Theoretical Study of Weakly Bound Dimers between Hydrogen Fluoride and Some Polar Molecules

Weakly bound linear and bent dimers, FH—X (where X = CO, OC, CNH, NCH, N₂O and ON₂), are investigated using the DFT B3LYP and ab initio MP2 methods with the same basis sets (6–311++G(3df,2pd)). The strengths of the H—C or H—N H‐bonds in dimers FH—CO, FH—CNH, and FH—N₂O are compared with those of the H—O or H—N H‐bonds in dimers FH—OC, FH—NCH, and FH—ON₂. The results obtained for the H‐bond distances, the elongation effect of the HF bond, the red shift of the HF stretching frequency, and the energy difference between the dimer and the charge transfer reveal that the H‐bonds of the first group of dimers are stronger than those of the second. The Gibbs energies calculated for the six dimer formations indicate that the weakly bound dimers are unstable at room temperature (T = 298 K) (FH—X's → FH + X's, ΔG < 0).     

Phototoxic effects of Hypericum extract in cultures of human keratinocytes compared with those of psoralen

Extracts of Hypericum perforatum (St. John's wort) are used in the treatment of depression. They contain the plant pigment hypericin and hypericin derivates. These compounds have light-dependent activities. In order to estimate the potential risk of phototoxic skin damage during antidepressive therapy, we investigated the phototoxic activity of hypericin extract using cultures of human keratinocytes and compared it with the effect of the well-known phototoxic agent psoralen. The absorbance spectrum of our Hypericum extract revealed maxima in the whole UV range and in parts of the visible range. We cultivated human keratinocytes in the presence of different Hypericum concentrations and irradiated the cells with 150 mJ/cm2 UVB, 1 J/cm2 UVA or 3 h with a white light of photon flux density 2.6 mumol m-2 s-1. The determination of the bromodeoxyuridine incorporation rate showed a concentration- and light-dependent decrease in DNA synthesis with high hypericin concentrations (> or = 50 micrograms/mL) combined with UVA or visible light radiation. In the case of UVB irradiation a clear phototoxic cell reaction was not detected. We found phototoxic effects even with 10 ng/mL psoralen using UVA with the same study design as in the case of the Hypericum extract. These results confirm the phototoxic activity of Hypericum extract on human keratinocytes. However, the blood levels that are to be expected during antidepressive therapy are presumably too low to induce phototoxic skin reactions.     

The effect of molecular planarity on crystal non‐centrosymmetry in benzyl­idene–aniline derivatives

In the title compound, N‐(2‐methoxy­phenyl)‐4‐nitro­benzyli­deneamine, C₁₄H₁₂N₂O₃, the two phenyl rings make a dihedral angle of 48.0 (2)° and the nitro group is at an angle of 6.5 (1)° with respect to its attached phenyl ring. In the crystal structure, mol­ecules are related as centrosymmetric pairs through π–π interactions and are further connected through strong C—H?O hydrogen bonds [C?O 3.4259 (17) Å and C—H?O 167°], forming molecular stacks along [100]. These stacks associate further through longer C—H?O interactions, forming two‐dimensional networks. In the c direction, there are only weak van der Waals interactions. The relationship between the molecular planarity and its centrosymmetry is also briefly described.     

A unique tautomer of 1‐n‐hexyl‐3‐phenyl‐1H‐pyrazol‐5‐ol

In the title compound, C₁₅H₂₀N₂O, the bond distances and angles are consistent with the presence of the hydroxy tautomer. This tautomer was unambiguously determined by the clear presence of a H atom bonded to oxygen, as well as the total absence of any residual electron density around the N atom in the heterocycle, thus precluding any possibility of desmotropism.     

Hydrogen bonding and π–π stacking in 6‐hydroxy­biochanin A monohydrate

In the lattice of the title compound (systematic name: 5,6,7‐trihydroxy‐4′‐meth­oxy­isoflavone monohydrate), C₁₆H₁₂O₆·H₂O, the isoflavone mol­ecules are linked into chains through R₄³(17) motifs composed via O—H⋯O and C—H⋯O hydrogen bonds. Centrosymmetric R₄²(14) motifs assemble the chains into sheets. Hydrogen‐bonding and aromatic π–π stacking inter­actions lead to the formation of a three‐dimensional network structure.     

A New Chemo‐Enzymatic Route to Side‐Chain Liquid‐Crystalline Polymers: The Synthesis and Polymerization of 6‐(4‐Methoxybiphenyl‐4′‐oxy)hexyl Vinyl Hexanedioate

A novel synthetic method combining chemo and enzymatic synthesis strategies was employed to prepare a vinyl acetate type monomer, 6‐(4‐methoxybiphenyl‐4′‐oxy)hexyl vinyl hexanedioate (VA‐LC). Homo‐ and copolymers of VA‐LC with maleic anhydride (MAn) were prepared by conventional free radical polymerization using 2,2′‐azobisisobutyronitrile (AIBN) and 1,1′‐azobis (cyclohexane carbonitrile) (AHCN) as an initiator at 95 and 60 °C, respectively. The thermal properties of the generated polymeric material were investigated by differential scanning calorimetry (DSC), and the optical texture was inspected by polarizing optical microscopy (POM). While the monomer VA‐LC does not exhibit liquid‐crystalline properties, poly(VA‐LC), and the alternating copolymer of VA‐LC with maleic anhydride both displayed such properties.

     

Signatures andS-discrete lattice-ordered groups

The central theme of this article is the approximation of lattice-ordered groups (l-groups) first by Specker groups and, subsequently, by the so-calledS-discretel-groups. The sense of these approximations is made precise via the notion of a signature, defined below, and by that ofa ^*-subgroups. Sample result: ifG is a projectablel-group then it has anl-subgroupH which is Specker and for which the mapPPH defines a boolean isomorphism between the algebras of polars ofG andH.Presented by L. Fuchs.This article was written while this author was a Stouffer Visiting Professor at the University of Kansas. He wishes to thank the members of the Mathematics Department of that institution for their hospitality.     

Achiral Calcium‐Oxalate Crystals with Chiral Morphology from the Leaves of Some Solanacea Plants

The leaves of some plants, particularly among the Solanacea, contain crystals of calcium oxalate with a peculiar chiral pseudo‐tetrahedral morphology, even though the calcium oxalate crystal structure is centrosymmetric, hence achiral. We studied the morphology of these crystals extracted from the leaves of three Solanacea plants: the potato, the hot pepper, and a species of wild Solanum. The crystal morphology was the same in all three species. Based on the examination of more than 100 crystals from each plant, we showed that the crystal morphology is chiral with invariant chirality. We suggest that morphological chirality is induced by macromolecules during nucleation from a specific, genetically encoded crystal plane, and is further established during subsequent controlled crystal growth. This is one of few examples where it is possible to deduce a molecular mechanism for biologically induced breaking of morphological symmetry in organisms. A very high level of recognition is required by the macromolecules to allow them to distinguish between symmetry‐related crystal planes. It is also surprising that this finely controlled mechanism of crystal formation, including the chiral morphology, has been conserved during evolution.