Functional group effects on metal-organic framework topology

Functionalization of the ligand 1,3,5-tris(4-carboxyphenyl)benzene (H(3)BTB) has been realized with methoxy (H(3)BTB-[OMe](3)) and hydroxy (H(3)BTB-[OH](3)) groups. Combining H(3)BTB-[OMe](3) and Zn(ii) results in the formation of the first isostructural, functionalized analogue of MOF-177 (MOF-177-OMe), while the combination of H(3)BTB-[OH](3) and Zn(ii) generates a rare, interpenetrated pcu-e framework.     

Communication: Identical temperature dependence of the time scales of several linear-response functions of two glass-forming liquids

The frequency-dependent dielectric constant, shear and adiabatic bulk moduli, longitudinal thermal expansion coefficient, and longitudinal specific heat have been measured for two van der Waals glass-forming liquids, tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl-4-ether. Within the experimental uncertainties the loss-peak frequencies of the measured response functions have identical temperature dependence over a range of temperatures, for which the Maxwell relaxation time varies more than nine orders of magnitude. The time scales are ordered from fastest to slowest as follows: Shear modulus, adiabatic bulk modulus, dielectric constant, longitudinal thermal expansion coefficient, and longitudinal specific heat. The ordering is discussed in light of the recent conjecture that van der Waals liquids are strongly correlating, i.e., approximate single-parameter liquids.     

A performance study of radio-opaque personal protective fabrics for the reduction of transmittance of gamma-rays and neutrons

Commercial radio-opaque combat (CRC) fabrics, for incorporation into personal protective equipment used by first responders and armed forces, are marketed as having the ability to provide a level of protection against specific types of radiation. For a CRC material, a standard combat uniform and a multi-layered chemical, biological, radiological, nuclear (CBRN) protective material, the present work examines chemical composition and radiation protection against gamma-rays and neutron fluxes. Significant reduction in gamma-ray transmittance occurs only for the CRC fabric (46–514 keV) with gamma-ray attenuation coefficients of 3.10 to <0.10 cm² g⁻¹. Reduction in neutron transmittance, for all three fabrics, could not be assessed with certainty as the measured transmittance was obscured by large statistical uncertainties.     

Fourteen-electron ring model and the anomalous magnetic circular dichroism of meso-triarylsubporphyrins

The MCD spectra of meso-triarylsubporphyrins show a sign anomaly which is correlated with the acceptor properties of the aryl substituent. From the spectra, magnetic moments of the excited states are determined. In the context of a simplified orbital model, the sign change is attributed to the quenching of the magnetic moment of the LUMO by acceptor orbitals of the substituent. The actual calculation of this moment presents a major challenge to computational methods. It is shown that wave function techniques based on CASSCF underestimate the covalency effects that are responsible for the quenching. In contrast, a CI method based on DFT orbitals yields excellent results, which fully support the orbital model.     

LIBS using dual- and ultra-short laser pulses

Pre-ablation dual-pulse LIBS enhancement data for copper, brass and steel using ns laser excitation are reported. Although large enhancements are observed for all samples, the magnitude of the enhancement is matrix dependent. Whereas all of the dual-pulse studies used ns laser excitation we see interesting effects when using ps and fs laser excitation for single-pulse LIBS. LIBS spectra of copper using 1.3 ps and 140 fs laser pulses show much lower background signals compared to ns pulse excitation. Also, the atomic emission decays much more rapidly with time. Because of relatively low backgrounds when using ps and fs pulses, non-gated detection of LIBS is shown to be very effective. The plasma dissipates quickly enough using ps and fs laser pulses, that high pulse rates, up to 1000 Hz, are effective for increasing the LIBS signal, for a given measurement time. Finally, a simple near-collinear dual-pulse fiber-optic LIBS probe is shown to be useful for enhanced LIBS measurements. Received: 1 August 2000 / Revised: 2 November 2000 / Accepted: 8 November 2000     

Increased xylose reductase activity in the xylose-fermenting yeastPichia stipitis by overexpression ofXYL1

Applied Biochemistry and Biotechnology - ThePichia stipitis xylose reductase gene (XYL1) was inserted into an autonomous plasmid thatP. stipitis maintains in multicopy. The plasmid pXOR with...     

