Blue luminescence of Au nanoclusters embedded in silica matrix

Photoluminescence study using the 325 nm He-Cd excitation is reported for the Au nanoclusters embedded in SiO2 matrix. Au clusters are grown by ion beam mixing with 100 KeV Ar+ irradiation on Au [40 nm]/SiO2 at various fluences and subsequent annealing at high temperature. The blue bands above ~3 eV match closely with reported values for colloidal Au nanoclusters and supported Au nanoislands. Radiative recombination of sp electrons above Fermi level to occupied d-band holes are assigned for observed luminescence peaks. Peaks at 3.1 eV and 3.4 eV are correlated to energy gaps at the X- and L-symmetry points, respectively, with possible involvement of relaxation mechanism. The blue shift of peak positions at 3.4 eV with decreasing cluster size is reported to be due to the compressive strain in small clusters. A first principle calculation based on density functional theory using the full potential linear augmented plane wave plus local orbitals (FP-LAPW+LO) formalism with generalized gradient approximation (GGA) for the exchange correlation energy is used to estimate the band gaps at the X- and L-symmetry points by calculating the band structures and joint density of states (JDOS) for different strain values in order to explain the blueshift of ~0.1 eV with decreasing cluster size around L-symmetry point.Comment: 13 pages, 7 Figures Only in PDF format; To be published in J. of Chem. Phys. (Tentative issue of publication 8th December 2004

A Strategy for the Sequential Recovery of Biomacromolecules from Red Macroalgae Porphyra umbilicalis Kützing

A nondestructive, multicomponent fractionation strategy has been developed to extract proteins and polysaccharides from the red macroalgae Porphyra umbilicalis collected along the west coast of Sweden and cultivated indoors under controlled conditions. First, a protein-rich fraction was extracted in an ice-cold alkaline solution. The overall protein content in Porphyra umbilicalis was estimated to be 30.6% of the dry weight, and out of that, 15.0% could be recovered. Water-soluble polysaccharides were then extracted from the insoluble residual fraction using sequential alkaline and acidic treatments at 90 °C for 4 h. Spectroscopic and chromatographic analyses of the polysaccharide fractions show that high-molecular-weight carrageenans were obtained from the alkaline extraction and a galactose-rich pectin substance was obtained from the acidic extraction. The insoluble fraction remaining after all extraction steps was rich in cellulose. An elemental analysis of Porphyra umbilicalis via scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDS) showed the presence of C, O, Na, Ca, Mg, Al, Cl, and S. However, no heavy metals or other toxic elements, such as Pb, Hg, and As, were found

Probing the Dynamic Nature of Water Molecules and Their Influences on Ligand Binding in a Model Binding Site

The model binding site of the cytochrome <i>c</i> peroxidase (CCP) W191G mutant is used to investigate the structural and dynamic properties of the water network at the buried cavity using computational methods supported by crystallographic analysis. In particular, the differences of the hydration pattern between the uncomplexed state and various complexed forms are analyzed as well as the differences between five complexes of CCP W191G with structurally closely related ligands. The ability of docking programs to correctly handle the water molecules in these systems is studied in detail. It is found that fully automated prediction of water replacement or retention upon docking works well if some additional preselection is carried out but not necessarily if the entire water network in the cavity is used as input. On the other hand, molecular interaction fields for water calculated from static crystal structures and hydration density maps obtained from molecular dynamics simulations agree very well with crystallographically observed water positions. For one complex, the docking and MD results sensitively depend on the quality of the starting structure, and agreement is obtained only after redetermination of the crystal structure and refinement at higher resolution

Lifetime and predissociation yield of ¹⁴N₂b¹Πu(v=1) revisited: effects of rotation

Coupled-channel Schrödinger-equation calculations of the lifetime of the b(v=1) level of ¹⁴N₂ reveal strong rotational effects, with extreme lifetime shortening at high J as a result of rapidly increasing predissociation. Comparison with experiment indicates that the apparent discrepancy between the results of Sprengers et al. [J. Chem. Phys.120, 8973 (2004)] and Oertel et al. [Chem. Phys. Lett.82, 552 (1981)] is an artifact, caused by the significant J dependence of the lifetime, together with differing experimental conditions. This comparison is facilitated by the analysis of further experimental data, not previously reported by Sprengers et al. The strong J dependence of the b(v=1)predissociation yield is likely to lead to significant consequences for the interpretation of N₂fluorescence spectra and aeronomical data.This work was supported by Australian Research Council Discovery Program Grant No. DP0558962 and the European Community, through the Access to Research Infrastructures action of the Improving Human Potential Program, Contract No. HPRI-CT-1999-00041

Consequences of High Adatom Energy during Pulsed Laser Deposition of La_0.7Sr_0.3MnO₃

