Microcrystallography, high-pressure cryocooling and BioSAXS at MacCHESS

The Macromolecular Diffraction Facility at the Cornell High Energy Synchrotron Source (MacCHESS) is a national research resource supported by the National Center for Research Resources of the US National Institutes of Health. MacCHESS is pursuing several research initiatives designed to benefit both CHESS users and the wider structural biology community. Three initiatives are presented in further detail: microcrystallography, which aims to improve the collection of diffraction data from crystals a few micrometers across, or small well diffracting regions of inhomogeneous crystals, so as to obtain high-resolution structures; pressure cryocooling, which can stabilize transient structures and reduce lattice damage during the cooling process; and BioSAXS (small-angle X-ray scattering on biological solutions), which can extract molecular shape and other structural information from macromolecules in solution.     

Infrared sensitive liquid crystal photorefractive hybrid cell with semiconductor substrates

Refractive index grating recording is achieved in the infrared in a nematic liquid crystal placed between semiconductor CdTe substrates. Light-induced space-charge field created in photorefractive semiconductor substrates varies an alignment of liquid crystal molecules inhomogeneously in space forming a spatial modulation of the refractive index in liquid crystal. Two-beam coupling is studied in the hybrid cell, in which a gain factor Γ=16 cm⁻¹ is achieved in the layer of liquid crystal of the sandwich.     

Oxygen reduction to water mediated by a dirhodium hydrido-chloride complex

The two-electron mixed-valence dirhodium complex Rh(2)(0,II)(tfepma)(2)(CN(t)Bu)(2)Cl(2) (tfepma = CH(3)N[P(OCH(2)CF(3))(2)](2)) reacts with HCl to furnish two isomeric dirhodium hydrido-chloride complexes, Rh(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(3)H. In the presence of HCl, the hydride complex effects the reduction of 0.5 equiv of O(2) to 1 equiv of H(2)O, generating Rh(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(4), which can be prepared independently by chlorine oxidation of the Rh(2)(0,II) precursor. The starting Rh(2)(0,II) complex is regenerated photochemically to close an oxygen-to-water reduction photocycle.     

Negative polaron and triplet exciton diffusion in organometallic "molecular wires"

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.     

Post-self-assembly covalent chemistry of discrete multicomponent metallosupramolecular hexagonal prisms

The multicomponent coordination-driven self-assembly of hexakis[4-(4-pyridyl)phenyl]benzene, cis-(PEt(3))(2)Pt(II)(OTf)(2), and amine- or maleimide-functionalized isophthalate forms discrete hexagonal prisms as single reaction products. The amino or maleimide groups decorating the isophthalate pillars of the prisms provide reactive sites for post-self-asssembly modifications. In this communication, we demonstrate that the hexagonal prisms can be functionalized without disrupting the prismatic cores, enabling the incorporation of new functionalities under mild conditions.     

Designed post-self-assembly structural and functional modifications of a truncated tetrahedron

Post-self-assembly modifications of a discrete metal-organic supramolecular structure have been developed. Such modifications allow the properties of the self-assembled supramolecular species to be changed in a simple and efficient manner (>90% yield). Initiated by the application of chemical stimuli, the post-self-assembly modifications described herein result in three distinct changes to the supramolecular system: an individual building-block component change, an overall structural modification, and a functional evolution of a [6+4] metal-organic supramolecular structure. The three modifications have been carefully examined by a range of characterization methods, including NMR and UV-vis spectroscopy, electrospray ionization mass spectrometry, pulsed field gradient spin echo NMR measurements, electrochemical analysis, and computational simulations.     

