o-hydroxylmethylphenylchalcogens: synthesis, intramolecular nonbonded chalcogen...OH interactions, and glutathione peroxidase-like activity

[Structure: see text]. The synthesis and characterization of a series of organochalcogen (Se, Te) compounds derived from benzyl alcohol 13 are described. The synthesis of the key precursor dichalcogenides 15, 22, and 29 was achieved by the ortho-lithiation route. Selenide 18 was obtained by the reaction of the dilithiated derivative 14 with Se(dtc)2. Oxidation of 15 and 22 with H2O2 afforded the corresponding cyclic ester derivatives 17 and 24, respectively. Oxidation of selenide 18 with H2O2 affords the spirocyclic compound 19. The presence of intramolecular interactions in dichalcogenides 15 and 22 has been proven by single-crystal X-ray studies. The cyclic compounds 17 and 19 have also been characterized by single-crystal X-ray studies. GP(X)-like antioxidant activity of selenium compounds has been evaluated by the coupled bioassay method. Density functional theory calculations at the mPW1PW91 level on ditelluride 22 have identified a fairly strong nonbonding interaction between the hydroxy oxygen and tellurium atom. The second-order perturbation energy obtained through NBO analysis conveys the involvement of n(O) --> sigma(Te-Te) orbital overlap in nonbonding interaction. Post wave function analysis with the Atoms in Molecules (AIM) method identified distinct bond critical point in 15 and 22 and also indicated that the nonbonding interaction is predominantly covalent. Comparison between diselenide 15 and ditelluride 22 using the extent of orbital interaction as well as the value of electron density at the bond critical points unequivocally established that a ditelluride could be a better acceptor in nonbonding interaction, when the hydroxy group acts as the donor.     

Grafting of monosaccharide derivatives to cellulose acetate

Grafting of 1,2-O-isopropylidene-α-D -xylofuranose to commercial cellulose diacetate has been accomplished by using a boron trifluoride catalyst. The reaction proceeds quickly at 25 and 40°C, resulting in degrees of molar substitution (MS) of 0.37 and 0.34, respectively. If monomer and catalyst are added over an extended period of time to maintain low concentrations, MS values as high as 0.89 and 0.85 are obtained at 25 and 40°C, respectively. Major side reactions are depolymerization of the cellulose acetate backbone and grafted D -xylose and the homopolymerization of the monomer. These side reactions may be minimized by conducting the reaction at 40°C for a short time or by adding monomer and catalyst over an extended period of time. Grafting has also been accomplished by using D -xylose derivatives with various reactive groups at the anomeric carbon atom. The grafted material of MS greater than 0.7 is insoluble in acetone and after deacetylation is soluble in water under alkaline, neutral and acidic conditions. Oxalic acid hydrolysis of the deacetylated material indicates that most of the grafted D -xylose units are in the furanose form. Methylation and hydrolysis of the methylated material shows that 75% of the D -xylose residues are terminal units and indicates the presence of many singly grafted D -xylose residues and a few di-and trisaccharide grafts.     

Enhancement of phosphoprotein analysis using a fluorescent affinity tag and mass spectrometry

A fluorescent affinity tag (FAT) was synthesized and was utilized to selectively modify phosphorylated serine and threonine residues via beta-elimination and Michael addition chemistries in a 'one-step' reaction. This labeling technique was used for covalent modification of both phosphoproteins and phosphopeptides, allowing identification of these molecular species by fluorescence imaging after solution- or gel-based separation methods. In addition to the strong fluorescence of the rhodamine tag, a commercially available antibody can be used to enrich low-abundance post-labeled phosphopeptides present in complex mixtures. Application of this methodology to phosphorylation-site mapping has been evaluated for a phosphoprotein standard, bovine beta-casein. Initial results demonstrated low femtomole detection limits after fluorescence image analysis of FAT-labeled proteins or peptides.     

Spectrophotometric determination of platinum with substituted pyrimidine-2-thiol

The yellow complex of Pt(IV) with 1-pyridyl-4,4,6-trimethyl-1H,4H-pyrimidine-2-thiol (PyTPT) which has maximum absorbance at 430 nm, is studied for the spectrophotometric determination of the metal. Molar absorptivity is 5000 liters mol^?1 cm^?1 and Sandell's sensitivity is 0.039 μg/cm². The determination of Pt(IV) (2.8–8.4 ppm) in the presence of diverse ions is described.     

Nonperturbative modeling of two-photon absorption in a three-state system

The physics of the two-photon absorption process is investigated for a three-state system. The density-matrix equations for the two-photon interaction are solved in the steady-state limit assuming that the pump laser radiation is monochromatic. Collisional broadening, saturation, and Stark shifting of the two-photon resonance are investigated in detail by numerical solution of the steady-state density-matrix equations. Analytical expressions for the saturation intensity and the Stark shift are derived for the case where the single-photon transitions between the intermediate state and the initial and final states are far from resonance with the pump laser. For this case, it is found that the direction of the Stark shift is dependent on the relative magnitudes of the dipole-moment matrix elements for the single-photon transitions that couple the intermediate state with the initial and final states. Saturation and Stark shifting are also investigated for the case where the single-photon transitions between the intermediate state and the initial and final states are close to resonance with the pump laser.     