The impact of the adatom energy on the stoichiometry, surface morphology, and crystalline twinning during pulsed laser deposition of La_0.7Sr_0.3MnO₃ is studied. We show that although nonthermal growth using highly energetic adatoms results in very smooth ultrathin films, it also causes preferential resputtering of Mn and a surface roughening transition with increasing film thickness. This can be circumvented by carefully tuning the adatom energy into thermal growth, resulting in more Mn rich samples and a delayed roughening transition. Furthermore, we demonstrate that the crystalline twinning can be controlled by controlling the adatom energy. Hence, a detailed control of the adatom energy during growth opens for better stoichiometry control as well as surface quality

Specific Binding of a β-Cyclodextrin Dimer to the Amyloid β Peptide Modulates the Peptide Aggregation Process

Alzheimer’s disease involves progressive neuronal loss. Linked to the disease is the amyloid β (Aβ) peptide, a 38–43-amino acid peptide found in extracellular amyloid plaques in the brain. Cyclodextrins are nontoxic, cone-shaped oligosaccharides with a hydrophilic exterior and a hydrophobic cavity making them suitable hosts for aromatic guest molecules in water. β-Cyclodextrin consists of seven α-d-glucopyranoside units and has been shown to reduce the level of fibrillation and neurotoxicity of Aβ. We have studied the interaction between Aβ and a β-cyclodextrin dimer, consisting of two β-cyclodextrin monomers connected by a flexible linker. The β-cyclodextrin monomer has been found to interact with Aβ(1–40) at sites Y10, F19, and/or F20 with a dissociation constant (<i>K</i>_D) of 3.9 ± 2.0 mM. Here ¹H–¹⁵N and ¹H–¹³C heteronuclear single-quantum correlation nuclear magnetic resonance (NMR) spectra show that in addition, the β-cyclodextrin monomer and dimer bind to the histidines. NMR translational diffusion experiments reveal the increased affinity of the β-cyclodextrin dimer (apparent <i>K</i>_D of 1.1 ± 0.5 mM) for Aβ(1–40) compared to that of the β-cyclodextrin monomer. Kinetic aggregation experiments based on thioflavin T fluorescence indicate that the dimer at 0.05–5 mM decreases the lag time of Aβ aggregation, while a concentration of 10 mM increases the lag time. The β-cyclodextrin monomer at a high concentration decreases the lag time of the aggregation. We conclude that cyclodextrin monomers and dimers have specific, modulating effects on the Aβ(1–40) aggregation process. Transmission electron microscopy shows that the regular fibrillar aggregates formed by Aβ(1–40) alone are replaced by a major fraction of amorphous aggregates in the presence of the β-cyclodextrin dimer

In Vitro and Mechanistic Studies of an Antiamyloidogenic Self-Assembled Cyclic d,l‑α-Peptide Architecture

Misfolding of the Aβ protein and its subsequent aggregation into toxic oligomers are related to Alzheimer’s disease. Although peptides of various sequences can self-assemble into amyloid structures, these structures share common three-dimensional features that may promote their cross-reaction. Given the significant similarities between amyloids and the architecture of self-assembled cyclic d,l-α-peptide, we hypothesized that the latter may bind and stabilize a nontoxic form of Aβ, thereby preventing its aggregation into toxic forms. By screening a focused library of six-residue cyclic d,l-α-peptides and optimizing the activity of a lead peptide, we found one cyclic d,l-α-peptide (<b>CP-2</b>) that interacts strongly with Aβ and inhibits its aggregation. In transmission electron microscopy, optimized thioflavin T and cell survival assays, <b>CP-2</b> inhibits the formation of Aβ aggregates, entirely disassembles preformed aggregated and fibrillar Aβ, and protects rat pheochromocytoma PC12 cells from Aβ toxicity, without inducing any toxicity by itself. Using various immunoassays, circular dichroism spectroscopy, photoinduced cross-linking of unmodified proteins (PICUP) combined with SDS/PAGE, and NMR, we probed the mechanisms underlying <b>CP-2</b>’s antiamyloidogenic activity. NMR spectroscopy indicates that <b>CP-2</b> interacts with Aβ through its self-assembled conformation and induces weak secondary structure in Aβ. Upon coincubation, <b>CP-2</b> changes the aggregation pathway of Aβ and alters its oligomer distribution by stabilizing small oligomers (1–3 mers). Our results support studies suggesting that toxic early oligomeric states of Aβ may be composed of antiparallel β-peptide structures and that the interaction of Aβ with <b>CP-2</b> promotes formation of more benign parallel β-structures. Further studies will show whether these kinds of abiotic cyclic d,l-α-peptides are also beneficial as an intervention in related in vivo models