A temperature-controlled electric field sample environment for small-angle neutron scattering experiments.

A new sample environment is introduced for the study of soft matter samples in electric fields using small-angle neutron scattering instruments. The sample environment is temperature controlled and features external electrodes, allowing standard quartz cuvettes to be used and conducting samples or samples containing ions to be investigated without the risk of electrochemical reactions occurring at the electrodes. For standard 12.5 mm quartz cuvettes, the maximum applied field is 8 kV/cm, and the applied field may be static or alternating (up to 10 kHz for 8 kV/cm and up to 60 kHz for 4 kV/cm). The electric fields within the sample are calculated and simulated under a number of different conditions, and the capabilities of the setup are demonstrated using a variety of liquid crystalline samples. Measurements were performed as a function of temperature and time spent in the electric field. Finally, the advantages, drawbacks, and potential optimization of the sample environment are discussed with reference to applications in the fields of complex soft matter, biology, and electrorheology.

Synthesis and application of resorufin ß-D-glucuronide, a low-cost chromogenic substrate for detecting Escherichia coli in drinking water.

The development of low-cost tests for Escherichia coli is hampered by the expense and limited choice of enzyme substrates. Most chromogenic substrates are required in costly amounts, while fluorogenic substrates require an additional apparatus (e.g., an ultraviolet lamp) to be detected. Herein, we propose an alternative chromogenic substrate, resorufin ß-d-glucuronide (REG), which is exceptionally sensitive and may be employed in very small amounts. We show that REG can be produced similarly to other simple glucuronides and should therefore be no more expensive. The compound is used by both healthy and injured E. coli, resulting in a pronounced color change from orange to a bright pink. Because the released dye (resorufin) has a high extinction coefficient, substantially lower amounts are needed than for commercially available substrates. The potential of this substrate is demonstrated by a presence/absence test requiring just 0.1 mg of REG/100 mL of water sample, one hundredth of the quantity needed for common chromogenic substrates, with an estimated bulk cost of ≤0.1 U.S. cents/test. REG shows promise as a chromogenic substrate for E. coli detection and should be considered in the development of new water tests, especially for low-income settings.

Tunable porous organic crystals: structural scope and adsorption properties of nanoporous steroidal ureas.

Previous work has shown that certain steroidal bis-(N-phenyl)ureas, derived from cholic acid, form crystals in the P6(1) space group with unusually wide unidimensional pores. A key feature of the nanoporous steroidal urea (NPSU) structure is that groups at either end of the steroid are directed into the channels and may in principle be altered without disturbing the crystal packing. Herein we report an expanded study of this system, which increases the structural variety of NPSUs and also examines their inclusion properties. Nineteen new NPSU crystal structures are described, to add to the six which were previously reported. The materials show wide variations in channel size, shape, and chemical nature. Minimum pore diameters vary from ~0 up to 13.1 Å, while some of the interior surfaces are markedly corrugated. Several variants possess functional groups positioned in the channels with potential to interact with guest molecules. Inclusion studies were performed using a relatively accessible tris-(N-phenyl)urea. Solvent removal was possible without crystal degradation, and gas adsorption could be demonstrated. Organic molecules ranging from simple aromatics (e.g., aniline and chlorobenzene) to the much larger squalene (M(w) = 411) could be adsorbed from the liquid state, while several dyes were taken up from solutions in ether. Some dyes gave dichroic complexes, implying alignment of the chromophores in the NPSU channels. Notably, these complexes were formed by direct adsorption rather than cocrystallization, emphasizing the unusually robust nature of these organic molecular hosts.

Vicinal diphosphoniums: electrostatic repulsion under covalent constraint.

