Hydrogen‐Bonding Interactions Trigger a Spin‐Flip in Iron(III) Porphyrin Complexes

A key step in cytochrome P450 catalysis includes the spin‐state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin‐state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen‐bonding interactions on the electronic structure of a five‐coordinate iron(III) octaethyltetraarylporphyrin chloride. The spin state of the metal was found to switch reversibly between high (S=⁵/₂) and intermediate spin (S=³/₂) with hydrogen bonding. Our study highlights the possible effects and importance of hydrogen‐bonding interactions in heme proteins. This is the first example of a synthetic iron(III) complex that can reversibly change its spin state between a high and an intermediate state through weak external perturbations.     

One‐Pot Synthesis of Highly Fluorescent Pyrido[1,2‐a]indole Derivatives through CH/NH Activation: Photophysical Investigations and Application in Cell Imaging

We describe a straightforward strategy for the synthesis of strongly fluorescent pyridoindoles by Pd‐catalyzed oxidative annulations of internal alkynes with C‐3 functionalized indoles through C?H/N?H bond activation in a one‐pot tandem process. Mechanistic investigations reveal the preferential activation of N?H indole followed by C?H activation during the cyclization process. Photophysical properties of pyridoindoles exhibited the highest fluorescence quantum yield of nearly 80 %, with emission color varying from blue to green to orange depending on the substructures. Quantum mechanical calculations provide insights into the observed photophysical properties. The strong fluorescence of the pyrido[1,2‐a]indole derivative has been employed in subcellular imaging, which demonstrates its localization in the cell nucleus.     

Limiting behavior of the ratio of 〈r〉 and 〈r〉 in the helium sequence

The limiting of the 〈r〈/〈r〈 ratio in the He sequence is critically examined at different levels of approximations leading to some interesting results.     

Phonon and thermal properties of achiral single wall carbon nanotubes

A detailed theoretical study of the phonon and thermal properties of achiral single wall carbon nanotubes has been carried out using force constant model considering up to third nearest-neighbor interactions. We have calculated the phonon dispersions, density of states, radial breathing modes (RBM) and the specific heats for various zigzag and armchair nanotubes, with radii ranging from 2.8 Å to 11.0 Å. A comparative study of phonon spectrum with measured Raman data reveals that the number of Raman active modes for a tube does not depend on the number of atoms present in the unit cell but on its chirality. Calculated phonon modes at the zone center more or less accurately predicted the Raman active modes. The radial breathing mode is of particular interest as for a specific radius of a nanotube it is found to be independent of its chirality. We have also calculated the variation of RBM and G-band modes for tubes of different radii. RBM shows an inverse dependence on the radius of the tube. Finally, the values of specific heat are calculated for various nanotubes at room temperature and it was found that the specific heat shows an exponential dependence on the diameter of the tube.     

1,3,5‐Triaza‐7‐phosphaadamantane (PTA) Derived Caged Phosphines for Palladium‐Catalyzed Selective Functionalization of Nucleosides and Heteroarenes

Phosphines have, in combination with transition metals, played a pivotal role in the rapid development of efficient catalytic processes. Caged phosphines constitute a class of three‐dimensional scaffolds providing unique control over steric and electronic properties. The versatility of the caged phosphine ligands has been demonstrated elegantly by the groups of Verkade, Gonzalvi as well as Stradiotto. Our research group has also been working extensively for the past several years in the development of 1,3,5‐triaza‐7‐phosphaadamantane‐based caged ligands and in this personal note we have summarized these applications pertaining to the modification of biologically useful nucleosides and heteroarenes.     

Formation of single-walled aluminosilicate nanotubes from molecular precursors and curved nanoscale intermediates

We report the identification and elucidation of the mechanistic role of molecular precursors and nanoscale (1-3 nm) intermediates with intrinsic curvature in the formation of single-walled aluminosilicate nanotubes. We characterize the structural and compositional evolution of molecular and nanoscale species over a length scale of 0.1-100 nm by electrospray ionization mass spectrometry, nuclear magnetic resonance spectroscopy ((27)Al liquid-state, (27)Al and (29)Si solid-state MAS), and dynamic light scattering. Together with structural optimization of key experimentally identified species by solvated density functional theory calculations, this study reveals the existence of intermediates with bonding environments, as well as intrinsic curvature, similar to the structure of the final nanotube product. We show that "proto-nanotube-like" intermediates with inherent curvature form in aqueous synthesis solutions immediately after initial hydrolysis of reactants, disappear from the solution upon heating to 95 °C due to condensation accompanied by an abrupt pH decrease, and finally form ordered single-walled aluminosilicate nanotubes. Detailed quantitative analysis of NMR and ESI-MS spectra from the relevant aluminosilicate, aluminate, and silicate solutions reveals the presence of a variety of monomeric and polymeric aluminate and aluminosilicate species (Al(1)Si(x)-Al(13)Si(x)), such as Keggin ions [AlO(4)Al(12)(OH)(24)(H(2)O)(12)](7+) and polynuclear species with a six-membered Al oxide ring unit. Our study also directly reveals the complexation of aluminate and aluminosilicate species with perchlorate species that most likely inhibit the formation of larger condensates or nontubular structures. Integration of all of our results leads to the construction of the first molecular-level mechanism of single-walled metal oxide nanotube formation, incorporating the role of monomeric and polymeric aluminosilicate species as well as larger nanoparticles.     

Fluorescent Probing of Protein Bovine Serum Albumin Stability and Denaturation Using Polarity Sensitive Spectral Response of a Charge Transfer Probe

The polarity sensitive photo-induced intra-molecular charge transfer (ICT) fluorescence probe (E)-3-(4-methylamino-phenyl)-acrylic acid ethyl ester (MAPAEE) has been used to study the model protein Bovine Serum Albumin (BSA) in its native and thermal and urea induced denatured states. The interaction between BSA and the regular surfactant Sodium Dodecyl Sulphate (SDS) as well as the biologically relevant steroid-based amphiphile Sodium Deoxycholate (NaDC) has also been very keenly followed using this ICT probe. The variation of micellar properties of both SDS and NaDC with increasing ionic strengths and in presence of the chaotrope urea has also been well documemted by the same probe. Steady-state spectroscopy, FRET, and fluorescence anisotropy measurements have been used to gain better insight into these processes and the molecule MAPAEE to be a full-bodied fluorescent probe for studying such intricate biological systems, their properties and interactions.     

A Rhodamine-Based Dual Chemosensor for Cu(II) and Fe(III)

An “off-on” rhodamine-based fluorescence probe for the selective signaling of Cu(II) and Fe(III) has been designed and synthesized. The optical properties of this compound have been investigated in acetonitrile-water (1:1) binary solution. Very interestingly, this compound showed sensitivity and selectivity towards Cu(II) during absorption process and towards Fe(III) during emission process. So this is a nice example of an excellent dual chemosensor for two biologically/physiologically very important transition metal ions using only the two very different techniques (absorption and emission); both cases displayed only intensity enhancement.