Base-pairing configurations between purines and pyrimidines in the solid state: V. Crystal and molecular structure of two 1:1 hydrogen-bonded complexes, 1-methyluracil: 9-ethyl-8-bromo-2,6-diaminopurine and 1-ethylthymine: 9-Ethyl-8-bromo-2,6-diaminopurine

Abstract

9-Ethyl-8-bromo-2,6-diaminopurine forms 1:1 complexes with 1-methyluracil and with 1-ethylthymine. Both structures have been determined by three-dimensional X-ray diffraction methods and refined by block diagonal least squares. The first complex, 9-ethyl-8-bromo-2,6-diaminopurine: 1-methyluracil, exhibits a Watson-Crick type base-pairing configuration, while the second complex, 9-ethyl-8-bromo-2,6-diaminopurine: 1-ethylthymine, involves a “reversed” Watson-Crick type hydrogen-bonded configuration. Finding these two different base-pairing configurations in different lattice environments suggests that the energy difference between them is small, probably indicating similar strengths for these associations in solution prior to co-crystallization. These findings are discussed in relation to previous findings in this series of studies.