pH-dependent sedimentation of DNA in the presence of divalent, but not monovalent, metal ions.

Precipitation of DNA is performed frequently in molecular biology laboratories for the purpose of purification and concentration of samples and also for transfer of DNA into cells. Metal ions are used to facilitate these processes, though their precise functions are not well characterized. In the current study we have investigated the precipitation of double-stranded DNA by group 1 and group 2 metal ions. Double-stranded DNAs were not sedimented efficiently by metals alone, even at high concentrations. Increasing the pH to 11 or higher caused strong DNA precipitation in the presence of the divalent group 2 metals magnesium, calcium, strontium and barium, but not group 1 metals. Group 2 sedimentation profiles were distinctly different from that of the transition metal zinc, which caused precipitation at pH 8. Analysis of DNAs recovered from precipitates formed with calcium revealed that structural integrity was retained and that sedimentation efficiency was largely size-independent above 400 bp. Several tests supported a model whereby single-stranded DNA regions formed by denaturation at high pH became bound by the divalent metal cations. Neutralization of negative surface charges reduced the repulsive forces between molecules, leading to formation of insoluble aggregates that could be further stabilized by cation bridging (ionic crosslinking).

Complexing Agent Directed Growth of α-Zirconium Phosphate-Based Hexagonal Prisms.

A layered prism is an ideal system for fundamental studies due to its unique structure with uniform-sized sheets. However, there are very limited reports in the last few decades on the preparation of such materials. In this contribution, we report for the first time the preparation of α-ZrP intercalation compound-based hexagonal prisms. Preferential crystal growth perpendicular to the (001) crystal plane of α-ZrP intercalation compounds was achieved by incorporating a complexing agent and a layer growth coordinator into a crystal growth reaction system. With the presence of a layer growth coordinator to coordinate the crystal growth perpendicular to the (001) crystal plane and the presence of a complexing agent to slow down the crystal growth rate, the previously unknown layer growth coordination effect is revealed. After a facile ion exchange treatment, pure α-ZrP hexagonal prisms can also be obtained.

Isolation and Crystallographic Characterization of Gd3N@D2(35)-C88 through Non-Chromatographic Methods.

While several nonchromatographic methods are available for the isolation and purification of endohedral fullerenes of the type M3N@Ih-C80, little work has been done that would allow other members of the M3N@C2n family to be isolated with minimal chromatography. Here, we report that Gd3N@D2(35)-C88 can be isolated from the multitude of endohedral and empty cage fullerenes present in carbon soot obtained by electric-arc synthesis using Gd2O3-doped graphite rods. The procedure developed utilizes successive precipitation with the Lewis acids CaCl2 and ZnCl2 followed by treatment with amino-functionalized silica gel. The structure of the product was identified by single-crystal X-ray diffraction.

Direct growth of layered intercalation compounds via single step one-pot in situ synthesis.

A single step one-pot in situ synthesis method was developed to directly grow layered intercalation compounds. This methdology is expected to be applicable to a wide range of layered materials.

One-step direct synthesis of layered double hydroxide single-layer nanosheets.

Layered double hydroxide (LDH) single-layer nanosheets were traditionally prepared through a multi-step exfoliation process which is very time-consuming and of low efficiency. Herein we report the preparation of LDH single-layer nanosheets through a facile direct synthesis method. By introducing a layer growth inhibitor, one can directly synthesize LDH single-layer nanosheets instead of LDH layered compounds. The inhibitor weakens the interactions between neighboring layers, thus preventing the interlayer growth. This investigation on blocking interlayer growth by weakening interlayer interactions to obtain inorganic single-layer nanosheets opens a new route for the synthesis of 2-dimensional materials.

Tuning the selectivity of Gd3N cluster endohedral metallofullerene reactions with Lewis acids.

We demonstrate the manipulation of the Lewis acid strength to selectively fractionate different types of Gd3N metallofullerenes that are present in complex mixtures. Carbon disulfide is used for all Lewis acid studies. CaCl2 exhibits the lowest reactivity but the highest selectivity by precipitating only those gadolinium metallofullerenes with the lowest first oxidation potentials. ZnCl2 selectively complexes Gd3N@C88 during the first 4 h of reaction. Reaction with ZnCl2 for an additional 7 days permits a selective precipitation of Gd3N@C84 as the dominant endohedral isolated. A third fraction is the filtrate, which possesses Gd3N@C86 and Gd3N@C80 as the two dominant metallofullerenes. The order of increasing reactivity and decreasing selectivity (left to right) is as follows: CaCl2 < ZnCl2 < NiCl2 < MgCl2 < MnCl2 < CuCl2 < WCl4 ≪ WCl6 < ZrCl4 < AlCl3 < FeCl3. As a group, CaCl2, ZnCl2, and NiCl2 are the weakest Lewis acids and have the highest selectivity because of their very low precipitation onsets, which are below +0.19 V (i.e., endohedrals with first oxidation potentials below +0.19 V are precipitated). For CaCl2, the precipitation threshold is estimated at a remarkably low value of +0.06 V. Because most endohedrals possess first oxidation potentials significantly higher than +0.06 V, CaCl2 is especially useful in its ability to precipitate only a select group of gadolinium metallofullerenes. The Lewis acids of intermediate reactivity (i.e., precipitation onsets estimated between +0.19 and +0.4 V) are MgCl2, MnCl2, CuCl2, and WCl4. The strongest Lewis acids (WCl6, ZrCl4, AlCl3, and FeCl3) are the least selective and tend to precipitate the entire family of gadolinium metallofullerenes. Tuning the Lewis acid for a specific type of endohedral should be useful in a nonchromatographic purification method. The ability to control which metallofullerenes are permitted to precipitate and which endohedrals would remain in solution is a key outcome of this work.

