Stacking interaction of indole ring with thiazolium ring and effect for H-D exchange reaction of thiamin

As part of a program to clarify the possible binding mode of tryptophan with thiamin coenzyme, the interaction between the indole and thiazolium rings has been studied by using a model compound, 2-(3,4-dimethylthiazolium-5-)ethyl indole-3-propionate, and the prominent stacking formation between both rings has been evidenced by the spectroscopic and X-ray crystallographic methods. The faster H-D exchange reaction of thiamin C2 proton observed in the presence of indole than in the lack would be resulted from the pi-pi charge-transfer interaction between the indole ring and the thiazolium ring of thiamin.