Diborane as a reducing agent - VI the novel reduction of indole-1-carboxaldehydes to 1-methyl- indoles,di(indolyimethyl) ethers and indolylmethyl indolines

Reduction of the indole-1-carboxaldehydes ($\text{1a}$-$\text{1f}$) with borane /THF gives the 1-methylindoles ($\text{4}$) in 42-91-% yields together with the di(indolylmethyl)ethers ($\text{8}$), the indolyl-methyl indolines ($\text{7}$), the unsymmetric ether($\text{10}$) and the indolenine ($\text{11}$) as the minor products, except $\text{7a}$. This appears to be the first report on the formation of symmetric ethers in the borane/THF reduction of an oxygen function. The formation of $\text{7a}$ and $\text{7b}$ from $\text{1a}$ and $\text{1b}$ implies that electrophilic substitution takes place primarily at position 3 of 3-substituted indoles, $\text{1c - 1f}$ did not form the corresponding $\text{7}$ probably because of steric hindrance. These results are discussed in relation to the mechanisms of borane/THF reduction, origin of the different products and electrophilic substitution in 3-substituted indoles.     

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 SiO(2) matrix. Au clusters are grown by ion beam mixing with 100 KeV Ar(+) irradiation on Au [40 nm]/SiO(2) at various fluences and subsequent annealing at high temperature. The blue bands above approximately 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 and 3.4 eV are correlated to energy gaps at the X- and L-symmetry points, respectively, with possible involvement of relaxation mechanism. The blueshift 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 formalism with generalized gradient approximation 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 for different strain values in order to explain the blueshift of approximately 0.1 eV with decreasing cluster size around L-symmetry point.     

Cancer Cell Imaging Using in Situ Generated Gold Nanoclusters

In situ generated fluorescent gold nanoclusters (Au‐NCs) are used for bio‐imaging of three human cancer cells, namely, lung (A549), breast (MCF7), and colon (HCT116), by confocal microscopy. The amount of Au‐NCs in non‐cancer cells (WI38 and MCF10A) is 20–40 times less than those in the corresponding cancer cells. The presence of a larger amount of glutathione (GSH) capped Au‐NCs in the cancer cell is ascribed to a higher glutathione level in cancer cells. The Au‐NCs exhibit fluorescence maxima at 490–530 nm inside the cancer cells. The fluorescence maxima and matrix‐assisted laser desorption ionization (MALDI) mass spectrometry suggest that the fluorescent Au‐NCs consist of GSH capped clusters with a core structure (Au_8‐13). Time‐resolved confocal microscopy indicates a nanosecond (1–3 ns) lifetime of the Au‐NCs inside the cells. This rules out the formation of aggregated Au–thiolate complexes, which typically exhibit microsecond (≈1000 ns) lifetimes. Fluorescence correlation spectroscopy (FCS) in live cells indicates that the size of the Au‐NCs is ≈1–2 nm. For in situ generation, we used a conjugate consisting of a room‐temperature ionic liquid (RTIL, [pmim][Br]) and HAuCl₄. Cytotoxicity studies indicate that the conjugate, [pmim][AuCl₄], is non‐toxic for both cancer and non‐cancer cells.     

Selective Killing of Breast Cancer Cells by Doxorubicin‐Loaded Fluorescent Gold Nanoclusters: Confocal Microscopy and FRET

Fluorescent gold nanoclusters (AuNCs) capped with lysozymes are used to deliver the anticancer drug doxorubicin to cancer and noncancer cells. Doxorubicin‐loaded AuNCs cause the highly selective and efficient killing (90 %) of breast cancer cells (MCF7) (IC₅₀=155 nm ). In contrast, the killing of the noncancer breast cells (MCF10A) by doxorubicin‐loaded AuNCs is only 40 % (IC₅₀=4500 nm ). By using a confocal microscope, the fluorescence spectrum and decay of the AuNCs were recorded inside the cell. The fluorescence maxima (at ≈490–515 nm) and lifetime (≈2 ns), of the AuNCs inside the cells correspond to Au_10–13. The intracellular release of doxorubicin from AuNCs is monitored by Förster resonance energy transfer (FRET) imaging.     

