Self-assembled growth of nanostructural Ge islands on bromine-passivated Si(111) surfaces at room temperature

We have deposited relatively thick (∼60 nm) Ge layers on Br-passivated Si(111) substrates by thermal evaporation under high vacuum conditions at room temperature. Ge has grown in a layer-plus-island mode although it is different from the Stranski-Krastanov growth mode observed in epitaxial growth. Both the islands and the layer are nanocrystalline. This appears to be a consequence of reduction of surface free energy of the Si(111) substrate by Br-passivation. The size distribution of the Ge nanoislands has been determined. The Br-Si(111) substrates were prepared by a liquid treatment, which may not produce exactly reproducible surfaces. Nevertheless, some basic features of the nanostructural island growth are reasonably reproducible, while there are variations in the details of the island size distribution.     

An extraction-based assay for neutral anionophores: the measurement of high binding constants to steroidal receptors in a nonpolar solvent

The extraction-based protocol for measuring binding constants, developed by Cram and co-workers, has been extended for use with anionic substrates. The method is especially useful for high-affinity receptors, allowing very high binding constants to be measured in nonpolar solvents. Distribution constants K(d) between chloroform and water have been obtained for tetraethylammonium chloride and bromide, thus calibrating the method for these two substrates. Application to steroidal podands 5-9 has confirmed the ability of electron-withdrawing groups to enhance hydrogen-bond donor capabilities. Binding constants of approximately 3 x 10(7) M(-1) have been measured for the most powerful receptor 7. An X-ray crystal structure of 15, the methyl ester analogue of 7, reveals a well-defined binding site preorganised for anion recognition.     

Preliminary Study on the Use of Water-in-Oil Microemulsion Eluents in HPLC

This paper describes the use of water-in-oil (W/O) microemulsion eluents to achieve unique normal phase HPLC separations. The effects of varying the oil type, co-surfactant, surfactant, use of mixed surfactant and water concentration upon the chromatographic performance was assessed. Other parameters such as temperature and flow rate were also investigated. An optimised set of W/O microemulsion HPLC (MELC) operating conditions was then applied to the separation of a range of acids, bases and neutral compounds. The more water soluble compounds were more highly retained. W/O MELC was found to be especially suitable for determination of water insoluble compounds. The drug content in bumetanide tablets was determined by W/O MELC with good linearity and accuracy. The solubilising ability of the W/O microemulsion reduced sample preparation (precipitation and extraction) requirements compared to conventional HPLC. The results obtained compared well with those obtained by a validated reverse phase HPLC method. It is recommended that W/O MELC should be considered for routine applications, especially for the analysis of water insoluble compounds in complex sample matrices. Further research is recommended to more definitely assess the operating parameters of W/O MELC and to determine other applications.     

An anchoring role for FeS clusters: chelation of the amino acid moiety of S-adenosylmethionine to the unique iron site of the [4Fe-4S] cluster of pyruvate formate-lyase activating enzyme

Pyruvate formate-lyase activating enzyme (PFL-AE) generates the catalytically essential glycyl radical on pyruvate formate-lyase via the interaction of the catalytically active [4Fe-4S]+ cluster with S-adenosylmethionine (AdoMet). Like other members of the Fe-S/AdoMet family of enzymes, PFL-AE is thought to function via generation of an AdoMet-derived 5'-deoxyadenosyl radical intermediate; however, the mechanistic steps by which this radical is generated remain to be elucidated. While all of the members of the Fe-S/AdoMet family of enzymes appear to have a unique iron site in the [4Fe-4S] cluster, based on the presence of a conserved three-cysteine cluster binding motif, the role of this unique site has been elusive. Here we utilize 35-GHz pulsed electron nuclear double resonance (ENDOR) studies of the [4Fe-4S]+ cluster of PFL-AE in complex with isotopically labeled AdoMet (denoted [1+/AdoMet]) to show that the unique iron serves to anchor the AdoMet for catalysis. AdoMet labeled with 17O at the carboxylate shows a coupling of A = 12.2 MHz, consistent with direct coordination of the carboxylate to the unique iron of the cluster. This is supported by 13C-ENDOR with the carboxylato carbon labeled with 13C, which shows a hyperfine coupling of 0.71 MHz. AdoMet enriched with 15N at the amino position gives rise to a spectrum with A(15N) = 5.8 MHz, consistent with direct coordination of the amino group to a unique iron of the cluster. Together, the results demonstrate that the unique iron of the [4Fe-4S] cluster anchors AdoMet by forming a classical N/O chelate with the amino and carboxylato groups of the methionine fragment.     

