dependence of the association constant between tryptophan and thiamin. Both electrostatic and dispersion forces are found to contribute to the stability of the complex.