Summary of reference materials for the determination of the nutrient composition of foods

Certified reference materials (CRMs) play a critical role in validating the accuracy of nutrient data for food samples. A number of available food CRMs of differing matrix composition have assigned concentrations for various nutrients, along with associated uncertainty intervals (UIs) for those values. These CRMs have been used extensively in the United States Department of Agriculture’s (USDA) ongoing National Food and Nutrient Analysis Program (NFNAP) to monitor the accuracy of assays of key foods and nutrients consumed in the United States. A total of 690 assigned values for individual nutrients, including proximates, vitamins, macroelements, microelements, fatty acids, amino acids, and selected phytochemicals (e.g., carotenoids), were compiled from the certificates of analysis for 63 CRMs, and the specified UI in each case was expressed as a percentage of the assigned certified or reference concentration. Across all nutrients, 63.5% of the UIs were less than 10% of the assigned value, 25.5% were 10–20%, and 11% were greater than 20% of the assigned value. The UIs for proximates, minerals, and trace elements were most consistently less than 10% of the assigned value. The relative uncertainties were significantly higher for vitamins, suggesting greater challenges in measuring and certifying these components. These high UIs (greater than 10% assigned value) in the best available reference materials are likely to be indicative of the precision and accuracy that can be obtained by current measurement systems for these components. These data suggest that care must be taken in choosing CRMs to monitor food composition analysis, including evaluating what levels of uncertainty are required in assigned values and which analytical measurement systems for food components need closer examination and improvement. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Presented at ‘BERM-10’, April 2006, Charleston, SC, USA.     

Electronic Modifications of Fluorescent Cytidine Analogues Control Photophysics and Fluorescent Responses to Base Stacking and Pairing

The rational design of fluorescent nucleoside analogues is greatly hampered by the lack of a general method to predict their photophysics, a problem that is especially acute when base pairing and stacking change fluorescence. To better understand these effects, a series of tricyclic cytidine (tC and tC^O) analogues ranging from electron-rich to electron-deficient was designed and synthesized. They were then incorporated into oligonucleotides, and photophysical responses to base pairing and stacking were studied. When inserted into double-stranded DNA oligonucleotides, electron-rich analogues exhibit a fluorescence turn-on effect, in contrast with the electron-deficient compounds, which show diminished fluorescence. The magnitude of these fluorescence changes is correlated with the oxidation potential of nearest neighbor nucleobases. Moreover, matched base pairing enhances fluorescence turn-on for the electron-rich compounds, and it causes a fluorescence decrease for the electron-deficient compounds. For the tC^O compounds, the emergence of vibrational fine structure in the fluorescence spectra in response to base pairing and stacking was observed, offering a potential new tool for studying nucleic acid structure and dynamics. These results, supported by DFT calculations, help to rationalize fluorescence changes in the base stack and will be useful for selecting the best fluorescent nucleoside analogues for a desired application.     

The homologous conjugate addition of thiols to electron-deficient cyclopropanes catalyzed by a calcium(II) complex

The synthesis of γ-sulfanyl malonates was achieved through the addition of thiols to electron deficient cyclopropanes. These reactions are catalyzed by calcium acetylacetonate, Ca(acac)₂. A variety of electron rich and electron deficient thiols were added without the need for prior activation or exogenous base. The thiol additions to donor–acceptor cyclopropanes bearing electron-rich and electron-deficient aromatic and heteroaromatic groups proceeded in good to excellent yields.     

Physical aging of molecular glasses studied by a device allowing for rapid thermal equilibration

Aging to the equilibrium liquid state of organic glasses is studied. The glasses were prepared by cooling the liquid to temperatures just below the glass transition. Aging following a temperature jump was studied by measuring the dielectric loss at a fixed frequency using a microregulator in which temperature is controlled by means of a Peltier element. Compared to conventional equipment, the new device adds almost two orders of magnitude to the span of observable aging times. Data for the following five glass-forming liquids are presented: dibutyl phthalate, diethyl phthalate, 2,3-epoxy propyl-phenyl-ether, 5-polyphenyl-ether, and triphenyl phosphite. The aging data were analyzed using the Tool-Narayanaswamy formalism. The following features are found for all five liquids: (1) The liquid has an "internal clock," a fact that is established by showing that aging is controlled by the same material time that controls the dielectric properties. (2) There are no so-called expansion gaps between the long-time limits of the relaxation rates following up and down jumps to the same temperature. (3) At long times, the structural relaxation appears to follow a simple exponential decay. (4) For small temperature steps, the rate of the long-time exponential structural relaxation is identical to that of the long-time decay of the dipole autocorrelation function.