A series of five vicinal bis(alkyl-triarylphosphoniums) derived from o-bis(diphenyl-phosphino)benzene (o-dppb) is described. Each of them have been prepared by specific methods, and their formal electrostatic and possible van der Waals strain is compared through the P+...P+ distances in the crystal state. According to X-ray diffraction analyses, while the conformations of the dimethyl dication is C2 symmetric, the conformation of alka-1,n-diyl-diphosphoniums (n = 1, 2, 3) is pseudo-Cs symmetric. The solution structure of the dimethyldiphosphonium 2 was studied by NMR techniques after metathesis of the triflate counterions with enantiomerically pure X-PHAT chiral anions (X = TRIS, BIN). [(Delta)-BINPHAT] was indirectly shown to discriminate between the enantiomers of the diphosphonium, but no effective enantio-differentiation could be measured down to -70 degrees C in CD2Cl2. When associated to [(Delta)-TRISPHAT] counterions, progressive cleavage of the diphosphonium afforded the corresponding methyl-triphenyl-monophosphonium salt. In the bridged series, the diphosphoniacyclopentene dication was found to be stable in spite of the very short transannular P+...P+ distance (2.83 A). The naturally more relaxed homologous diphosphoniacyclohexene dication (P+...P+ = 3.28 A) was found to be flexible in solution, as indicated by the equivalence of the four protons of the +P-CH2CH2-P+ bridge from 20 degrees C to -80 degrees C in acetone-d6. The two diphosphoniacycloheptene dications exhibit no P+...P+ steric compression (ca 3.64 A), just as the dimethyl dication (3.70 A). In solution, the seven-membered ring of the +P-CH2CR2CH2-P+ dications (R = H, Me) remains rigidly Cs symmetric: no interconversion of the pseudo-axial and pseudo-equatorial R groups is evidenced at the NMR time scale at -95 degrees C (coalescence at -20 degrees C for R = H, at -65 degrees C for R = Me). According to DFT calculations at the B3PW91/6-31G** level in 1,2-dichloroethane (DCE: PCM, epsilon = 10.36), the lowest energy conformation is indeed Cs-symmetric. However a quasi-isoenergetic C2-symmetric conformation which is the most stable in the gas phase is also found, and is thus a likely intermediate in the observed Cs <==> Cs interconversion. This +P-CH2CMe2CH2-P+ diphosphonium was obtained by dialkylation of o-dppb with 2,2-dimethylpropan-1,3-diyl bistriflate in DCE at 80 degrees C. The use of this solvent was the key for the synthesis of this dication, which was however competitively produced along with some +P-CH2CH2-P+ dication. The latter was also obtained by a totally different route, by heating a sulfinylethyl monophosphonium salt of o-dppb in the presence of the cationic complex [Rh(cod)2][PF6]. A mechanism for this peculiar process is proposed.

Structure-activity relationships in cholapod anion carriers: enhanced transmembrane chloride transport through substituent tuning.

Chloride transport by a series of steroid-based "cholapod" receptors/carriers was studied in vesicles. The principal method involved preincorporation of the cholapods in the vesicle membranes, and the use of lucigenin fluorescence quenching to detect inward-transported Cl-. The results showed a partial correlation between anion affinity and transport activity, in that changes at the steroidal 7 and 12 positions affected both properties in concert. However, changes at the steroidal 3-position yielded irregular effects. Among the new steroids investigated the bis-p-nitrophenylthiourea 3 showed unprecedented activity, giving measurable transport through membranes with a transporter/lipid ratio of 1:250 000 (an average of <2 transporter molecules per vesicle). Increasing transporter lipophilicity had no effect, and positively charged steroids had low activity. The p-nitrophenyl monourea 25 showed modest but significant activity. Measurements using a second method, requiring the addition of transporters to preformed vesicle suspensions, implied that transporter delivery was problematic in some cases. A series of measurements employing membranes of different thicknesses provided further evidence that the cholapods act as mobile anion carriers.

The "triamino-analogue" of methyl allocholate; a rigid, functionalised scaffold for supramolecular chemistry.

Cholic acid has been converted into triamine with the all-trans polycyclic allocholanoyl skeleton and co-directed, axial amino groups; the potential of this system as a scaffold is illustrated by conversion to a preorganised anion receptor.

Substrate discrimination by cholapod anion receptors: geometric effects and the "affinity-selectivity principle".

Cholapod anion receptors can achieve high affinities while maintaining compatibility with nonpolar media. Previously they have been shown to transport anions across cell and vesicle membranes. In the present work, the scope of the architecture is expanded and structure-selectivity relationships are investigated. Eight new receptors have been synthesized, with up to six H-bond donor centers. Using Cram's extraction method, these compounds plus five known examples have been tested for binding to seven monovalent anions (tetraethylammonium salts, wet chloroform as solvent). Association constants in excess of 10(10) M(-1) have been measured for several pairings. Selectivities vary with receptor geometry, as expected. More remarkably, they also depend on receptor strength: more powerful receptors show a wider range of binding free energies, and therefore a greater spread of Ka(X-)/Ka(Y-). This "affinity-selectivity" effect can be derived from empirical relationships for H-bond strengths, and could prove widely operative in supramolecular chemistry.

The first linear multiphosphazene having five different types of side groups and its use as the core of a dendrimeric species.

The step-by-step synthesis of the first oligophosphazenes having more than two types of side functions is described. These multifunctionalized oligophosphazenes possess up to four phosphazene linkages and up to four types of functional side groups. These compounds may serve as models for a better understanding of electronic delocalization phenomena in linear inorganic chains, and constitute novel cores for the synthesis of new dendrimeric species possessing several types of functions as end groups, at the level of the core, and within the branches.