Geometry dependence of spin-spin couplings in cyanamide by DFT analysis.

There have been numerous theoretical and experimental investigations examining NMR parameters related to non-amino N-H...N H-bonded moieties in both biological and chemical contexts. In contrast, little information on the geometry dependence of NMR parameters related to the biologically important H-bond donor amino group is available. Herein, the geometric dependencies of the one-bond amino N-H spin-spin coupling constants [(1)J(NH)] in the cyanamide monomer and dimer have been computed with B3LYP and the aug-cc-pVTZ-su0 basis set. In an isolated planar cyanamide molecule, the |(1)J(NH)| couplings were found to increase as the N-H bond lengthened. In contrast, in the planar cyanamide dimer the size of the H-bonded amino N-H coupling (|(1)J(N(d)H(d))|) decreased with increasing N(d)H(d) bond length. The |(1)J(N(d)H(d))| coupling was larger than the |(1)J(N(d)H(free))| coupling for N(d)H(d) distances up to 1.18 A (for a fixed N(d)H(free) distance of 1.006 A). Hence, the decrease of |(1)J(NH)| with increasing N-H distance, as well as the larger value of |(1)J(N(d)H(d))| compared to |(1)J(N(d)H(free))|, were only observed for situations where the amino group is involved in an H-bonding interaction. This is attributed to electron redistribution induced by the presence of the second cyanamide molecule. Similar electron-redistribution effects are thought to be responsible for the observed distance dependence of computed (1)J(NH) couplings of H-bonded amino groups in near-planar G-quartet structures. Here, the |(1)J(NH)| couplings of the amino N-H bonds decreased with increasing N-H bond length whereas the |(1)J(N(d)H(d))| couplings are approximately 7 Hz larger than the |(1)J(N(d)H(free))| couplings, despite the longer N(d)-H(d) bond length.     

Amplification of chirality in hydrogen-bonded tetrarosette helices

The amplification of chirality in hydrogen-bonded tetrarosette assemblies under thermodynamic equilibrium is described. The extent of the chiral amplification obtained by means of "sergeants-and-soldiers" experiments depends only on the structure of the assembly and it is independent of the methodology used for the formation of the tetrarosette assemblies. The difference in free energy (deltaG(o)(M/P)) between the M- and P-diastereomeric helices is up to 40 times higher than for double rosette assemblies.     

Self-assembly of folic acid derivatives: induction of supramolecular chirality by hierarchical chiral structures

Hierarchical chiral structures made up of dendritic oligo(L- or D-glutamic acid) moieties of folic acid derivatives induce supramolecular chirality in the self-assembled columnar structures of the folic acids. These folic acids self-assemble through the intermolecular hydrogen bonds of the pterin rings to form disklike tetramers. In the neat states, the stacked tetramers form thermotropic hexagonal columnar phases over wide temperature ranges, including room temperature. Addition of alkali metal salts induces chirality in the columnar phases. In dilute solution states in a relatively polar solvent (chloroform), the folic acid derivatives form non-chiral, self-assembled structures. In the presence of sodium triflate, the folic acid forms chiral columnar assemblies through the oligo(L-glutamic acid) moiety, similar to those formed in the liquid-crystalline (LC) states. The enantiomer of the folic acid induces columnar assemblies with reversed helicity. In the case of the diastereomer, no induced helicity is observed. Application of an apolar solvent (dodecane) drives the folic acid derivatives to form chiral assemblies in the absence of ions. In this case, lipophilic interactions promote nanophase segregation, which enhances the formation of chiral columns. Interestingly, the chiral supramolecular structure of the diastereomer induces the most intense circular dichroism. In both cases, the molecular chirality in the oligo(glutamate) moieties yields supramolecular chirality of the folic acids that self-assemble through cooperative molecular interactions.