Cooperative face-to-face and edge-to-face aromatic interactions of tryptophan indole ring with N7-quarternized guanine and neutral cytosine bases.

In order to investigate the effect of cytosine base upon the stacking interaction of N7-quarternized guanine base with tryptophan indole ring, the X-ray crystal structure of a 1:1 complex of model compounds 1 and 2 was carried out. Contrary to the expectation of the interaction of both molecules in aqueous solution, the crystal structure showed the first example of the simultaneous recognition of the Trp indole ring by guanine and cytosine bases by the coupling of the face-to-face and edge-to-face aromatic interactions, respectively.