SummaryThe 15N-NMR spectra of azoles, with natural isotope abundance, have been measured under different experimental conditions, and chemical shifts are reported for imidazoles, pyrazoles, oxazoles, isoxazoles, thiazoles, and isothiazoles. General trends of substituent effects in this heterocyclic series are discussed based on the data of 67 substituted azoles, dihydro-and tetrahydroazoles. "N, 'H spin-coupling constants have been determined from spectra obtained by ['HI 4 I5N polarizationtransfer experiments, i.e. an application of INEPT and DEPT pulse sequences. Two-bond and three-bond coupling constants are fully assigned and are discussed in terms of the specific pathways in azoles. The potential of structural applications of the new data is illustrated for isomeric nitro-imidazoles and highly-substituted pyrazoles, and in the case of ring-chain tautomerism of 2-substituted tetrahydrooxazoles.

A method has been developed to solve structures of DNA oligomers in solution from the experimental NOE data. The method is a combination of two approaches: (1) full matrix NOESY simulations and (2) conformational calculations of DNA double helix based on generalized helical parameters. The process of the refinement of a solution structure does not involve NMR-derived interproton distance constraints; rather it consists of a direct fitting of a structure to the experimental NOE data, a weighted sum of energy, and R factor being under minimization. A helical parameters-based generation of DNA forms makes it possible to organize the search for the optimal structure more effectively, systematically varying starting conformations. The method has been used to calculate a structure for the self-complementary DNA hexamer GGATCC, which is consistent with the available experimental data. The structure belongs to the B family of forms, although the local structural heterogeneity is very strong. Sugar puckers vary from O4'-exo to C3'-exo; helical steps are open with different magnitudes toward the minor groove. Next, we have addressed the question of how uniquely the structure is defined by the existing NMR data. Different structural parameters have been systematically varied, and their effect on individual NOE's and the R factor has been studied. Two energetically conjugated parameters, sugar puckers and glycosidic angles, can be determined very reliably, because of the strong dependences of the intraresidue H6/H8 to H2'/H2''/H3' NOE's. In contrast, the local helical conformation of DNA and the geometry of base pairs proved to be underdetermined by the existing NOE information, because the effect of any helical parameter on interproton distances can be compensated by the concerted changes in other parameters.

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SummaryThe 'H-NMR spectra of 2-(nitromethy1idene)pyrrolidine (7), l-methy1-2-(nitromethy1idene)imidazolidine (10) and 3-(nitromethy1idene)tetrahydrothiazine (1 1) in CDC1, and (CDJ,SO indicate that these compounds have the intramolecularly H-bonded structures (2)-7, (E)-10 and (2)-11 while the N-methyl derivative 8 of 7 is (E)-configurated in both solvents. 1 -Benzylamino-l-(methylthio)-2-nitroethylene (13), an acylic model, has the H-bonded configuration (E)-13 in CDC1, and in (CD,),SO. 2-(Nitromethy1idene)thiazolidine (3) has the (E)-configuration in CDC1, but exists in (CD,),SO as a mixture of (2)-and (E)-isomers with the former predominating. Both species are detected to varying proportions in a mixture of the two solvents. "N-NMR spectroscopy of 3 ruled out unambiguously the nitronic acid structure 6 and the nitromethyleneimine structure 5. The N-methyl derivative 4 of 3 is (Z)-configurated in (CD,),SO. Comparison of the olefinic proton shifts of (2)-3 and (2)-4 with those of analogues and also of 1,1-bis(methylthio)-2-nitroethylene (12) shows decreased conjugation of the lone pair of electrons of the ring N-atom in (2)-3 and (2)-4. This is also supported by T -N M R studies. Plausible explanations for the phenomenon are offered by postulating that the ring N-atoms are pyramidal in (2)-3 and (2)-4 and planar in other cases or, alternatively, that the conjugated nitroenamine system gets twisted due to steric interaction between the NO,-group and the ring S-atom. Single-crystal X-ray studies of 3 and 8 show that the former exists in the (Z)-configuration and the latter in (E)-configuration; the ring N-atom in the former has slightly more pyramidal character than in the latter.