FLUORESCENCE STUDIES WITH HUMAN EPIDERMAL GROWTH FACTOR

Steady-state and time-resolved fluorescence studies have been performed with human epidermal growth factor, a small globular protein having two adjacent tryptophan residues near its C-terminus. Based on the relatively red fluorescence and accessibility to solute quenchers, the two tryptophan residues are found to be exposed to solvent. Anisotropy decay measurements show the dominant depolarizing process to have a sub-nanosecond rotational correlation time indicating the existence of rapid segmental motion of the fluorescing tryptophan residues. From an analysis of the low-temperature excitation anisotropy spectrum of the protein (and in comparison with that of tryptophan, the peptide melittin, and the dipeptide trp-trp), it is concluded that homo-energy transfer and/or exciton interaction occurs between the adjacent tryptophan residues. A thermal transition in the structure of the protein, which is observed by circular dichroism measurements, is not sensed by the steady-state fluorescence of the protein. This result, in conjunction with the anisotropy decay results, indicates that the two tryptophan residues are in a highly flexible C-terminus segment, which is not an integral part of the three-dimensional structure of the protein. Fluorescence measurements with three site-directed mutants also show very little variation.     

Porous Tungsten Oxide: Recent Advances in Design,Synthesis, and Applications

Tungsten oxide (WO₃) has received ever more attention and has been highly researched over the last decade due to its being a low-cost transition metal semiconductor with tunable, yet widely stable, band gaps. This minireview briefly highlights the challenges in the design and synthesis of porous WO₃ including methods, precursors, solvent effects, crystal phases, and surface activities of the porous WO₃ base material. These topics are explored while also drawing a connection of how the morphology and crystal phase affect the band gap. The shifts in band gap not only impact the optical properties of tungsten but also allow tuning to operate on different energy levels, which makes WO₃ highly desirable in many applications such as supercapacitors, batteries, solar cells, catalysts, sensors, smart windows, and bioapplications.     

Kinetics of thermal decomposition of a synthetic K–H3O jarosite analog

The thermal decomposition kinetics of a synthetic K–H₃O jarosite analog was determined from thermogravimetric analysis at various heating rates in air. A thermal decomposition mechanism was proposed based on X-ray analysis of partially decomposed material and distinct features observed during thermal decomposition analysis. The decomposition path is complex. The material was treated as a composite of K-jarosite, H₃O-jarosite, and a “vacancy component”. The evolution of (OH)^? and SO₃ from these individual components was modeled. The decomposition is broken into subreactions according to distinct features in the thermoanalytical measurements. The subreactions are arranged sequentially and in parallel according to the evolution of the participating phases. A set of associated apparent activation energies was determined using isoconversion analysis. Kinetic triplets were assigned to each subreaction. A reasonable match with the observed decomposition was achieved by varying pre-exponential factors.     

Novikov-type coordinates for HJ separable metrics

Synchronous frames are known to remove (at least partially) deficiencies inherent in Killing coordinates. Strangely enough, no constructive general method of finding these frames has been formulated, though it follows from the definition of synchronous coordinates; We propose here such a method and apply it to the Kerr metric to obtain a Novikov-type synchronous frame. The removal of nonphysical singularities is discussed in the context of 3-metric and 3-curvature on the spatial hypersurfaces in this frame.We are thankful to Dr. I. D. Novikov and Professor R. Gautreau for the interest shown in the work, and whose ideas helped to bring this work to completion.     

DCMU-INDUCED STIMULATION OF PHOTOSYSTEM I ELECTRON TRANSPORT. INVOLVEMENT OF MEMBRANE STRUCTURAL CHANGES

DCMU-induced stimulation of the rate of photosystem I (PS I) electron transport in DCIPH₂→ MV photoreaction occurs through the action of DCMU on the rate-limiting step which contains the site of electron donation of DCIPH₂ (Ramanujam et al. , 1981). The magnitude of stimulation of the rate by 50 μ M DCMU decreased with increasing concentration of chlorophyll (Chl), implying that DCMU is stoichiometrically related to Chl with respect to the stimulation of the PS I rate.
DCMU-induced stimulation was sensitive to the ionic condition of the thylakoids, the effect being reduced at low cation concentration. Cation-induced scattering changes in thylakoid suspension were partially reversed by DCMU, and the percent Chl in the 10 K fraction of the thylakoid decreased upon addition of DCMU, indicating that grana structure is disrupted by DCMU. Hydroquinone-mediated reduction of cytochrome f in thylakoids in the dark was accelerated in the presence of DCMU. The DCMU effect was not observed in isolated PS I particles.
It is concluded that DCMU binds to the thylakoid membranes and brings about structural changes leading to unstacking of the thylakoids accompanied by an altered interaction of the electron transfer chain components with the added electron donor. This binding of DCMU must have an affinity lower than the well-known binding of DCMU to photosystem II (PS II), because the concentration required is markedly higher.     

Elasticity of adsorbent beds

The practical application of adsorbents with the desired separation properties depends not only upon the adsorption characteristics of the material but also upon the mechanical properties of the packed bed. The packed bed, the vessel surrounding the bed and any internal structure that supports the bed are subjected to both static and cyclic loads during an adsorption process. In order to properly design the vessel and its internal structure, the bulk mechanical properties (most particularly the elastic properties) of the adsorbent bed must be known. The primary focus of this study was to determine the elastic properties of adsorbent beds packed with activated alumina, synthetic molecular sieve 13X or natural zeolite clinoptilolite. The bulk modulus of elasticity was found to be a linear function of applied stress for each of these materials in a range of stresses lower than the bulk crush strength. The Poisson’s ratio for the packed bed was also deduced from these results.     

A Cs2CO3-mediated simple and selective method for the alkylation and acylation of 3,4-dihydropyrimidin-2(1H)-thiones

Alkyl and acyl derivatives of 3,4-dihydropyrimidin-2(1H)-thiones were synthesized in good to excellent yields in the presence of Cs₂CO₃, a mild base. The method evidences a selective S-alkylation when using acyl chlorides as efficient acylating agents at room temperature on the 2-thioxo-dihydropyrimidone moiety. A possible mechanistic interpretation of the different selectivities in case of alkylation and acylation was done with the help of a geometry optimization process.     

The probe technique far from equilibrium: Magnetic field symmetries of nonlinear transport

The probe technique is a simple mean to incorporate elastic and inelastic processes into quantum transport problems. Using numerical simulations, we demonstrate that this tool can be employed beyond the analytically tractable linear response regime, providing a stable solution for the probe parameters: temperature and chemical potential. Adopting four probes: dephasing, voltage, temperature, and voltage-temperature, mimicking different elastic and inelastic effects, we provide a systematic analysis of magnetic field and gate voltage symmetries of charge current and heat current in Aharonov-Bohm interferometers, potentially far from equilibrium. Considering electron current, we prove that in the linear response regime inelastic scattering processes do not break the Onsager symmetry. Beyond linear response, even (odd) conductance terms obey an odd (even) symmetry with the threading magnetic flux, as long as the system acquires a spatial inversion symmetry. When spatial asymmetry is introduced, particle-hole symmetry assures that nonlinear conductance terms maintain certain symmetries with respect to magnetic field and gate voltage. These analytic results are supported by numerical simulations. Analogous results are obtained for the electron heat current. Finally, we demonstrate that a double-dot Aharonov-Bohm interferometer can act as a charge rectifier when two conditions are met simultaneously: (i) many-body effects are included, here in the form of inelastic scattering; and (ii) time reversal symmetry is broken.