Development of a novel single sensor multiplexed marker assay

There is an increasing desire to measure multiple analytes simultaneously for disease management and detection. However, in the case of invasive devices, it would be better to obtain one small sample and immediately be able to detect the analytes rapidly, as in the case of self-monitoring blood glucose, without the need for additional steps, arrays, or reagents. Electrochemical impedance spectroscopy is used to measure the interaction between ultralow levels of analyte and molecular recognition element in a label-free and rapid manner. Gold nanoparticles were attached to antibodies against interleukin-12 and tumor necrosis factor-α, typical inflammatory markers found with near overlapping responses, on an impedance spectroscopy based biosensor. Cross-reactivity and specificity of tuned antibodies were verified using ELISA. Impedance frequency was quantified by concentration gradients of marker against the device. The natural impedance frequency for interleukin-12 (5.00 Hz) was tuned to a lower frequency four Hertz away from one another for better signal processing. This was accomplished without significantly altering the lower limits of detection (<4 pg ml(-1) and ～60 pg ml(-1) for interleukin-12 and tumor necrosis factor-α, respectively), no cross-reactivity and specificity as determined by ELISAs. With modeling the nanoscale effects and further development, a larger tuning will be possible for making a better multiplexed sensor. Although interleukin-12 and TNF-α equivalent circuit calculations were modeled here, a sensor with the potential to measure multiple markers at once might offer a solution on the sensor front for simplified management of conditions such as diabetes, where both glucose and hemoglobin A1c values could be obtained.     

Investigation of noble metal nanoparticle ζ-potential effects on single-cell exocytosis function in vitro with carbon-fiber microelectrode amperometry

Since noble metal nanoparticles are increasingly found in consumer goods, there is a need for information about potential impacts of these nanoparticles on cellular function to avoid environmental and health risks associated with exposure. In this study, spherical Au and Ag nanoparticles of similar size were synthesized and modified to assess the effects of ζ-potential on immune cell function. Nanoparticle ζ-potential was controlled by employing surfactant exchange to generate nanoparticles with positive or negative surface charge. Mouse peritoneal mast cells (MPMCs) were then exposed to 5-15 μg ml(-1) of these nanomaterials, and uptake was assessed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Uptake for positively charged nanoparticles was more efficient than for negatively charged nanomaterials, and all nanoparticles were taken up in a concentration-dependent manner. Following uptake, MPMC degranulation function was assessed using carbon-fiber microelectrode amperometry (CFMA), showing decreased quantal secretion of serotonin by MPMCs exposed to the positively charged Au nanoparticles and negatively charged Ag nanoparticles. The overall efficiency of the degranulation process (indicated by amperometric spike frequency) decreased for all Au-exposed MPMCs. However, only the negatively charged version of the Ag nanomaterial resulted in decreased MPMC degranulation efficiency. Further studies revealed that ionic Ag was partially responsible for the observed effects. Overall, these studies reveal the complex nature of interactions between noble metal nanomaterials and cells that result in perturbed cellular function and illustrate the necessity of thorough nanoparticle characterization for interpretation of cellular function assays.     

Halogen oxidation and halogen photoelimination chemistry of a platinum-rhodium heterobimetallic core

The heterobimetallic complexes, PtRh(tfepma)(2)(CN(t)Bu)X(3) (X = Cl, Br), are assembled by the treatment of Pt(cod)X(2) (cod =1,5-cyclooctadiene) with {Rh(cod)X}(2), in the presence of tert-butylisonitrile (CN(t)Bu) and tfepma (tfepma = bis(trifluoroethoxyl)phosphinomethylamine). The neutral complexes contain Pt-Rh single bonds with metal-metal separations of 2.6360(3) and 2.6503(7) ? between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPt(I)Rh(II)X(2) cores with suitable halide sources (PhICl(2) or Br(2)) furnishes PtRh(tfepma)(2)(CN(t)Bu)X(5), which preserves a Pt-Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a meridional geometry, which slowly transforms to a facial arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the mer- or fac-Cl(3)Pt(III)Rh(II)Cl(2) isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPt(I)Rh(II)Cl(2) core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the fac-ClPt(I)Rh(II)Cl(2) isomer, details of the X(2) photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (<8 ns), and halogen is eliminated from it with a rate constant of 3.6 × 10(4) s(-1). As M-X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage.     

Indefinite quadratic polynomials of small signature

LetF()=Q()+L() be a real quadratic polynomial with no constant term. Suppose that the quadratic partQ() is indefinite of type (r, n-r). For an integerk4 we show that if min (r, n-r) >-k there exists a functionf (n, k)=–1/2+3/(4k+2)+O _k (1/n) with the following property. For any >0 and all large enoughX there is an integer vector 0 such that || X and.