Spectrophotometric and Nuclear Magnetic Resonance Study of Temperature- and Pressure-induced Changes in the Intimate Solvation of Neodymium (III) in N,N-Dimethylformamide and Trimethylphosphate

The temperature- and pressure-dependent equilibria for the addition of an extra N, N-dimethylforrnamide (DMF) or trimethylphosphate (TMP) ligand onto [Nd (DMF)₈]³⁺- and [Nd(TMP)₆]³⁺-species respectively have been measured by visible spectrophotometry. Complementary NMR. studies on other lanthanide ions show a gradual shift in preference for the lower coordination number across the lanthanide series.     

High-Pressure 17O-NMR. Study of Vanadium (II) in Water: A Second Example of an Associative Interchange Mechanism (Ia) for Solvent Exchange on an Octahedral Divalent Transition-Metal Ion

The water exchange of [V(H₂O)₆]Cl₂ in aqueous solution has been studied as a function of temperature and pressure (up to 250 MPa), by measuring the ¹⁷O-FT-NMR. line-widths of the free water resonance at 8.13 MHz. The kinetic parameters obtained are K = 87±4 s^?1, ΔH^* = +61.8 ± 0.7 kJ mo1^?1 and ΔS^* = ?0.4±1.9 J mol^?1 K^?1. A pressure-independent volume of activation ΔV^* = ?4.1±0.1 cm³ mol^?1 is obtained, suggesting an associative interchange (I^a) mechanism for this early divalent metal ion.     

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Influence of the shallow impurity environment on the luminescence of single-quantum dots

The evolution of low-temperature photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power. At very low powers, where multi-exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed. In this weak excitation regime, a significant difference in the fine structure of single-QD spectra is observed not only among different, structurally identical QDs of the same sample, but also among spectra taken from the same single QD excited above and below the AlGaAs barrier. A time-resolved, two-color pump and probe PL experiment on a single QD indicates relaxation times between the different spectral shapes in the ms-range. A model, taking into account the influence of the shallow impurities in the environment of each QD, explains the experimental results.     

Few-particle effects in semiconductor quantum dots: observation of multicharged excitons

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.     

NMR characterization of 1,1′‐binaphthyl‐2,2′‐diyl hydrogen phosphate binding to chiral molecular micelles

NMR spectroscopy was used to characterize the binding of the chiral compound 1,1′‐binaphthyl‐2,2′‐diyl hydrogen phosphate (BNP) to five molecular micelles with chiral dipeptide headgroups. Molecular micelles have covalent linkages between the surfactant monomers and are used as chiral mobile phase modifiers in electrokinetic chromatography. Nuclear overhauser enhancement spectroscopy (NOESY) analyses of (S)‐BNP:molecular micelle mixtures showed that in each solution the (S)‐BNP interacted predominately with the N‐terminal amino acid of the molecular micelle's dipeptide headgroup. NOESY spectra were also used to generate group binding maps for (S)‐BNP:molecular micelle mixtures. In these maps, percentages are assigned to the (S)‐BNP protons to represent the relative strengths of their interactions with a specified molecular micelle proton. All maps showed that (S)‐BNP inserted into a previously reported chiral groove formed between the molecular micelle's dipeptide headgroup and hydrocarbon chain. In the resulting intermolecular complexes, the (S)‐BNP protons nearest to the analyte phosphate group were found to point toward the N‐terminal Hα proton of the molecular micelle headgroup. Finally, pulsed field gradient NMR diffusion experiments were used to measure association constants for (R) and (S)‐BNP binding to each molecular micelle. These K values were then used to calculate the differences in the enantiomers' free energies of binding, Δ(ΔG). The NMR‐derived Δ(ΔG) values were found to scale linearly with electrokinetic chromatography (EKC) chiral selectivities from the literature. Copyright © 2010 John Wiley & Sons, Ltd.     

High-Pressure Stopped-Flow Study of the Kinetics of Base Hydrolysis of the Dichromate Ion by Hydroxide Ion,Ammonia, Water,and 2,6-Lutidine in Aqueous Solution

Volumes of activation for the base hydrolysis of the dichromate anion have been measured at 298.2 K, using high-pressure stopped-flow spectrophotometry. The values of ΔV* (cm³ · mol^?1), ? 17.9 ± 0.6, ? 19.2 ± 0.9, ? 24.9 ± 0.9 and ? 26.0 ± 0.7 for OH^?, NH₃, H₂O and 2,6-lutidine, respectively, are consistent with an interchange mechanism with associative activation mode (I^a).     

Receptor-based virtual ligand screening for the identification of novel CDC25 phosphatase inhibitors

CDC25 phosphatases play critical roles in cell cycle regulation and are attractive targets for anticancer therapies. Several small non-peptide molecules are known to inhibit CDC25, but many of them appear to form a covalent bond with the enzyme or act through oxidation of the thiolate group of the catalytic cysteine. Structure-based virtual ligand screening computations were performed with FRED, Surflex, and LigandFit, a compound collection of over 310,000 druglike molecules and the crystal structure of CDC25B in order to identify novel classes of ligands. In vitro experiments carried out on a selected list of 1500 molecules led to the discovery of 99 compounds able to inhibit CDC25B activity at 100 microM. Further docking computations were applied, allowing us to propose a binding mode for the most potent molecule (IC50 = 13 microM). Our best compounds represent promising new classes of CDC25 inhibitors that also exhibit antiproliferative properties.     

High Pressure 17O-FT-NMR. Evidence for the Associative Nature of Substitution Reactions on Mn2+ in Water

The water exchange on [Mn(H₂O)₆](ClO₄)₂ in aqueous solution has been studied as a function of pressure (up to 250 MPa), measuring ¹⁷O-FT-NMR. linewidths of the free water resonance at 8.13 MHz. A pressure independent volume of activation, ΔV≠ = ?6.2 ± 0.2 cm³ mol^?1 is obtained, producing a clear evidence that, contrary to general belief, the mechanism for solvent exchange on Mn²⁺ is an associative interchange (I_a).