Coenzyme Interactions

Complex formation between thiamine or thiamine pyrophosphate and various indolyl compounds has been studied by observing changes in the nuclear magnetic resonance and ultraviolet spectra of solutions of the complex-forming pairs. The 1:1 stoichiometry of each of the complexes was shown by the linearity of double reciprocal plots relating change in chemical shift or in ultraviolet absorbance and concentration of either of the components. The values of apparent association constant for all of the complexes obtained from such double reciprocal plots, and the values of ΔG, ΔH, and ΔS calculated for the indoleacetate-thiamine complex are typical for weak complexes in solution. The predominant mode of association of the indolyl-thiamine complexes seems to be the π-electron donor-acceptor interaction. This conclusion was based on the following observations: (a) the magnitude of the association constants is decreased by increased ionic strength, (b) there is little difference among the association constants for a series of complexes in which only the net charge of each indolyl compound was different (+, 0, -), (c) the association constants for the complexes involving 5-methoxyindoleacetate and 5-hydroxytryptophan, in which the donor capacity of the indole ring is enhanced by the electronsupplying substituents, are significantly greater than those for all of the other complexes.