A simple class of photorheological fluids: surfactant solutions with viscosity tunable by light

Photorheological (PR) fluids, i.e., those with light-tunable rheological properties, may be useful in a variety of applications, such as in sensors and microfluidic devices. Currently, the need to synthesize complex photosensitive molecules hampers the applicability of these fluids. Here, we report a simple class of PR fluids that require no special synthesis and can be easily replicated in any lab from inexpensive chemicals. The fluids consist of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), and the photoresponsive organic derivative, trans-ortho-methoxycinnamic acid (OMCA). Aqueous mixtures of CTAB and OMCA in basic solution self-assemble into long, wormlike micelles. Upon irradiation by UV light (<400 nm), OMCA undergoes a photoisomerization from its trans to its cis form, which alters the molecular packing at the micellar interface. The result is to transform the long micelles into much shorter entities and, in turn, the solution viscosity decreases by more than 4 orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. The extent of viscosity reduction in these PR fluids can be tuned based on the composition of the mixture as well as the duration of the irradiation.     

Unexpected steric effects of "remote" alkyl groups on the rate of conjugate additions to alkyl alpha,beta-ethylenic sulfones, sulfoxides, and esters

Examination of conjugated ethylenic sulfones, sulfoxides, and esters in Michael-type addition reactions reveals, for the first time, that the size of the heteroatom-attached alkyl group affects the rate of conjugate addition. Molecular modeling strongly suggests that what are generally considered to be "remote" alkyl groups in -CbetaH=CalphaHS(O)n-alkyl systems and -CH2CbetaH=CalphaHCOO-alkyl systems are actually not remote from the beta-carbon atom of the Michael accepting unit. Molecular modeling clearly shows that the alkyl groups in these Michael acceptors shield the beta-carbons in the following order: Eti-Pr>t-Bu.     

The Instability of Ni{N(SiMe3)2}2: A Fifty Year Old Transition Metal Silylamide Mystery

The characterization of the unstable Ni^II bis(silylamide) Ni{N(SiMe₃)₂}₂ ( 1 ), its THF complex Ni{N(SiMe₃)₂}₂(THF) ( 2 ), and the stable bis(pyridine) derivative trans‐Ni{N(SiMe₃)₂}₂(py)₂ ( 3 ), is described. Both 1 and 2 decompose at ca. 25 °C to a tetrameric Ni^I species, [Ni{N(SiMe₃)₂}]₄ ( 4 ), also obtainable from LiN(SiMe₃)₂ and NiCl₂(DME). Experimental and computational data indicate that the instability of 1 is likely due to ease of reduction of Ni^II to Ni^I and the stabilization of 4 through dispersion forces.     

Indole Synthesis Using Silver Catalysis

Indoles are amongst the most important classes of heteroaromatics in organic chemistry, commonly found in biologically active natural products and therapeutically useful compounds. The synthesis of indoles is therefore important and several methods for their synthesis that make use of silver(I) catalysts and reagents have been developed in recent years. This Minireview contains, to the best of our knowledge, a comprehensive coverage of silver‐mediated indole forming reactions since the first reaction of this type was reported in 2004.     

Bi-directional interhemispheric inhibition during unimanual sustained contractions

Background  

The interaction between homologous muscle representations in the right and left primary motor cortex was studied using a paired-pulse transcranial magnetic stimulation (TMS) protocol known to evoke interhemispheric inhibition (IHI). The timecourse and magnitude of IHI was studied in fifteen healthy right-handed adults at several interstimulus intervals between the conditioning stimulus and test stimulus (6, 8, 10, 12, 30, 40, 50 ms). IHI was studied in the motor dominant to non-dominant direction and vice versa while the right or left hand was at rest, performing isometric contraction of the first dorsal interosseous (FDI) muscle, and isometric contraction of the FDI muscle in the context of holding a pen.     

Friend or foe? Turning a host defense protein into a pathogen's accomplice

Cystatins are cysteine protease inhibitors that are at the front-line of defense against pathogens that secrete proteases as virulence factors. In this issue, Vincents et al. (2008) reveal how the bacterial protease IdeS from Streptococcus pyogenes hijacks normal cystatin C function to convert it into a cofactor that enhances proteolytic destruction of host-defense antibodies.     

