Multivariate optimization and validation of an analytical methodology by RP-HPLC for the determination of losartan potassium in capsules

This paper describes the optimization and validation of an analytical methodology for the determination of losartan potassium in capsules by HPLC using 2^5-1 fractional factorial and Doehlert designs. This multivariate approach allows a considerable improvement in chromatographic performance using fewer experiments, without additional cost for columns or other equipment. The HPLC method utilized potassium phosphate buffer (pH 6.2; 58 mmol L⁻¹)-acetonitrile (65:35, v/v) as the mobile phase, pumped at a flow rate of 1.0 mL min⁻¹. An octylsilane column (100 mm × 4.6 mm i.d., 5 μm) maintained at 35 °C was used as the stationary phase. UV detection was performed at 254 nm. The method was validated according to the ICH guidelines, showing accuracy, precision (intra-day relative standard deviation (R.S.D.) and inter-day R.S.D values <2.0%), selectivity, robustness and linearity (r = 0.9998) over a concentration range from 30 to 70 mg L⁻¹ of losartan potassium. The limits of detection and quantification were 0.114 and 0.420 mg L⁻¹, respectively. The validated method may be used to quantify losartan potassium in capsules and to determine the stability of this drug.     

Particle size and concentration of poly(-caprolactone) and adipate modified starch blend on mineralization in soils with differing textures

The objective of this study was to evaluate the effect of particle size and concentration of poly(-caprolactone) and adipate modified starch blend on mineralization in soils with differing textures, comparing it with polyethylene under the same experimental conditions. Two soil types were used: a Kandiudalfic Eutrudox with a clayey texture and an Arenic Hapludult with a sandy texture. The two different plastic specimens were incorporated in the form of plastic films with three increasing particle sizes and six doses, from 0 to 2.5 mg C g⁻¹ soil. Each plastic dose was incorporated into 200 g of soil placed in a hermetically closed jar at 28 °C, and incubated for a 120-day period to determine CO₂ evolution. Once again it was confirmed that polyethylene is almost non-biodegradable, in contrast to PCL/S, which can be defined as a biodegradable material. Soil texture affected the mineralization kinetics of the plastic specimens, with higher values for the clayey soil. No changes in soil microbial biomass-C or -N were observed by adding polyethylene and PCL/S to the soil. Also, no significant differences were observed on seed emergence and development of rice seedlings (Oryza sativa L.) in plastic modified soil.     

Influence of Endohedral Confinement on the Electronic Interaction between He atoms: A He2@C20H20 Case Study

The shortest He–He distance! r(He–He)=1.265 Å. The electronic interaction between confined pairs of He atoms in the C₂₀H₂₀ dodecahedrane cage is analyzed (see figure). A key observation about bonding that emerges uniquely from endohedral complexes is that a short internuclear separation does not necessarily imply the existence of a chemical bond.

     

Hematite reaction with tar to produce carbon/iron composites for the reduction of Cr(VI) contaminant

In this work, highly reactive carbon–iron composites were prepared using a waste, i.e. tar, as carbon precursor and a simple iron oxide, i.e. hematite. Tar was impregnated on Fe₂O₃ with different tar/hematite weight ratios of 1:1; 2:1 and 4:1, and thermally treated under N₂ atmosphere (400°C, 600°C and 800°C). Mössbauer, XRD and magnetization measurements suggested that treatment at 400°C and 600°C produces Fe₃O₄ but treatment at 800°C produced mainly Fe°. Raman and TG analyses of the different composites suggested the formation of carbon contents of 18, 24 and 32 wt.% as amorphous and graphitic highly dispersed on the Fe surface. The composites obtained at 800°C showed high efficiency to reduce Cr(VI) as CrO $_{4}^{2-}$ in aqueous medium with much better results compared to finely ground commercial Fe°.     

Electronic Structure of Fe3+ at a Metal-Binding Site Introduced in Modified Bacterial Reaction Centers

The electronic structure of Fe³⁺ was studied in a mutant that has been modified to bind manganese or iron at a site corresponding to the manganese-binding site of photosystem II (Kálmán et al., Biochemistry 45:13869–13874, 2006). Using electron paramagnetic resonance spectroscopy, the presence of the oxidized state of the bacteriochlorophyll dimer, P^·+, was detected in the light when no metal was added. When iron was bound to the modified reaction centers in the presence of bicarbonate, the contribution of P^·+ was greatly reduced and a signal characteristic of Fe³⁺ was evident. To characterize the electronic structure of this ferric ion, the electron paramagnetic resonance spectrum was measured at X-band at temperatures from 4 to 200 K. The major contribution to the spectrum at 4 K is from Fe³⁺ with a spin 3/2 in a rhombic coordination and E/D ratio of 0.1914 and g _eff values of 6.0, 2.9, and 2.0. As the temperature increases from 4 to 200 K, the signal shifts, with the central g _eff value changing from 2.9 to 2.2. This change with temperature may result from alterations in the interaction with the bicarbonate coordinated to the iron as the temperature increases.     

