GIAO/DFT studies on 1,2,4-triazole-5-thiones and their propargyl derivatives.

Density functional theory (DFT)/Becke-Lee-Yang-Parr (B3LYP) and gauge-including atomic orbital (GIAO) calculations were performed on a number of 1,2,4-triazole derivatives, and the optimized structural parameters were employed to ascertain the nature of their predominant tautomers. (13)C and (15)N NMR chemical shifts of 3-substituted 1,2,4-triazole-5-thiones and their propargylated derivatives were calculated via GIAO/DFT approach at the B3LYP level of theory with geometry optimization using a 6-311++G** basis set. A good agreement between theoretical and experimental (13)C and (15)N NMR chemical shifts could be found for the systems investigated. The data generated were useful in predicting (15)N chemical shifts of all the nitrogen atoms of the triazole ring, some of which could not be obtained in solution state (15)N HMBC/HSQC NMR measurements. The energy profile computed for the dipropargylated derivatives was found to follow the product distribution profile of regioisomers formed during propargylation of 1,2,4-triazole thiones.

New dipyranocoumarin from the leaves of Calophyllum apetalum Willd.

A new dipyranocoumarin, α-hydroxytomentolide A (1) was isolated from the leaves of Calophyllum apetalum together with the known compounds friedelin (2), apetalactone (3), inophyllum C (4) and canophyllol (5). The structure of the new compound was established by spectroscopic studies which include (1)H NMR, (13)C NMR, NOESY, HetCOSY, COLOC experiments and single crystal X-ray diffraction analysis.

Concurrent display of both α- and ß-turns in a model peptide.

This article describes a model peptide that concurrently displays both α- and ß-turns, as demonstrated by structural investigations using single crystal X-ray crystallography and solution-state NMR studies. The motif reported herein has the potential for the design of novel conformationally ordered synthetic oligomers with structural architectures distinct from those classically observed.

Synthesis and RNA binding selectivity of oligonucleotides modified with five-atom thioacetamido nucleic acid backbone structures.

Convenient chemical synthesis and incorporation of dithymidine and thymidine-cytidine dimer blocks connected with a five-atom amide linker N3'-CO-CH2-S-CH2 into oligonucleotides (ONs) are reported. The UV-Tm experiments for binding affinities of these mixed backbone ONs with complementary DNA and RNA sequences revealed important results such as significantly higher RNA-binding selectivity as compared with complementary DNA. NMR studies of the dimer blocks suggested a marginal increase in the N-type sugar conformations over that of the native DNA.

Self-assembly with degenerate prototropy.

This work describes a rational approach for addressing the prototropy-related problems in heterocycle-based self-assembling systems by the use of degenerate prototropy. As a proof of principle, the utility of degenerate prototropy is demonstrated herein by developing heterocycle-based AADD-type self-assembling modules that exist as "single set of protameric pair (duplex)" in both solution and solid states. These self-assembling modules are quickly accessible in good yield by reacting 2-amino-5,5-disubstituted-1H-pyrimidine-4,6-diones, available in one step by the condensation of alpha,alpha-dialkyl malonates and free guanidine, with isocyanates. Evidence from NMR spectroscopy, ESI mass spectrometry, and single-crystal X-ray diffraction studies confirmed the formation of molecular duplexes. The effect of electronic repulsion in duplex formation is also investigated. Their ready synthetic accessibility, remarkably high propensity to crystal formation, and the novel property of degenerate prototropy would make these novel self-assembling molecules promising candidates for many proposed applications.