Phase prediction of the response of choked nozzles to entropy and acoustic disturbances

The development and transmission of sound through the exit of an aero-engine combustor is often investigated by modelling the complex geometry as a convergent-divergent nozzle. However, these analytical acoustic predictions are usually limited to the compact case, where the length of the nozzle is insignificant compared to the wavelength of the flow perturbations, or to cases where the variation of the mean velocity through the nozzle may be treated as linear or piece-wise linear. Considering terms up to first order in frequency for the conservation of mass, momentum and energy, this paper investigates an alternative approach by deriving effective lengths for the passage of the flow perturbations through a supercritical convergent-divergent nozzle. The effects due to the presence of a normal shock wave are also studied using a linearised form of the Rankine-Hugoniot relations. The analyses lead to predictions for the phase and magnitude of the transmitted acoustic waves from finite-length nozzles, and are valid for low non-dimensional frequencies. It has been found that these predictions agree well with the numerical results from inviscid simulations.     

Rational design of inhibitors and activity-based probes targeting Clostridium difficile virulence factor TcdB

Clostridium difficile is a leading cause of nosocomial infections. The major virulence factors of this pathogen are the multi-domain toxins TcdA and TcdB. These toxins contain a cysteine protease domain (CPD) that autoproteolytically releases a cytotoxic effector domain upon binding intracellular inositol hexakisphosphate. Currently, there are no known inhibitors of this protease. Here, we describe the rational design of covalent small molecule inhibitors of TcdB CPD. We identified compounds that inactivate TcdB holotoxin function in cells and solved the structure of inhibitor-bound protease to 2.0??. This structure reveals the molecular basis of CPD substrate recognition and informed the synthesis of activity-based probes for this enzyme. The inhibitors presented will guide the development of therapeutics targeting C. difficile, and the probes will serve as tools for studying the unique activation mechanism of bacterial toxin CPDs.     

Size discrimination in the coordination chemistry of an isoindoline pincer ligand with CdII and ZnII

The reactions of Cd2+ and Zn2+ with the pyridine-arm isoindoline ligand 4'-MeLH = 1,3-bis[2-(4-methylpyridyl)imino]isoindoline produced the series of octahedrally coordinated complexes M(4'-MeL)2, [M(4'-MeLH)2]2+, and [M(4'-MeL)(4'-MeLH)]+. The complexes M(4'-MeL)2 resulted from reactions of the respective metal perchlorates with deprotonated ligand, whereas the complexes [M(4'-MeLH)2](ClO4)2 resulted from reactions with ligand in the absence of added base. The mixed-ligand complexes [M(4'-MeL)(4'-MeLH)]+ were generated in solution by reactions of equimolar quantities of M(4'-MeL)2 and [M(4'-MeLH)2]2+. Whereas [Cd(4'-MeL)(4'-MeLH)]+ is stable in solution, [Zn(4'-MeL)(4'-MeLH)]+ converts to and establishes equilibrium with the tetrahedrally coordinated, trinuclear complex [Zn3(4'-MeL)4]2+. The complexes Cd(4'-MeL)2 (1), Zn(4'-MeL)2 (2), and [Cd(4'-MeL)(4'-MeLH)]ClO4 (5) were characterized by single-crystal X-ray diffraction, with the latter complex being shown to contain 4'-MeLH coordinated as a protonated iminium zwitterionic ligand. The [M(4'-MeLH)2]2+ and [M(4'-MeL)(4'-MeLH)]+ complexes are tautomeric in solution because of the shuttling of the iminium protons between imine N atoms. The rate of prototropic tautomerism in [Cd(4'-MeLH)2]+ was followed by 1H NMR spectroscopy. Over the temperature range 276-312 K, a linear Eyring plot with the activation parameters DeltaG++ = 16.0 +/- 0.1 kcal/mol, DeltaH++ = 2.9 +/- 0.1 kcal/mol, and DeltaS++ = -44.0 +/- 0.3 cal/mol.K was obtained.     

De novo designed peptides for biological applications

In recent years our ability to design and assemble peptide-based materials and objects de novo (i.e. from first principles) has improved considerably. This brings us to a point where the resulting assemblies are quite sophisticated and amenable to engineering in new functions. Whilst such systems could be used in a variety of ways, biological applications are of particular interest because of the demand for biocompatible, readily produced systems with potential as drug-delivery agents, components of biosensors and scaffolds for 3D cell and tissue culture. This tutorial review describes the building blocks (or tectons) that are being used in peptide assembly, highlights a range of materials and objects that have been produced, notably hydrogels and virus-like particles, and introduces a number of potential applications for the designs.