Restricted rotation of the amino group of nucleic acid base derivatives and a study of structures of base pairs by the use of the phenomena

Summary

¹ H NMR spectra of complementary base derivative pairs, 9-ethyladenine (9EA):1-cyclohexyl-5-bromouracil(BrU),9-ethyl-8-bromoadenine(BrA):BrU and 9-ethylguanine(9EG):1-methylcytosine(1MC) were measured at low temperature in chloroform. Restricted rotation about 6N-6C bond of 9EA and 4N-4C bond of 1MC at low temperatures allows us to observe the separated amino proton signals at about -20°C for 9EA-BrU system and at about 0°C for 9EG-1MC. When the mixed ratio of the base derivatives were varied, the signal position chanqes in a different manner for each separated amino proton signal. The behaviors of the chemical shifts give us the information about the base pair structures, i.e., the Hoogsteen type base pair is initially formed at lower ratio of BrU to 9EA, and then the Watson-Crick type base pair appears with the increase of the ratio. In contrast to this result, the Watson-Crick type base pair is dominantly present in the BrA-BrU system.