1999/12/01 by Andreas Schmidt, Markus Karl Kindermann, Markus Kindermann +2
Chemistry · Biochemistry, Genetics and Molecular Biology · #Crystallography and molecular interactions #DNA and Nucleic Acid Chemistry #Chemical Synthesis and Analysis
paper · doi:10.1021/jo991125t
Depending on the conditions, reaction of 6-chloropyrimidine-2,4(1 H,3 H )-dione 1 with heteroaromatics such as 4-(dimethylamino)pyridine, 4-(pyrrolidin-1-yl)pyridine, pyridine, and 1-methylimidazole, respectively, results in the formation of uracil-6-ylhetarenium salts or cross-conjugated uracilylbetaines, the intramolecular interactions of which are examined. In addition, 1 forms uncharged 1:1 π-stacks with DMAP and PPY, the electronic properties of which are inverted in relation to natural nucleobase−heteroaromatic systems. The geometry of the π-complexes is governed by steric effects as well as by the frontier orbitals of the uracil and the heteroaromatic, whereas the dipole moments are almost completey uncoupled. 1 H NMR experiments in DMSO- d 6 at room temperature as well as ESI- and MALDIMS prove homo -intramolecular hydrogen bonding of the uracilylhetarenium salts and the uracilylbetaines, respectively. An X-ray single-crystal analysis shows the uracilylbetaine 7 associated with a second molecule via two strong hydrogen bonds, thus forming a centrosymmetric dimer. In contrast to the Watson−Crick pairing mode of natural pyrimidine nucleobases, O(2) and N(3)-H of the uracilylbetaines are involved in hydrogen bonding. This geometry of the dimer 7 7 gives rise to a coupling of the semiempirically calculated dipole moments of the monomeric cross-conjugated betaines.