First structural example of a metal uncoordinated mesoionic imidazo[1,5-a]pyridine and its precursor intermediate copper complex: an insight to the catalytic cycle

Four copper complexes of a tridentate Schiff base ligand, 2-pyridyl-N-(2'-methylthiophenyl) methyleneimine, L(1) have been synthesized. All theses species, namely, [L(1)Cu(2)(SCN)(3)](n) (1), [Cu(SCN)(CH(3)CN)](n) (3), [(L(1))Cu(N(3))(Cl)] (4) and [(L(1))Cu(N(3))(SCN)] (5) have been structurally characterized. Complex 1 in acetonitrile promotes cycloaddition of a Cu(II) bound SCN(-) ion to L(1) that exclusively and stoichiometrically forms a mesoionic imidazo[1,5-a]pyridine, namely, 3-(imino-N'-2-methylthiophenyl)imidazo[1,5-a]pyridinium-1-thiolate (2) and a thiocyanato bridged Cu(i) complex, [Cu(SCN)(CH(3)CN)](n) (3). The X-ray crystal structure of 1 confirms the presence of square-pyramidal Cu(II) and tetrahedral Cu(I) ions in N(3)S(2) and N(2)S(2) coordination environments, respectively, bridged to each other via thiocyanate anion. The Cu(II) ions are bonded to a tridentate ligand L(1) and two SCN(-) ions occupy the remaining equatorial and an axial coordination site to adopt a square-pyramidal coordination geometry. To investigate which SCN(-) ion, axially or equatorially bound to Cu(II) center, underwent cycloaddition to L(1) to form 2, two mononuclear Cu(II) complexes 4 and 5 have been synthesized and their reactivity towards externally added KSCN was studied. The molecular structures of 4 and 5 feature a meridionally bound L(1) and an azide ion (N(3)(-)) in the square plane, while a Cl(-) or SCN(-) ion are occupying the axial site, respectively, to fulfill square-pyramidal coordination geometry. Complex 4 reacts with SCN(-) ion to form 5. That an MeCN solution of 5 itself, or of 5 in the presence of KSCN, does not produce 2, supports that possibly the Cu(II) bound equatorial SCN(-) ion is responsible for cycloaddition to L(1). Dark purple solid 2 has also been prepared (turnover number ~4 or 41% yield) efficiently following an alternative and easier one-pot synthesis procedure, that is from a mixture of KSCN and L(1) in the presence of a catalytic amount of anhydrous CuCl(2) (10 mol%) in MeCN in air. The X-ray crystal structure, (1)H NMR spectrum and solution conductivity measurements strongly support that 2 is mesoionic. An MeCN solution of 2 fluoresces at room temperature upon excitation at 240 nm with an emission maximum λ(em) at 470 nm, associated with a quantum yield of 0.16 with respect to a standard Rhodamine-6G fluorophore.     

Self-assembling dipeptide-based nontoxic vesicles as carriers for drugs and other biologically important molecules

Self-assembling short peptides can offer an opportunity to make useful nano-/microstructures that find potential application in drug delivery. We report here the formation of multivesicular structures from self-assembling water-soluble synthetic amphiphilic dipeptides containing a glutamic acid residue at the C-terminus. These vesicular structures are stable over a wide range of pH (pH 2-12). However, they are sensitive towards calcium ions. This causes the rupturing of these vesicles. Interestingly, these vesicles can not only encapsulate an anticancer drug and a fluorescent dye, but also can release them in the presence of calcium ions. Moreover, these multivesicular structures have the potential to carry biologically important molecules like cyclic adenosine monophosphate (cAMP) within the cells keeping their biological functions intact. A MTT cell-survival assay suggests the almost nontoxic nature of these vesicles. Thus, these peptide vesicles can be used as biocompatible delivery vehicles for carrying drugs and other bioactive molecules.     

Application of HR-MAS NMR spectroscopy for studying chemotype variations of Withania somnifera (L.) Dunal

Withania somnifera (L.) Dunal (Solanaceae), commonly known as Ashwagandha, is one of the most valued Indian medicinal plants with a number of pharmaceutical and nutraceutical applications. Metabolic profiling has been performed by HR-MAS NMR spectroscopy on fresh leaf and root tissue specimens from four chemotypes of W. somnifera. The HR-MAS NMR spectroscopy of lyophilized defatted leaf tissue specimens clearly distinguishes resonances of medicinally important secondary metabolites (withaferin A and withanone) and its distinctive quantitative variability among the chemotypes. A total of 41 metabolites were identified from both the leaf and root tissues of the chemotypes. The presence of methanol in leaf and root tissues of W. somnifera was detected by HR-MAS NMR spectroscopy. Multivariate principal component analysis (PCA) on HR-MAS (1) H NMR spectra of leaves revealed clear variations in primary metabolites among the chemotypes. The results of the present study demonstrated an efficient method, which can be utilized for metabolite profiling of primary and secondary metabolites in medicinally important plants.     