EPR and ENDOR Investigation of Rhodosemiquinone in Bacterial Reaction Centers Formed by B-Branch Electron Transfer

In photosynthetic bacteria, light-induced electron transfer takes place in a protein called the reaction center (RC) leading to the reduction of a bound ubiquinone molecule, Q_B, coupled with proton binding from solution. We used electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) to study the magnetic properties of the protonated semiquinone, an intermediate proposed to play a role in proton coupled electron transfer to Q_B. To stabilize the protonated semiquinone state, we used a ubiquinone derivative, rhodoquinone, which as a semiquinone is more easily protonated than ubisemiquinone. To reduce this low-potential quinone we used mutant RCs modified to directly reduce the quinone in the Q_B site via B-branch electron transfer (Paddock et al. in Biochemistry 44:6920–6928, 2005). EPR and ENDOR signals were observed upon illumination of mutant RCs in the presence of rhodoquinone. The EPR signals had g values characteristic of rhodosemiquinone (g _x  = 2.0057, g _y  = 2.0048, g _z  ~ 2.0018) at pH 9.5 and were changed at pH 4.5. The ENDOR spectrum showed couplings due to solvent exchangeable protons typical of hydrogen bonds similar to, but different from, those found for ubisemiquinone. This approach should be useful in future magnetic resonance studies of the protonated semiquinone.     

4- vs. 5-phenylquinolinolate aluminum (III) isomers

A method for the synthesis of 4-arylquinolinolate ligand and their Al^III complexes based on Michael reaction of 2-methoxyaniline with 1-phenylpropenones was developed. The resulting 4-aryl-8-methoxyquinoline was demethylated and converted to corresponding Al^III complexes. Photophysical properties of two 4-aryl-Alq₃ derivatives were then compared with properties of the parent Alq₃ and a 5-phenyl-Alq₃ congener. It appears that the 5-aryl derivatives show improved luminescence but also decreased physical stability. Electroluminescence of the prepared materials is presented and compared to Alq₃ and a 5-phenyl-Alq₃.     

Returning to Basics: Direct Integration of the Full Molecular‐Weight Distribution Equations in Addition Polymerization

Direct integration of the equations describing the full MWD in addition polymerization is shown to be feasible in reasonable computing times using standard computers for a considerable number of realistic systems. This is possible if the stiffness of the system is removed (if present) by the application of the QSSA, and the use of suitable numerical methods. This communication provides the basic ideas of a general method, based on the chemical nature of the system and the structure of the MWD equations. The application of the method provides additional insight on the dynamics and the underlying chemistry of the system. Examples of free radical polymerizations including controlled radical polymerization are offered, which can be solved in less than an hour on a PC.

     

Wannier-stark ladders in one-dimensional elastic systems

The optical analogues of Bloch oscillations and their associated Wannier-Stark ladders have been recently analyzed. In this Letter we propose an elastic realization of these ladders, employing for this purpose the torsional vibrations of specially designed one-dimensional elastic systems. We have measured, for the first time, the ladder wave amplitudes, which are not directly accessible either in the quantum-mechanical or optical cases. The wave amplitudes are spatially localized and coincide rather well with theoretically predicted amplitudes. The rods we analyze can be used to localize different frequencies in different parts of the elastic systems and vice versa.     

Ionoluminescence of trivalent rare-earth-doped strontium barium niobate

Ionoluminescence spectra for different rare-earth ion (Pr³⁺ and Eu³⁺)-activated Sr_xBa_1?xNb₂O₆ strontium barium niobate crystals (x=0.33 and 0.60) have been induced with a 3 MeV proton beam for a variety of beam current intensities (45, 40 and 20 nA). The proton-beam induced luminescent spectra have shown features associated with the presence of the rare-earth ion and some spectral features mostly related to the host crystal, which appear only for high beam current intensities. We have compared the ionoluminescence results to those obtained under UV light excitation (photoluminescence technique) where a direct excitation of the band gap would occur.