Spectroscopic approaches to elucidating novel iron-sulfur chemistry in the "radical-Sam" protein superfamily

Electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and M?ssbauer spectroscopies and other physical methods have provided important new insights into the radical-SAM superfamily of proteins, which use iron-sulfur clusters and S-adenosylmethionine to initiate H atom abstraction reactions. This remarkable chemistry involves the generation of the extremely reactive 5'-deoxyadenosyl radical, the same radical intermediate utilized in B12-dependent reactions. Although early speculation focused on the possibility of an organometallic intermediate in radical-SAM reactions, current evidence points to novel chemistry involving a site-differentiated [4Fe-4S] cluster. The focus of this forum article is on one member of the radical-SAM superfamily, pyruvate formate-lyase activating enzyme, and how physical methods, primarily EPR and ENDOR spectroscopies, are contributing to our understanding of its structure and mechanism. New ENDOR data supporting coordination of the methionine moiety of SAM to the unique site of the [4Fe-4S]2+/+ cluster are presented.     

Nonlinear propagation effects in an AlGaAs Bragg grating filter

We present experimental observations of nonlinear propagation effects in an integrated AlGaAs waveguide filter. We demonstrate pulse shaping, pulse compression, and the production of gap solitons within the stop band of the grating for switching powers of ~130 W . This nonlinear behavior is associated with the counterbalancing effects of self-phase modulation and the large dispersion effects introduced by the grating itself.     

Combinatorial Methods and Informatics Provide Insight into Physical Properties and Structure Relationships during IPN Formation

Combinatorial methods and informatics are applied to the study of complex property‐structure‐processing relationships during interpenetrating polymer network formation in an epoxy/acrylate system. Principal component analysis of a dataset that covers different compositions and process sequences successfully identifies the most unique samples as well as relationships between material properties. The relationships between material properties can be exploited in future investigations by allowing high throughput screening and as a guide for engineering materials. The use of combinatorial methods, high throughput screening, and informatics will lead to accelerated material design.

     

The Effects of Streptozotocin-Induced Diabetes and Insulin Treatment on Carnitine Biosynthesis and Renal Excretion

Carnitine insufficiency is reported in type 1 diabetes mellitus. To determine whether this is accompanied by defects in biosynthesis and/or renal uptake, liver and kidney were obtained from male Sprague-Dawley rats with streptozotocin-induced diabetes. Diabetic rats exhibited the metabolic consequences of type 1 diabetes, including hypoinsulinemia, hyperglycemia, and increased urine output. Systemic hypocarnitinemia, expressed as free carnitine levels, was evident in the plasma, liver, and kidney of diabetic rats. Compared to control rats, the low free carnitine in the plasma of diabetic rats was accompanied by decreased expression of γ-butyrobetaine hydroxylase in liver and kidney, suggesting impaired carnitine biosynthesis. Expression of organic cation transporter-2 in kidney was also reduced, indicating impaired renal reabsorption, and confirmed by the presence of elevated levels of free carnitine in the urine of diabetic rats. Insulin treatment of diabetic rats reversed the plasma hypocarnitinemia, increased the free carnitine content in both kidney and liver, and prevented urinary losses of free carnitine. This was associated with increased expression of γ-butyrobetaine hydroxylase and organic cation transporter-2. The results of our study indicate that type 1 diabetes induced with streptozotocin disrupts carnitine biosynthesis and renal uptake mechanisms, leading to carnitine insufficiency. These aberrations in carnitine homeostasis are prevented with daily insulin treatment.     

Impact of average free‐volume element size on transport in stereoisomers of polynorbornene. I. Properties at 35 °C

This article demonstrates that transport of gases through glassy polymers is significantly influenced not only by the absolute amount but also by the distribution of free volume. Two stereoisomers of polynorbornene with nearly equivalent total free volume, but markedly different average free‐volume sizes, were evaluated. The free‐volume element size was probed with positron annihilation lifetime spectroscopy, wide‐angle X‐ray scattering, gas sorption, and molecular modeling. The permeation, sorption, and diffusion of light gases were measured in each stereoisomer at 35 °C. All analytical techniques indicated that one isomer (labeled as Architecture II) had a larger average free‐volume element size but fewer elements. This isomer also had a very slightly higher bulk density (1.000 vs 0.992 g/cm³ for the other stereoisomer). Architecture II also had gas sorption and diffusion coefficients that were two to three times those of the less dense counterpart. These differences have been attributed to differences in the free‐volume element size available within the polymer matrix. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2185–2199, 2003