Modelling precipitation kinetics and investigating age-hardening behaviour of 2219Al alloys microalloyed with Cd

Journal of Thermal Analysis and Calorimetry - 2219Al alloys microalloyed with varying Cd contents were processed by casting technique. Differential scanning calorimetry (DSC) was performed from 323...     

Mapping the Transformation [{RuII(CO)3Cl2}2]→[RuI2(CO)4]2+: Implications in Binuclear Water–Gas Shift Chemistry

The complete sequence of reactions in the base‐promoted reduction of [{Ru^II(CO)₃Cl₂}₂] to [Ru^I₂(CO)₄]²⁺ has been unraveled. Several μ‐OH, μ:κ²‐CO₂H‐bridged diruthenium(II) complexes have been synthesized; they are the direct results of the nucleophilic activation of metal‐coordinated carbonyls by hydroxides. The isolated compounds are [Ru₂(CO)₄(μ:κ²‐C,O‐CO₂H)₂(μ‐OH)(NP^F‐Am)₂][PF₆] ( 1 ; NP^F‐Am=2‐amino‐5,7‐trifluoromethyl‐1,8‐naphthyridine) and [Ru₂(CO)₄(μ:κ²‐C,O‐CO₂H)(μ‐OH)(NP‐Me₂)₂][BF₄]₂ ( 2 ), secured by the applications of naphthyridine derivatives. In the absence of any capping ligand, a tetranuclear complex [Ru₄(CO)₈(H₂O)₂(μ³‐OH)₂(μ:κ²‐C,O‐CO₂H)₄][CF₃SO₃]₂ ( 3 ) is isolated. The bridging hydroxido ligand in 1 is readily replaced by a π‐donor chlorido ligand, which results in [Ru₂(CO)₄(μ:κ²‐C,O‐CO₂H)₂(μ‐Cl)(NP‐PhOMe)₂][BF₄] ( 4 ). The production of [Ru₂(CO)₄]²⁺ has been attributed to the thermally induced decarboxylation of a bis(hydroxycarbonyl)–diruthenium(II) complex to a dihydrido–diruthenium(II) species, followed by dinuclear reductive elimination of molecular hydrogen with the concomitant formation of the Ru^I? Ru^I single bond. This work was originally instituted to find a reliable synthetic protocol for the [Ru₂(CO)₄(CH₃CN)₆]²⁺ precursor. It is herein prescribed that at least four equivalents of base, complete removal of chlorido ligands by Tl^I salts, and heating at reflux in acetonitrile for a period of four hours are the conditions for the optimal conversion. Premature quenching of the reaction resulted in the isolation of a trinuclear Ru^I₂Ru^II complex [{Ru(NP‐Am)₂(CO)}{Ru₂(NP‐Am)₂(CO)₂(μ‐CO)₂}(μ₃:κ³‐C,O,O′‐CO₂)][BF₄]₂ ( 6 ). These unprecedented diruthenium compounds are the dinuclear congeners of the water–gas shift (WGS) intermediates. The possibility of a dinuclear pathway eliminates the inherent contradiction of pH demands in the WGS catalytic cycle in an alkaline medium. A cooperative binuclear elimination could be a viable route for hydrogen production in WGS chemistry.     

Characterization of the tryptophan residues of human placental ribonuclease inhibitor and its complex with bovine pancreatic ribonuclease A by steady-state and time-resolved emission spectroscopy

Human placental ribonuclease inhibitor (hRI) containing six tryptophan (Trp) residues located at positions 19, 261, 263, 318, 375, and 438 and its complex with RNase A have been studied using steady-state and time-resolved fluorescence (298 K) as well as low-temperature phosphorescence (77 K). Two Trp residues in wild-type hRI and also in the protein-protein complex with RNase A are resolved optically. The accessible surface area values of Trp residues in the wild-type hRI and its complex and consideration of inter-Trp energy transfer in the wild-type hRI reveal that one of the Trp residues is Trp19, which is located in a hydrophobic buried region. The other Trp residue is tentatively assigned as Trp375 based on experimental results on wild-type hRI and its complex. This residue in the wild-type hRI is more or less solvent exposed. Both the Trp residues are perturbed slightly on complex formation. Trp19 moves slightly toward a more hydrophobic region, and the environment of Trp375 becomes less solvent exposed. The complex formation also results in a more heterogeneous environment for both the optically resolved Trp residues.     

Quantum well engineering of InAlAs/InGaAs HEMTs for low impact ionization applications

The performance of InAlAs/InGaAs quantum well field effect transistors are subject to high impact ionization and band-to-band tunneling (BTBT) due to its narrow bandgap feature. In this work, the energy gap is engineered using strain and quantization techniques to increase the effective energy gap leading to low impact ionization and BTBT leakage current. It is shown that the impact ionization is reduced in 5 nm channel device as compared to 13 nm device with onset at approximately E_geff/q. Also the band-to-band-tunneling current is reduced due to the increase in effective energy gap. We have also investigated the effects of quantum well engineering on the dc performance of InGaAs HEMTs.     

