Modeling side-on NO coordination to type 2 copper in nitrite reductase: structures, energetics, and bonding

DFT calculations reveal the existence of metastable side-on {CuNO}10 and {CuNO}11 species relevant to the type 2 copper site of nitrite reductase (CuNIR). Side-on NO coordination seems especially favorable in energy terms for the {CuNO}11 species. The {CuNO}11 geometry parameters also seem to be in better agreement with those reported for a crystallographically characterized CuNIR intermediate, relative to the {CuNO}10 parameters.     

Metal-promoted aromatic ring amination and deamination reactions at a diazo ligand coordinated to rhodium and ruthenium

Reactions of MCl(3).3H(2)O (M = Rh and Ru) with the ligand 2-[(2-N-arylamino)phenylazo]pyridine [HL(1); NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(1a)), NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(1b)), and NH(4)C(5)N=NC(6)H(4)N(H)C(5)H(4)N (HL(1c))] in the presence of dilute NEt(3) afforded multiple products. In the case of rhodium, two green compounds, viz. [Rh(L(1))(2)](+) ([2](+)) and [RhCl(pap)(L(1))](+) ([3](+)), where L(1) and pap stand for the conjugate base of [HL(1)] and 2-(phenylazo)pyridine, respectively, were separated on a preparative thin layer chromatographic plate. The reaction of RuCl(3).3H(2)O, on the other hand, produced two brown compounds, viz. [RuCl(HL(1))(L(1))] (4) and [RuCl(pap)(L(1))] (5), respectively, as the major products. The X-ray structures of the representative complexes are reported. Except for complex 2, and 4, the products are formed due to the cleavage of an otherwise unreactive C(phenyl)-N(amino) bond. In complex 4, one of the tridentate ligands (HL(1)) does not use its maximum denticity and coordinates as a neutral bidentate donor. Plausible reasons for the differences in their modes of coordination of the ligands as in 2 and 4 have been discussed. The ligand pap in the cationic mixed ligand complex [3](+) reacts instantaneously with ArNH(2) to produce an ink-blue compound, [RhCl(HL(2))(L(1))](+) ([6](+)) in a high yield. The ligand HL(2) is formed due to regioselective fusion of ArNH(2) residue at the para carbon of the phenyl ring (with respect to the azo fragment) of pap in [3](+). The above complexes are generally intensely colored and show strong absorptions in the visible region, which are assigned to intraligand charge transfer transitions. These complexes undergo multiple and successive one-electron-transfer processes at the cathodic potentials. Electrogenerated cationic complexes of ruthenium(III), [4](+) and [5](+), showed rhombic EPR spectra at 77 K.     

Conformational study of papain in the presence of sodium dodecyl sulfate in aqueous medium

The interactions between a globular protein, papain and the anionic surfactant, sodium dodecyl sulfate (SDS) have been investigated in aqueous medium using fluorimetric, circular dichroism, Fourier transform infra-red, UV-vis spectrophotometric, dynamic light scattering, and nuclear magnetic resonance techniques. The conformational change of papain in aqueous solution has been studied in the presence of SDS. The results show the high alpha-helical content and unfolded structure of papain in the presence of SDS due to strong electrostatic repulsion leading to a "necklace and bead model" in protein-surfactant complexes.     

Asymmetric synthesis of (-)-tetrahydrolipstatin: an anti-aldol-based strategy

A stereoselective synthesis of (-)-tetrahydrolipstatin is described. The synthesis involves an asymmetric ester derived titanium enolate anti-aldol reaction, a nitro-aldol reaction to append the C-2' C(11) side chain, and a diastereoselective reduction of a beta-hydroxy ketone to an anti-1,3-diol functionality followed by its elaboration to (-)-tetrahydrolipstatin.     