Benzobisoxazole cruciforms: heterocyclic fluorophores with spatially separated frontier molecular orbitals.

We report the synthesis of nine conjugated cruciform-shaped molecules based on the central benzo[1,2-d:4,5-d']bisoxazole nucleus, at which two conjugated currents intersect at a ~90° angle. Cruciforms' substituents were varied pairwise among the electron-neutral phenyl groups, electron-rich 4-(N,N-dimethylamino)phenyl substituents, and electron-poor pyridines. Hybrid density functional theory calculations revealed that the highest occupied molecular orbitals (HOMOs) are localized (24-99%) in all cruciforms, in contrast to the lowest unoccupied molecular orbitals (LUMOs) which are strongly dependent on the substitution and less localized (6-64%). Localization of frontier molecular orbitals (FMOs) along different axes of these cruciforms makes them promising as sensing platforms, since analyte binding to the cruciform should mandate a change in the HOMO-LUMO gap and the resultant optical properties. This prediction was verified using UV/vis absorption and emission spectroscopy: cruciforms' protonation results in hypsochromic and bathochromic shifts consistent with the preferential stabilization of HOMO and LUMO, respectively. In donor-acceptor-substituted systems, a two-step optical response to protonation was observed, wherein an initial bathochromic shift is followed by a hypsochromic one with continued acidification. X-ray diffraction studies of three selected cruciforms revealed the expected ~90° angle between the cruciform's substituents, and crystal packing patterns dominated by [π···π] stacking and edge-to-face [C-H···π] contacts.

Synthesis of a family of solids through the building-block approach: a case study with Ag(+) substitution in the ternary Na-Ge-Se system.

A new family of Ag-substituted pseudoquaternary alkali-seleno-germanates has been synthesized by two solid-state routes: the conventional flux method and metathesis. This family includes a series of semiconductors with varying amounts of Ag+ substituted for Na+ in Na8Ge4Se10 to form AgxNa(8-x)Ge4Se10, [x = 0.31 (I), 0.67 (II), 0.77 (III), 0.87 (IV), 1.05 (V), 1.09 (VI)] and another phase with a different composition AgxNa(6-x)Ge2Se7 (x = 1.76), VII, related to Na6Ge2Se7. In I-VI, Ge4Se10(8-) constitutes a 6-membered chairlike unit with a Ge-Ge bond, while in VII, a corner-shared dimer of GeSe4 tetrahedra (Ge2Se76-) acts as the building unit. The single-crystal structure analysis indicates that there is a phase transition from P to C2/c, in changing from pure Na8Ge4Se10 to AgxNa(8-x)Ge4 Se10 (I-VI), while there is no phase transition between pure Na6Ge2Se7 and AgxNa(6-x)Ge2Se7 (x = 1.76). The structures of I-VI may be described in terms of layers of cubic close-packed Se2- anions. In between the Se layers, octahedral holes fully occupied by Na+ and mixed Ag+/Na+ cations alternate with layers formed of octahedral holes fully occupied by Na+ and Ge26+ cations. Two adjacent Ge26+ cations form a chairlike Ge4Se10(8-) anion in which Ge-Ge bonds are oriented almost parallel to the Se layers. In contrast, VII does not have close-packed anions. Corner-shared GeSe4 tetrahedra (Ge2Se7(6-) dimer) and AgSe4 tetrahedra form layers that are cross-linked by Na/AgSe4 tetrahedra to form a 3-dimensional (3-D) structure. An optical property investigation indicates a red shift in the band gap of AgxNa(8-x)Ge4Se10 (x = 0.67)(II) as compared to that of pure Na8Ge4Se10. Raman data also indicate a red shift of the Ge-Se stretching mode in the Ag+-substituted phase II (x = 0.67) compared to that of Na8Ge4Se10.

Molten flux synthesis of an analogous series of layered alkali samarium selenogermanate compounds.

In this work, we used the molten chalcogenide flux synthetic method to form an analogous series of alkali samarium selenogermanates, with the general formula ASmGeSe(4) (A = K, Rb, Cs). Using a constant reactant stoichiometry, we relate the monoclinic KLaGeSe(4) structure type to the orthorhombic CsSmGeS(4) structure type. KSmGeSe(4) [in space group P2(1) with cell parameters a = 6.774(1) A, b = 6.994(1) A, c = 8.960(2) A, beta = 108.225(3) degrees, and V = 403.2(1) A(3) (Z = 2)], RbSmGeSe(4) [in space group P2(1)2(1)2(1) with cell parameters a = 6.7347(8) A, b = 7.0185(9) A, c = 17.723(2) A, and V = 837.7(2) A(3) (Z = 4)], and CsSmGeSe(4) [in space group P2(1)2(1)2(1) with cell parameters a = 6.707(2) A, b = 7.067(2) A, c = 18.334(6) A, and V = 869.1(5) A(3) (Z = 4)] were formed under identical synthetic conditions by changing the identity of the alkali ion from K to Rb or Cs, respectively. Additionally, with the substitution of sodium into the reaction, a triclinic structure with the approximate formula NaSmGeSe(4) was found with the cell parameters a = 6.897(2) A, b = 9.919(2) A, c = 11.183(2) A, alpha = 84.067(4) degrees, beta = 88.105(4) degrees, gamma = 73.999(4) degrees, and V = 731.5(3) A(3). In addition to single-crystal diffraction, Raman and diffuse reflectance UV-visible spectroscopic measurements have been used to characterize these compounds.