Coenzyme Interactions

Proton magnetic resonance and ultraviolet spectroscopic data have shown that the coordination of thiamine and thiamine pyrophosphate with indolyl compounds centers upon the positively charged quaternary nitrogen atom of the thiamine moiety and the benzene ring of the indole moiety. In ultraviolet studies a marked increase in absorbance over the sum of the absorbances of the individual solutions was found when a solution of a thiazolium derivative was mixed with that of an indolyl compound, whereas no interaction was indicated between the indolyl compounds and uncharged thiazole compounds. The proton magnetic resonance signals which show the greatest effect of the interaction are those of the protons which surround the quaternary nitrogen atom of thiamine and those of the protons attached to the benzene ring of the indolyl compounds. The signal of the bridge methylene protons of thiamine in the pure 1:1 complex with indoleacetate appears approximately 2.5 ppm upfield from the signal of the bridge methylene protons of free thiamine. From the magnitude of this change in chemical shift, the distance between the apex of the complexed thiamine molecule and the plane of the indole ring can be estimated as 0.37 nm. This small separation and the relative magnitude of the changes in chemical shift of the various thiamine signals suggest that the average conformation of thiamine in the complex has a large (g90°) inter-ring dihedral angle. Such a conformation places the pyrimidine amino group close to the thiazolium proton.