Hydromagnetic laminar flow of a conducting fluid in an annulus in presence of a radial magnetic field

The object of the present paper is to study the MHD effects on the laminar flow of a viscous, incompressible and conducting fluid in an annulus with arbitrary time-varying pressure gradient and arbitrary initial velocity in presence of a radial magnetic field. Using finite Hankel transform, solutions for both the unsteady and steady flows under different prescribed pressure gradients have been found out.Notation H a constant characterising the intensity of the magnetic field - p hydrostatic pressure - _e magnetic permeability - coefficient of viscosity - kinematic coefficient of voscosity - conductivity of the medium - density - a radius of the inner cylinder - b radius of the outer cylinder - parameter - s positive root - J (sr) Bessel's function of first kind of ordergl - Y (sr) Bessel's function of second kind of order     

Equilibrium studies in solution involving nickel(II) complexes of flexidentate Schiff base ligands: isolation and structural characterization of the planar red and octahedral green species involved in the equilibrium

Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L1-OH-L3-OH) based on 1-(2-aminoethyl)piperazine (X=H, L1-OH; X=NO2, L2-OH; and X=Br, L3-OH) and their nickel(II) complexes (1a, 1b, 2, and 3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformations that allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedral complexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in the complexes have profound influences on the coordination geometry of the isolated products. With the parent ligand L1-OH, the product obtained is either a planar red compound [Ni(L1-O)]+, isolated as tetraphenylborate salt (1a), or an octahedral green compound [Ni(L1-NH)(H2O)3](2+), isolated with sulfate anion (1b); both have been crystallographically characterized. In aqueous solution, both these planar (S=0) and octahedral (S=1) forms are in equilibrium that has been followed in the temperature range 298-338 K by 1H NMR technique using the protocol of Evans's method. The large exothermicity of the equilibrium process [Ni(L1-O)]+ + 3H2O + H+<=>[Ni(L1-NH)(H2O)3](2+) (DeltaH degrees=-46 +/- 0.2 kJ mol(-1) and DeltaS degrees=-133 +/- 5 J K(-1) mol(-1)) reflects formation of three new Ni-OH2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L2-OH, the sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L3-OH, the exclusive product is a planar molecule 3 with associated tetraphenylborate anion. Both 2 and 3 have been structurally characterized by X-ray diffraction analysis.     

Unraveling the effect of defects,domain size,and chemical doping on photophysics and charge transport in covalent organic frameworks

Understanding the underlying physical mechanisms that govern charge transport in two-dimensional (2D) covalent organic frameworks (COFs) will facilitate the development of novel COF-based devices for optoelectronic and thermoelectric applications. In this context, the low-energy mid-infrared absorption contains valuable information about the structure–property relationships and the extent of intra- and inter-framework “hole” polaron delocalization in doped and undoped polymeric materials. In this study, we provide a quantitative characterization of the intricate interplay between electronic defects, domain sizes, pore volumes, chemical dopants, and three dimensional anisotropic charge migration in 2D COFs. We compare our simulations with recent experiments on doped COF films and establish the correlations between polaron coherence, conductivity, and transport signatures. By obtaining the first quantitative agreement with the measured absorption spectra of iodine doped (aza)triangulene-based COF, we highlight the fundamental differences between the underlying microstructure, spectral signatures, and transport physics of polymers and COFs. Our findings provide conclusive evidence of why iodine doped COFs exhibit lower conductivity compared to doped polythiophenes. Finally, we propose new research directions to address existing limitations and improve charge transport in COFs for applications in functional molecular electronic devices.

This study highlights the importance of mid-infrared spectral signatures and discusses the fundamental mechanisms driving charge transport in COFs. Our analysis can hopefully guide the rational design of new COFs yielding higher conductivities.     

Hexafluoroisopropanol: the magical solvent for Pd-catalyzed C–H activation

Among numerous solvents available for chemical transformations, 1,1,1,3,3,3-hexafluoro-2-propanol (popularly known as HFIP) has attracted enough attention of the scientific community in recent years. Several unique features of HFIP compared to its non-fluoro analogue isopropanol have helped this solvent to make a difference in various subdomains of organic chemistry. One such area is transition metal-catalyzed C–H bond functionalization reactions. While, on one side, HFIP is emerging as a green and sustainable deep eutectic solvent (DES), on the other side, a major proportion of Pd-catalyzed C–H functionalization is heavily relying on this solvent. In particular, for distal aromatic C–H functionalizations, the exceptional impact of HFIP to elevate the yield and selectivity has made this solvent irreplaceable. Recent research studies have also highlighted the H-bond-donating ability of HFIP to enhance the chiral induction in Pd-catalyzed atroposelective C–H activation. This perspective aims to portray different shades of HFIP as a magical solvent in Pd-catalyzed C–H functionalization reactions.

Among numerous solvents available for chemical transformations, 1,1,1,3,3,3-hexafluoro-2-propanol (popularly known as HFIP) has attracted enough attention of the scientific community in recent years.