A comparison of three SPRITE techniques for the quantitative 3D imaging of the 23Na spin density on a 4T whole-body machine

Sodium density maps acquired with three SPRITE-based methods have been compared in terms of the resulting quantitative information as well as image quality and acquisition times. Consideration of factors relevant for the clinical implementation of SPRITE shows that the Conical-SPRITE variant is preferred because of a 20-fold reduction in acquisition time, slightly improved image quality, and no loss of quantitative information. The acquisition of a 3D data set (32x32x16; FOV=256x256x160 mm) for the quantitative determination of sodium density is demonstrated. In vivo Conical-SPRITE 23Na images of the brain of a healthy volunteer were acquired in 30 min with a resolution of 7.5x7.5x7.5 mm and a signal-to-noise ratio of 23 in cerebrospinal fluid and 17 in brain tissue.     

Poly(dimethylsiloxane) cross-linked carbon paste electrodes for microfluidic electrochemical sensing

Recently, the development of electrochemical biosensors as part of microfluidic devices has garnered a great deal of attention because of the small instrument size and portability afforded by the integration of electrochemistry in microfluidic systems. Electrode fabrication, however, has proven to be a major obstacle in the field. Here, an alternative method to create integrated, low cost, robust, patternable carbon paste electrodes (CPEs) for microfluidic devices is presented. The new CPEs are composed of graphite powder and a binder consisting of a mixture of poly(dimethylsiloxane) (PDMS) and mineral oil. The electrodes are made by filling channels molded in previously cross-linked PDMS using a method analogous to screen printing. The optimal binder composition was investigated to obtain electrodes that were physically robust and performed well electrochemically. After studying the basic electrochemistry, the PDMS-oil CPEs were modified with multi-walled carbon nanotubes (MWCNT) and cobalt phthalocyanine (CoPC) for the detection of catecholamines and thiols, respectively, to demonstrate the ease of electrode chemical modification. Significant improvement of analyte signal detection was observed from both types of modified CPEs. A nearly 2-fold improvement in the electrochemical signal for 100 μM dithiothreitol (DTT) was observed when using a CoPC modified electrode (4.0 ± 0.2 nA (n = 3) versus 2.5 ± 0.2 nA (n = 3)). The improvement in signal was even more pronounced when looking at catecholamines, namely dopamine, using MWCNT modified CPEs. In this case, an order of magnitude improvement in limit of detection was observed for dopamine when using the MWCNT modified CPEs (50 nM versus 500 nM). CoPC modified CPEs were successfully used to detect thiols in red blood cell lysate while MWCNT modified CPEs were used to monitor temporal changes in catecholamine release from PC12 cells following stimulation with potassium.     

A surface energy transfer nanoruler for measuring binding site distances on live cell surfaces

Measuring distances at molecular length scales in living systems is a significant challenge. Methods like Fo?rster resonance energy transfer (FRET) have limitations due to short detection distances and strict orientations. Recently, surface energy transfer (SET) has been used in bulk solutions; however, it cannot be applied to living systems. Here, we have developed an SET nanoruler, using aptamer-gold nanoparticle conjugates with different diameters, to monitor the distance between binding sites of a receptor on living cells. The nanoruler can measure separation distances well beyond the detection limit of FRET. Thus, for the first time, we have developed an effective SET nanoruler for live cells with long distance, easy construction, fast detection, and low background. This is also the first time that the distance between the aptamer and antibody binding sites in the membrane protein PTK7 was measured accurately. The SET nanoruler represents the next leap forward to monitor structural components within living cell membranes.     

Butyl acrylate porous polymer monoliths in fused-silica capillaries for use in capillary electrochromatography

Capillary electrochromatography incorporates features of both capillary electrophoresis and liquid chromatography. Butyl acrylate polymers, cast in-situ with heat initiated polymerization and no retaining frits have been made. Van Deemter plots of chrysene have been examined at a variety of operating temperatures to examine column behavior. Hmin moves to faster flow-rates and increases slightly in magnitude as temperature is increased. The longevity and reproducibility of the columns have been examined with a homologous series. Performance is very reproducible between two different columns of different diameters, operated on different systems and prepared from the same polymeriation batch. The relative standard deviation of retention factors is a maximum of 3.1% with most values calculated at less than 1%. The uniformity of the polymers as a function of length has also been studied with a series of polycyclic aromatic hydrocarbons, and the columns have proved to be very uniform across their length as measured by the consistency of retention factors with a maximum relative standard deviation of 3.4% and most values calculated between 1 and 2%. Plate numbers of between 65000 and 80000 plates/m have been attained for compounds with retention factors of 3 to 12. These columns have proved easy to make, are quite reproducible, and long lived.     

