Detection of separated amino proton resonance signals of adenine derivatives of low temperature and its application to estimation of population of the adenine-uracil dimers in solution.

Splitting of the amino proton signals of 9-ethyladenine derivatives was found in proton nuclear magnetic resonance spectra at low temperature (ca. -30 degrees C). One of the separated signals corresponds to the syn amino proton relative to the N(1) nitrogen in the adenine ring and the other to the anti one. The phenomenon is ascribable to slowing down of the hindered rotation around the N(6)-C(6) bond, which has partial double bond character. On the addition of 1-cyclohexyluracil derivatives, one of the separated signals shifts downfield. From the analysis of the concentration dependence of the signals we could estimate the population of two kinds of adenine-uracil (AU) dimers that employ the syn and anti protons, respectively. i.e. the Watson-Crick-type and the Hoogsteen-type dimers. Independent of the substitution on the uracil ring, the Hoogsteen type is predominant at 70% and the Watson-Crick type at 30% (at -56 degrees C). On the other hand, with mixtures of general kinds of 9-ethyladenine derivatives with 1-cyclohexyluracil. the substituents on the adenine ring cause the population to deviate to extreme values; i.e., either the Watson-Crick-type or the Hoogsteen-type dimer predominates. 2-Chloro-9-ethyladenine and N2-(dimethylamino)-9-ethyladenine take almost completely the Hoogsteen-type dimers, while 8-bromo-9-ethyladenine, N2-(methylamino)-9-ethyladenine, and 2-amino-9-ethylpurine predominant in the Watson-Crick-type dimers.