Approaching exponential growth with a self-replicating peptide

Self-replicating peptide systems hold great promise for a wide range of technological applications, as well as to address fundamental questions pertaining to the molecular origins of life. The development of self-replicating compounds capable of high efficiency, however, has remained elusive. Here we disclose a successful strategy whereby modulation of coiled-coil stability results in remarkable catalytic efficiency for self-replication. By shortening the peptide to the minimum length necessary for coiled-coil formation a highly efficient self-replicating system was obtained due to very low background reaction rates, bringing the efficiency close to naturally occurring enzymes.     

On the nature of the fluorescent state of methylated indole derivatives

A quantitative study has been made of the solvent effects on the fluorescence properties of 1- and 3-methyl indole, with the aim of further understanding the origin of the unusually large Stokes shift in polar solvents. For the derivatives considered here the fluorescence transition probability is decreased in solvents of moderate and high polarities, and the spectrum shifts to the red. The data (in two-component, solute and solvent, systems) can be interpreted on the basis of the stabilisation, by solvent-solute relaxation, of a state with an increased charge-transfer character, relative to the initially excited state. Å consideration of the decay data for other indole derivatives suggests that this state has its origin in the ¹L₄ state (S₂ in non-polar media). Thus we conclude that the appropriate label of the fluorescent state of many substituted indoles in polar solvents is ¹L_a/CT. This is consistent with the observed solvent, temperature, time and substituent dependence of the decay kinetics of these derivatives.     

Intramolecular nitrone cycloaddition reaction on carbohydrate-based precursors: application in the synthesis of spironucleosides and spirobisnucleosides

A simple synthesis of chiral spironucleosides and spirobisnucleosides is described. Intramolecular 1,3-dipolar nitrone cycloaddition reaction of d-glucose-derived precursors having olefin at C-3 and nitrone at C-5, C-1, or C-2 (in nor-series) furnished bisisoxazolidinospirocycles 4-7, 11, and 12 in good yields. Reductive ring opening of the isoxazolidine moieties in 4-6 followed by construction of a nucleoside base upon the generated amino groups smoothly yielded spirobisnucleosides 17 and 18 and spironucleosides 20 and 21.     

Amino Acid based cationic surfactants in aqueous solution: physicochemical study and application of supramolecular chirality in ketone reduction

The present study provides a molecular understanding of the origin of the chirality in aqueous micelles and its correlation with the proficiency of stereoselective ketone reduction. The effects of varied headgroup architecture on the surface-active properties as well as on other microstructural parameters were studied and correlated to the structural differences of these naturally occurring amino acid containing surfactants (1-4). Micropolarity sensed by pyrene showed that the micelles prepared using 1-4 are mostly hydrated; particularly large headgroup size surfactant produces more polar environment. A theoretical study was done to quantify the varied spatial dissymmetry for all four surfactants. Asymmetric reduction of prochiral ketones was carried out at the aqueous micellar interface of these chiral amphiphiles by exploiting the supramolecular chirality as evidenced from a circular dichroism study. The enantioselectivity of the reduction process is rationally improved through increase in spatial dissymmetry and steric constraint imposed at the micellar interface by the polar head of surfactants.     

Extraction of manganese(II) with dithizone and potassium thiocyanate on foam sorbents for spectrophotometric determination in silicates

A method for selective extraction of Mn(II) with dithizone and potassium thiocyanate has been described. The method involves formation of a Mn(II)-thiocyanate-dithizone complex in a hexamine medium containing potassium thiocyanate (2.8M), dithizone (5.5-6.5 x 10(-5)M) and hydroxylamine hydrochloride (0.25%) at pH approximately 6 followed by extraction of the complex on polyurethane foam using batch squeezing mode within 1 hr. The sorbed Mn-thiocyanate-dithizone complex is eluted with acetone and made alkaline with 0.5 ml of a stabilizer solution (19 ml 2M NH(3) solution + 1 ml 5% hydroxylamine hydrochloride). The absorbance of the solution is measured at 506 nm. The adverse effect due to Pb may be obviated by separating the Pb as the sulphate during decomposition of sample and that due to iron may be removed before extraction of Mn by any suitable method. The other interfering elements (Cd, Zn, Ni, Co, Cu, etc.) are masked with KCN (6 x 10(-3)M optimum) solution. The method obeys Beer's Law from 0.1 to 2.0 mug Mn/ml. The method has been applied to various silicates, carbonates and glasses.