Inversion symmetry and local vs. dispersive interactions in the nucleation of hydrogen bonded cyclic n-mer and tape of imidazolecarboxamidines.

Substitutional changes to imidazolecarboxamidine that preserved intermolecular hydrogen bonding in the solid state were used to study the relationship between packing and the hydrogen bond motif. Various motifs competed, but the most common imidazolecarboxamidine crystalline phase was a C(i) symmetric dimer that established inversion centers by associating enantiomeric tautomers. Counter to intuition, the calculated gas-phase energies per molecule of the solid state atomic coordinates of the C(i) dimer motifs were higher than those of the C(1) dimer, trimer, tetramer and tape motifs, while the packing densities of C(i) dimers were found to be higher. This result was interpreted as an enhanced ability of the C(i) dimers to pack. If other motifs competed, the hydrogen bonds and conformations should be lower in energy than the C(i) dimer. The results detail the effect of packing on the conformation in these molecules. The results are interpreted as a rough measure of the energetic compromise between packing and the energies related to the coordinates involving one dihedral angle and hydrogen bonding. The results establish a connection between solution and solid phase conformation.

De novo design and spectroscopic characterization of a dinucleating copper-binding pentadecapeptide.

A spectroscopic study of aqueous solutions of Ac-WGHGHGHGPGHGHGH-NH(2) (HGP) indicates that copper(II) binds to the peptide to form a 2:1 Cu(2+)/HGP complex with four nitrogen atoms in the copper coordination environment. Electron paramagnetic resonance (EPR) and UV-visible data suggest copper binding through the peptide backbone and imidazole nitrogen donors. Circular dichroism data show that HGP is unbound below pH 5.5 and is copper-saturated at pH 9 and above. The apo form of the peptide is unstructured in solution and is organized into a turn conformation in the presence of 2 mol equiv of Cu(2+) at basic pH. EPR measurements for 2:1 Cu(2+)/HGP solutions in the g = 2 region and within the pH range 7-11 exhibit axial spectra. A molecular-mechanics-minimized model of the Cu(2+)/HGP complex gave a Cu...Cu separation of 8 A.

A helical structure with Z' = 10.

The structure of N''-cyano-N,N-diisopropylguanidine, a push-pull nitrile that includes the H(2)N-C=N-CN fragment, has been determined. Although the number of molecules in the asymmetric unit is exceptionally high (Z' = 10), there were no special crystallographic difficulties associated with data collection, structure solution or structure refinement because the individual molecules are small (12 non-H atoms) and because there is no important crystallographic pseudosymmetry. A complete set of N-H...N bonds is formed. Pairs of molecules form dimers, which associate to form ribbons that twist into a helix having five dimers per turn. The helices lie parallel to [101] and fit together with close contacts in three different directions perpendicular to that axis. The problems with optimizing so many different intermolecular contacts lead to a loosely packed structure.

Monitoring the growth of polyoxomolybdate nanoparticles in suspension by flow field-flow fractionation.

We follow the evolution of polyoxomolybdate nanoparticles in suspensions derived from the keplerate (NH4)42[MoVI72MoV60O372(CH3CO2)30(H2O)72].ca..300H2O.ca..10CH3CO2NH4 ({Mo132}) by flow field-flow fractionation (FlFFF) to monitor the particle-size distribution in situ, atomic force and high-resolution transmission electron microscopy (AFM, SEM, and HRTEM) to confirm particle sizes, inductively coupled plasma-optical emission spectrometry (ICP-OES) to determine the Mo content of the FlFFF-separated fractions, and UV/visible spectroscopy to confirm the identity of the species in suspension. We observe the formation of 3-75-nm polyoxomolybdate particles in suspension and the dynamic growth of {Mo132} crystals.