Nonsolvent application of ionic liquids: organo-catalysis by 1-alkyl-3-methylimidazolium cation based room-temperature ionic liquids for chemoselective N-tert-butyloxycarbonylation of amines and the influence of the C-2 hydrogen on catalytic efficiency

1-Alkyl-3-methylimidazolium cation based ionic liquids efficiently catalyze N-tert-butyloxycarbonylation of amines with excellent chemoselectivity. The catalytic role of the ionic liquid is envisaged as "electrophilic activation" of di-tert-butyl dicarbonate (Boc(2)O) through bifurcated hydrogen bond formation with the C-2 hydrogen of the 1-alkyl-3-methylimidazolium cation and has been supported by a downfield shift of the imidazolium C-2 hydrogen of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][NTf(2)]) from δ 8.39 to 8.66 in the presence of Boc(2)O in the (1)H NMR and a drastic reduction of the catalytic efficiency with 1-butyl-2,3-dimethylimidazolium ionic liquids that are devoid of the C-2 hydrogen. The differential time required for reaction with aromatic and aliphatic amines has offered means for selective N-t-Boc formation during inter and intramolecular competitions. Preferential N-t-Boc formation with secondary aliphatic amine has been achieved in the presence of primary aliphatic amine. Comparison of the catalytic efficiency for N-t-Boc formation with a common substrate revealed that [bmim][NTf(2)] is superior to the reported Lewis acid catalysts.     

Extended structure design with simple molybdenum oxide building blocks and urea as a directing agent

We report here a simple one-pot directed synthesis of an oxomolybdate urea composite in which elementary molybdenum oxide building blocks are linked together with the aid of urea. This type of directed material design resulted in large rod-like crystals of an inorganic-organic hybrid extended structure of {MoO 3(NH 2-CO-NH 2)} infinity consisting of right- and left-handed helical units. In the crystal structure urea acts both as a glue that links the inorganic molybdenum units into a helix and as a supramolecular linker for the stabilization of the crystal structure as a whole. This type of molecular topology resulted in an unexpectedly high thermal stability.     

Path integral Monte Carlo study of CO2 solvation in 4He clusters

We present a finite temperature quantum mechanical study of the dynamical and structural properties of small (4)He(N)-CO(2) clusters (N< or =17) using a path integral Monte Carlo (PIMC) method. The simulations were based on a He-CO(2) interaction potential with explicit dependence on the asymmetric stretch of the CO(2) molecule obtained at the CCSD(T) level. The shift of the CO(2) antisymmetric stretching (nu(3)) band origin and effective rotational constant were calculated as a function of the cluster size. In excellent agreement with experimental observations, the CO(2) vibrational band origin shifts and rotational constant show a turnaround near N=5, corresponding to a donut structure with the He atoms in equatorial positions of the linear dopant molecule.     

Perturbative theory and modeling of electronic-resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy of nitric oxide

A theory is developed for three-laser electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitric oxide (NO). A vibrational Q-branch Raman polarization is excited in the NO molecule by the frequency difference between visible Raman pump and Stokes beams. An ultraviolet probe beam is scattered from the induced Raman polarization to produce an ultraviolet ERE-CARS signal. The frequency of the ultraviolet probe beam is selected to be in electronic resonance with rotational transitions in the A (2)Sigma(+)<--X (2)Pi (1,0) band of NO. This choice results in a resonance between the frequency of the ERE-CARS signal and transitions in the (0,0) band. The theoretical model for ERE-CARS NO spectra has been developed in the perturbative limit. Comparisons to experimental spectra are presented where either the probe laser was scanned with fixed Stokes frequency or the Stokes laser was scanned with fixed probe frequency. At atmospheric pressure and an NO concentration of 100 ppm, good agreement is found between theoretical and experimental spectral peak locations and relative intensities for both types of spectra. Factors relating to saturation in the experiments are discussed, including implications for the theoretical predictions.     

Cytotoxic cholic acid type sterones from a marine soft coral <Emphasis Type="Italic">Paraminabea</Emphasis> sp.

Chemical investigation of an Okinawan marine soft coral resulted in the isolation of one new compound (1) and five known cholic acid type keto-steroids 2–6 possessing enone or dienone A-rings and desmosterol (7). The structures of all compounds were elucidated from spectral data, mainly by 1D and 2D NMR techniques, mass spectra, comparison of NMR data with those of reported compounds, and by chemical transformations. Compounds 1–7 showed potent cytotoxic activity.     

NMR chromatography using microemulsion systems

NMR spectroscopy is an excellent tool for structural analysis of pure compounds. However, for mixtures, it performs poorly because of overlapping signals. Diffusion ordered NMR spectroscopy (DOSY) can be used to separate the spectra of compounds with widely differing molecular weights, but the separation is usually insufficient. NMR "chromatographic" methods have been developed to increase the diffusion separation but these usually introduced solids into the NMR sample that reduce resolution. Using nanostructured dispersed media, such as microemulsions, eliminates the need for suspensions of solids and brings NMR chromatography into the mainstream of NMR analytical techniques. DOSY was used in this study to resolve spectra of mixtures with no increase in line-width as compared to regular solutions. Components of a mixture are differentially dissolved into the separate phases of the microemulsions. Several examples of previously reported microemulsions and those specifically developed for this purpose were used here. These include a fully dilutable microemulsion, a fluorinated microemulsion, and a fully deuterated microemulsion. Log(diffusion) difference enhancements of up to 1.7 orders of magnitude were observed for compounds that have similar diffusion rates in conventional solvents. Examples of commercial pharmaceutical drugs were also analyzed via this new technique, and the spectra of up to six components were resolved from one sample.     

Engineering arginine cross-linked mercaptoundecanoic acid CdSe/CdS/ZnS quantum dots for two-photon imaging of live cancer cells

Low toxicity arginine cross-linked mercaptoundecanoic acid CdSe/CdS/ZnS quantum dots were synthesized for two-photon imaging of live cancer cells.