Interaction energies between tetrahydrobiopterin analogues and aromatic residues in tyrosine hydroxylase and phenylalanine hydroxylase

The phenylalanine residues 300 and 309 in the enzyme tyrosine hydroxylase are known to aid in the positioning and binding of tetrahydrobiopterin (BH4) to the enzyme active site. The residues phenylalanine 254 and tyrosine 325 similarly aid in binding BH4 in phenylalanine hydroxylase. BH4 is a cofactor necessary for enzyme function, and mutations in these residues have been shown to cause a decrease in enzyme function. We examine the pairwise interactions between each aromatic residue and BH4 using second-order Moller Plesset theory and density functional theory to determine the amount of binding due to these aromatic residues. Further, we perform in silico point mutations of these residues to determine if several likely mutations can cause a decrease in protein function. Our results show that dispersion dominates these interactions, and electrostatics alone is not enough to bind the BH4.     

Molecular heterogeneous catalysis: a single-site zeolite-supported rhodium complex for acetylene cyclotrimerization

By anchoring metal complexes to supports, researchers have attempted to combine the high activity and selectivity of molecular homogeneous catalysis with the ease of separation and lack of corrosion of heterogeneous catalysis. However, the intrinsic nonuniformity of supports has limited attempts to make supported catalysts truly uniform. We report the synthesis and performance of such a catalyst, made from [Rh(C(2)H(4))(2)(CH(3)COCHCOCH(3))] and a crystalline support, dealuminated Y zeolite, giving {Rh(C(2)H(4))(2)} groups anchored by bonds to two zeolite oxygen ions, with the structure determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and the uniformity of the supported complex demonstrated by (13)C NMR spectroscopy. When the ethylene ligands are replaced by acetylene, catalytic cyclotrimerization to benzene ensues. Characterizing the working catalyst, we observed evidence of intermediates in the catalytic cycle by NMR spectroscopy. Calculations at the level of density functional theory confirmed the structure of the as-synthesized supported metal complex determined by EXAFS spectroscopy. With this structure as an anchor, we used the computational results to elucidate the catalytic cycle (including transition states), finding results in agreement with the NMR spectra.     

Binary and gray-scale patterning of chemical functionality on polymer films

We present a facile technique for the gray-scale chemical functionalization of polymer surfaces with high dynamic range. We demonstrate the use of this technique to create amine-functionalized substrates that are used for the patterned binding of fluorophores and the patterned synthesis of peptides. Studies of the behavior of the model organism Dictyostelium discoideum indicate the biocompatibility of the functionalized substrates.     

Cross-cultural generalizability of a cusp catastrophe model for binge drinking among college students

We examined whether a cusp catastrophe model for binge alcohol consumption by college students that was reported earlier (Smerz & Guastello, 2008) could generalize to another culture. Participants were 130 undergraduates enrolled in economics courses at a private urban Japanese university. They completed the same questionnaire items that were used in the previous US study. For some analyses, a stratified random subsample was taken from the earlier US data that was comparable in number, age, and gender proportions (N = 132). Results for the combined sample showed essentially the same results that were obtained from the US sample: Binge drinking can be modeled as a cusp catastrophe with two stable states of behavior - low to moderate consumption and binge level consumption. The two control parameters were peer influence (bifurcation) and attitude toward alcohol use (asymmetry). The nonlinear models (average R2 = .74) accounted for considerably more variance in binge drinking and other alcohol consumption indices than the comparison linear models (average r2 = .18 ). There were some subtle differences between the two samples of students, however.     

Chemistry of the t-butoxyl radical: evidence that most hydrogen abstractions from carbon are entropy-controlled

Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical ((t) BuO.) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an analysis of all the available data reveals that most hydrogen abstractions (from carbon) by (t) BuO. are entropy controlled (i.e., TdeltaS > deltaH, in solution at room temperature). For substrates with C-H bond dissociation energies (BDEs) > 92 kcal/mol, the activation energy for hydrogen abstraction decreases with decreasing BDE in accord with the Evans-Polanyi equation, with alpha approximately 0.3. For substrates with C-H BDEs in the range from 79 to 92 kcal/mol, the activation energy does not vary significantly with C-H BDE. The implications of these results in the context of the use of (t) BuO. as a chemical model for reactive oxygen-centered radicals is discussed.