X-Ray crystallographic, spectroscopic, and energy calculation studies of the 3-carbamoyl-1-[3-(indol-3-yl) propyl]pyridinium ion, a charge-transfer interaction model between tryptophan and pyridine coenzymes

Abstract

The title compound was synthesized in order to elucidate the intramolecular stacking geometry between the indole and pyridinium rings, and its crystal structure was determined by X-ray analysis. The structure was solved by direct methods and refined by least-squares to a final R index of 0.105. In the crystal, the prominent stacking interaction between the indole ring and its nearest neighbouring pyridinium ring, which is favourable for the charge-transfer interaction of both aromatic rings, was observed, although the molecules took a fully extended conformation. This stacking mode, the parallel alignment of both rings, and the spacing near to the normal van der Waals separation distance, was stabilized by Coulombic attraction forces and the interaction between the highest occupied molecular orbital of the indole ring and the lowest unoccupied one of the pyridinium ring. However, the absorption spectra showed the existence of the intramolecularly stacked charge-transfer interaction of both aromatic rings. This stacked conformation was also suggested by fluorescence and ¹H n.m.r. spectra. Empirical potential-energy calculations, using the minimization technique, showed the four kinds of intramolecularly stacked conformers to be energetically stable.