Posterior Sensitivity to the Sampling Distribution and the Prior: More than One Observation

Sensitivity of a posterior quantity (f, P) to the choice of the sampling distribution f and prior P is considered. Sensitivity is measured by the range of (f, P) when f and P vary in nonparametric classes ^f and ^P respectively. Direct and iterative methods are described which obtain the range of (f, P) over f ^f when prior P is fixed, and also the overall range over f ^f and P ^P . When multiple i.i.d. observations X ₁,...,X _k are observed from f, the posterior quantity (f, P) is not a ratio-linear function of f. A method of steepest descent is proposed to obtain the range of (f, P). Several examples illustrate applications of these methods.     

Environment-Sensitive Fluorescence of 7-Nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-Labeled Ligands for Serotonin Receptors

Serotonin is a neurotransmitter that plays a crucial role in the regulation of several behavioral and cognitive functions by binding to a number of different serotonin receptors present on the cell surface. We report here the synthesis and characterization of several novel fluorescent analogs of serotonin in which the fluorescent NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl) group is covalently attached to serotonin. The fluorescent ligands compete with the serotonin_1A receptor specific radiolabeled agonist for binding to the receptor. Interestingly, these fluorescent ligands display a high environmental sensitivity of their fluorescence. Importantly, the human serotonin_1A receptor stably expressed in CHO-K1 cells could be specifically labeled with one of the fluorescent ligands with minimal nonspecific labeling. Interestingly, we show by spectral imaging that the NBD-labeled ligand exhibits a red edge excitation shift (REES) of 29 nm when bound to the receptor, implying that it is localized in a restricted microenvironment. Taken together, our results show that NBD-labeled serotonin analogs offer an attractive fluorescent approach for elucidating the molecular environment of the serotonin binding site in serotonin receptors. In view of the multiple roles played by the serotonergic systems in the central and peripheral nervous systems, these fluorescent ligands would be useful in future studies involving serotonin receptors.     

Stereoselective aldol-type cyclization reaction mediated by dibutylboron triflate/diisopropylethylamine

[reaction: see text] Dibutylboron triflate/diisopropylethylamine mediated aldol-type cyclization provides an expedient route for the stereoselective synthesis of cyclic ethers in a single step. The method is highly efficient for the stereoselective synthesis of 4-cis-tetrahydropyranones. The reaction is proposed to proceed via an S(N)1-type mechanism through a chair-like transition state, in which both substituents occupy equatorial positions.     

Energy transfer photophysics from serum albumins to sequestered 3-hydroxy-2-naphthoic acid, an excited state intramolecular proton-transfer probe

The steady-state and time-resolved studies of the sensitized emission of the excited-state proton transfer (ESIPT) probe 3-hydroxy-2-naphthoic acid (3HNA) when bound to bovine serum albumin (BSA) and human serum albumin (HSA) indicate that the nonradiative dipole-dipole F?rster type energy transfer from Trp singlet state of proteins to the ESIPT singlet state of 3HNA is greater in the case of HSA. This is supported by the distance and the orientation of the donor-acceptor pair obtained from the protein-ligand docking studies. The docking studies of the complex of BSA-3HNA also indicate that Trp 134 rather than Trp 213 is involved in the energy transfer process. The local environment of Trp 134 in BSA rather than that of Trp 213 is perturbed because of interaction with 3HNA as revealed by the optical resolution of Trp 134 phosphorescence in the complex at 77 K. Docking studies support the larger rotational correlation time, thetac (approximately 50 ns), observed for Trp residue/residues in the complexes of HSA and BSA compared with that in the free proteins.     

A new coordination mode of the photometric reagent glyoxalbis(2-hydroxyanil) (H2gbha): bis-bidentate bridging by gbha2- in the redox series [(mu-gbha)[Ru(acac)2]2]n (n = -2, -1, 0, +1, +2), including a radical-bridged diruthenium(III) and a Ru(III)/Ru(IV) intermediate

The bis-bidentate bridging function of gbha2- with N,O-/N,O- coordination was observed for the first time in the complex (mu-gbha)[Ru(III)(acac)2]2 (1). Density functional theory calculations of 1 yield a triplet ground state with a large (deltaE > 6000 cm(-1)) singlet-triplet gap. Intermolecular antiferromagnetic coupling was observed (J approximately -5.3 cm(-1)) for the solid. Complex 1 undergoes two one-electron reduction and two one-electron oxidation steps; the five redox forms [(mu-gbha)[Ru(acac)2]2]n (n = -2, -1, 0, +1, +2) were characterized by UV-vis-NIR spectroelectrochemistry (NIR = near infrared). The paramagnetic intermediates were also investigated by electron paramagnetic resonance (EPR) spectroscopy. The monoanion with a comproportionation constant K(c) of 2.7 x 10(8) does not exhibit an NIR band for a Ru(III)/Ru(II) mixed-valent situation; it is best described as a 1,4-diazabutadiene radical anion containing ligand gbha*3-, which binds two ruthenium(III) centers. A Ru(III)-type EPR spectrum with g1 = 2.27, g2 = 2.21, and g3 = 1.73 is observed as a result of antiferromagnetic coupling between one Ru(III) and the ligand radical. The EPR-active monocation (K(c) = 1.7 x 10(6)) exhibits a broad (deltanu(1/2) = 2600 cm(-1)) intervalence charge-transfer band at 1800 nm, indicating a valence-averaged (Ru3.5)2 formulation (class III) with a tendency toward class II (borderline situation).