Competing Excited-State Hydrogen and Proton-Transfer Processes in 6-Azaindole-S3,4 and 2,6-Diazaindole-S3,4 Clusters (S=H2O,NH3)

Excited state hydrogen (ESHT) and proton (ESPT) transfer reaction pathways in the three and four solvent clusters of 6-azaindole (6AI-S_3,4) and 2,6-diazaindole (26DAI-S_3,4)(S=H₂O, NH₃) were computationally investigated to understand the fate of photo-excited biomolecules. The ESHT energy barriers in (H₂O)₃ complexes (39.6–41.3 kJmol⁻¹) were decreased in (H₂O)₄ complexes (23.1–20.2 kJmol⁻¹). Lengthening the solvent chain lowered the barrier because of the relaxed transition states geometries with reduced angular strains. Replacing the water molecule with ammonia drastically decreased the energy barriers to 21.4–21.3 kJmol⁻¹ in (NH₃)₃ complexes and 8.1–9.5 kJ mol⁻¹ in (NH₃)₄ complexes. The transition states were identified as H_a atom attached to the first solvent molecule. The formation of stronger hydrogen bonds in (NH₃)_3,4 complexes resulted in facile ESHT reaction than that in the (H₂O)_3,4 complexes. The ESPT energy barriers in 6AI-S_3,4 and 26DAI-S_3,4 were found to range between 40–73 kJmol⁻¹. The above values were significantly higher than that of the ESHT processes and hence are considered as a minor channel in the process. The effect of N(2) insertion was explored for the very first time in the isolated solvent clusters using local vibrational mode analysis. In DAI-S₄, the higher K_a(H_a⋯S_a) values depicted the increased photoacidity of the N(1)-H_a group which may facilitate the hydrogen transfer reaction. However, the increased N(6)⋯H_b bond length elevated the reaction barriers. Therefore, in the ESHT reaction channel, the co-existence of two competing factors led to a marginal/no change in the overall energy barriers due to the N(2) insertion. In the ESPT reaction pathway, the energy barriers showed notable increase upon N(2) insertion because of the increased N(6)⋯H_b bond length.     

Nickel(II) complexes of tetradentate NSNO pyridylthioazophenol ligands: Synthesis,characterization and crystal structure

Two sets of nickel(II) complexes of a series of tetradentate NSNO ligands were synthesized and isolated in their pure form. All these complexes, formulated as [Ni(L)Cl]₂ and [Ni(L)(N₃)]₂ [HL = pyridylthioazophenols], were characterized using physicochemical and spectroscopic tools. The solid-state structures of two complexes (1a and 2a) were established by X-ray crystallography. The geometry about the nickel ion of the complexes is octahedral and the complexes are dimeric in nature. In 1, two Ni(II) ions are bridged by two Cl⁻ anions while in 2 they are bridged by two azide ions in a μ-1,1-bridging fashion.     

Solvent-mediated morphological transformations in peptide-based soft structures

Self-assembly is routinely used for the design and fabrication of new and advanced materials. Biological building blocks such as short peptides undergo self-assembly to reveal a variety of hierarchical structures such as nanotubes, fibers, and spherical structures. Peptide-based soft spherical structures are potentially useful for a variety of applications including vehicles for drug delivery and biological molecules. This report describes a tripalmitoylated triskelion ditryptophan peptide conjugate (5) that self-assembles to form hollow spheres along with pores on the surface when higher concentration of the dichloromethane was used, but forms fiber structures when heated in toluene solvent.     

Water complexes of styrene and 4-fluorostyrene: a combined electronic, vibrational spectroscopic and ab-initio investigation

The binary complexes of water with styrene and fluorostyrene were investigated using LIF and FDIR spectroscopic techniques. The difference in the shifts of S 1 <-- S 0 electronic transitions clearly points out the disparity in the intermolecular structures of these two binary complexes. The FDIR spectra in the O-H stretching region indicate that water is a hydrogen bond donor in both complexes. The formation of a single O-H...pi hydrogen-bonded complex with styrene and an in-plane complex with fluorostyrene was inferred based on the analysis of the FDIR spectra in combination with ab initio calculations. The in-plane complex with fluorostyrene is characterized by the presence of O-H...F and C-H...O hydrogen bonds, leading to formation of a stable six-membered ring. The synergistic effect of O-H...F and C-H...O hydrogen bonds overwhelms the O-H...pi interaction in fluorostyrene-water complexes.