Crystalline structure and thermotropic behavior of alkyltrimethylphosphonium amphiphiles.

Quaternary organophosphonium salts bearing long alkyl chains are cationic surfactants of interest owing to their physical and biological properties. In the present work, the crystal structure and thermotropic behavior of the homologous series of alkyltrimethylphosphonium bromides (nATMP·Br), with the alkyl chain containing an even number (n) of carbon atoms from 12 to 22, have been examined within the 0-300 °C range of temperatures. These compounds were shown to be resistant to heat up to ∼390 °C. The phases adopted at different temperatures were detected by DSC, and the structural changes involved in the phase transitions have been characterized by simultaneous WAXS and SAXS carried out in real-time, and by polarizing optical microscopy as well. Three or four phases were identified for n = 12 and 14 or n ≥ 16, respectively, in agreement with the heat exchange peaks observed by DSC. The phase existing at room temperature (Ph-I) was found to be fully crystalline and its crystal lattice was determined by single-crystal X-ray diffraction methods. Ph-II consisted of a semicrystalline structure that can be categorized as Smectic-B with the crystallized ionic pairs hexagonally arranged in layers and the molten alkyl chain confined in the interlayer space. Ph-II of 12ATMP·Br and 14ATMP·Br directly isotropicized upon heating at ∼220 °C, whereas for n ≥ 16, it converted into a Smectic-A phase (Ph-III) that needed to be heated above ∼240 °C to become isotropic (Ph-Is). The correlation existing between the thermal behavior, phase structure and length of the alkyl side chain has been demonstrated.

Stabilization by extra-helical thymines of a DNA duplex with Hoogsteen base pairs.

We present the crystal structure of the DNA duplex formed by d(ATATATCT). The crystals contain seven stacked antiparallel duplexes in the asymmetric unit with A.T Hoogsteen base pairs. The terminal CT sequences bend over so that the thymines enter the minor groove and form a hydrogen bond with thymine 2 of the complementary strand in the Hoogsteen duplex. Cytosines occupy extra-helical positions; they contribute to the crystal lattice through various kinds of interactions, including a unique CAA triplet. The presence of thymine in the minor groove apparently contributes to the stability of the DNA duplex in the Hoogsteen conformation. These observations open the way toward finding under what conditions the Hoogsteen duplex may be stabilized in vivo. The present crystal structure also confirms the tendency of A.T-rich oligonucleotides to crystallize as long helical stacks of duplexes.

Ionic coupling of hyaluronic acid with ethyl N-lauroyl l-arginate (LAE): Structure, properties and biocide activity of complexes.

Ethyl αN-lauroyl l-arginate hydrochloride (LAE) was coupled with hyaluronic acid (HyA) to form ionic complexes with LAE to HyA ratios of 1:1 and 1:2. The complexes were extensively characterized by FTIR and NMR spectroscopies and their thermal properties evaluated by thermogravimetry and calorimetry. Thin films prepared from these complexes by casting displayed a smectic-like structure based on an ordered arrangement of LAE and HyA layers with a nanometric periodicity of 3.8-3.9 nm. Films immersed in water at pH 7.4 and 5.5 dissociated to deliver free LAE to the environment and reaching the equilibrium in few hours. The biocide activity of these films against both Gram-positive and Gram-negative bacteria was preliminary assessed by the liquid medium method, and shown to be notable in both cases. The antibacterial property of the complexes was found to increase with the content of LAE and to be particularly efficient against Gram-negative S. enterica bacteria.

Morpholine-2,5-dione.

In the title compound, C4H5NO3, the morpholine ring adopts a boat conformation that is distorted towards twist-boat, the boat ends being the two Csp3 atoms of the ring. The molecular packing is stabilized by the establishment of strong intermolecular NH...OC hydrogen bonds, which give rise to centrosymmetric dimers, and a network of weak CH2...OC hydrogen bonds, where each dimer interacts with eight neighbouring morpholinedione rings.

Overview of the structure of all-AT oligonucleotides: organization in helices and packing interactions.

We present the crystalline organization of 33 all-AT deoxyoligonucleotide duplexes, studied by x-ray diffraction. Most of them have very similar structures, with Watson-Crick basepairs and a standard average twist close to 36 degrees. The molecules are organized as parallel columns of stacked duplexes in a helical arrangement. Such organization of duplexes is very regular and repetitive: all sequences show the same pattern. It is mainly determined by the stacking of the terminal basepairs, so that the twist in the virtual TA base step between neighbor duplexes is always negative, approximately -22 degrees. The distance between the axes of parallel columns is practically identical in all cases, approximately 26 A. Interestingly, it coincides with that found in DNA viruses and fibers in their hexagonal phase. It appears to be a characteristic distance for ordered parallel DNA molecules. This feature is due to the absence of short range intermolecular forces, which are usually due to the presence of CG basepairs at the end of the oligonucleotide sequence. The duplexes apparently interact only through their diffuse ionic atmospheres. The results obtained can thus be considered as intermediate between liquid crystals, fibers, and standard crystal structures. They provide new information on medium range DNA-DNA interactions.

N,N'-Butane-1,4-diylbis(bromoacetamide).

The title compound, C8H14Br2N2O2, lies about an inversion centre and adopts a pleated conformation, with the C(O)-NH-CH2-CH2 and NH-CH2-CH2-CH2 torsion angles of the butanediamine residue being -89.5 (6) and -62.1 (7) degrees, respectively. These data are useful in discerning the structure of polymers containing such a unit. A skew conformation is found for the Br-CH(2)-C(O)-NH torsion angle [-124.2 (4) degrees]. The molecular packing is stabilized by strong hydrogen bonds between amide groups and also by weak CH2...OC interactions. In this way, each molecule interacts with its six closest neighbours through eight hydrogen bonds.

Butane-1,4-diyl bis(chloroacetate).

The title compound, C(8)H(12)Cl(2)O(4), lies about an inversion centre. The molecular conformation is characterized by a tgt-gt conformation for the butanedioxy moiety and a trans conformation for the ClCH(2)-C(=O)O bond. The molecular packing is stabilized by a network of weak CH(2)...O=C intermolecular hydrogen bonds, where each molecule interacts with its four closest neighbours.

N-chloroacetyl-beta-alanine.

The beta-alanine residue of the title compound, C(5)H(8)ClNO(3), has a ggt folded conformation, which is mainly stabilized through intermolecular N-H...O=C (amide-acid) and O-H...O=C (acid-amide) hydrogen bonds. In addition, a cis conformation is found for the Cl-CH(2)-C(=O)-NH torsion angle, which is associated with the presence of an intramolecular hydrogen bond.