NMR study of stacking interactions between adenine and xanthine rings

Abstract

A relationship between the chemical shifts of adenine and xanthine ring protons of 7-[ω-(6-aminopurin-9-yl)alkyl]-1,3-dimethylxanthines (1) and the number of carbons (n = 2–10) in their polymethylene chains has been compared with that of 1-[ω-(6-aminopurin-9-yl)alkyl]-3,7-dimethylxanthines (2) in the buffer solutions at pD 7.0, 1.0 and 13.0 and in organic solvents. The relationship of 1 is clearly distinct from that of 2. The concentration dependence and the effects of temperature on the chemical shifts of 1 and 2 have also been investigated. While the upfield shifts of the ring protons of 1 and 2 in the buffer solutions at pD 7.0 and 13.0 are explained in terms of stacking interactions between adenine and xanthine rings, the results in the buffer solution at pD 1.0 may be due to cation–π interactions. On the basis of these data, it can be assumed that the stacking interactions do not only consist of interactions between adenine and xanthine rings.