A pH dependent transport and back transport of americium(III) through the cellulose triacetate composite polymer membrane of cyanex-301 and TBP: role of H-bonding interactions

Transport of Am(III) was studied through the composite polymer membrane of Cyanex-301 [bis(2,4,4-trimethylpentyl)dithiophosphinic acid] and tri-n-butylphosphate (TBP). Depending on the pH of the strip solution containing alpha-hydroxyisobutyric acid (AHIBA), the transport behaviour of Am(III) was changed significantly. After approximately 70% of the Am(III) transported to the strip side, interestingly, back transport of Am(III) was observed at a pH of 3.5. The back transport phenomenon was not so significant at pH 1 and 5.7. The back transport of Am(III) was attributed to the transport of AHIBA from strip to the feed side due to its interaction with TBP in the membrane and the attainment of Donnan equilibrium because of the presence of Na(+) in the feed as the driving ion. The experimental observations were rationalized using the hydrogen bonding interaction energies obtained through ab initio molecular orbital and DFT calculations.     

Molecular model for toughening in double-network hydrogels

A molecular mechanism is proposed for the toughness enhancement observed in double-network (DN) hydrogels prepared from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) polyelectrolyte network and poly(acrylamide) (PAAm) linear polymer. It is an extension of the phenomenological model set forth recently by Gong et al. ( Macromolecules 2007, 40, 6658- 6664 ). This mechanism rationalizes the changes in molecular structure of the DN gel constituents observed via in situ neutron scattering measurements, the composition dependence of the solution viscosity, and the thermodynamic interaction parameters of PAMPS and PAAm molecules obtained previously from neutron scattering studies. More specifically, this proposed mechanism provides an explanation for the observed periodic compositional fluctuations in the micrometer range induced by large strain deformation.     

On the cooperativity of cation-pi and hydrogen bonding interactions

Quantum chemical calculations are performed to gauge the effect of cation-pi and hydrogen bonding interactions on each other. M-phenol-acceptor (M = Li (+) and Mg (2+); acceptor = H(2)O, HCOOH, HCN, CH(3)OH, HCONH(2) and NH(3)) is taken as a model ternary system that exhibits the cation-pi and hydrogen bonding interactions. Cooperativity is quantified and the computed positive cooperativity between cation-pi and hydrogen bonding interactions is rationalized through reduced variational space (RVS) and charge analyses.     

Purification and characterization of zinc alpha2-glycoprotein-prolactin inducible protein complex from human seminal plasma

Zinc alpha2-glycoprotein (ZAG) is present at high concentration in human seminal plasma, and considered as soluble homologue of MHC-I. ZAG is a well-known biomarker for prostate and breast carcinomas. We have purified a naturally occurring complex of ZAG with Prolactin inducible protein (PIP), which is also a well-known biomarker for the same. The ZAG-PIP complex has been isolated and purified by simple chromatographic techniques in a reproducible two-step process, using ion exchange and gel-permeation chromatography and subsequently identified by MS. The complex between ZAG and PIP is formed by non-covalent interactions. The purity and molecular mass was determined by SDS-PAGE, which shows the bands corresponding to 40 kDa and 14 kDa, which were also confirmed by MALDI-TOF. Dynamic light scattering (DLS) experiments also showed hydrodynamic radii corresponding to 54, 40 and 14 kDa for ZAG-PIP complex, ZAG and PIP respectively. The concentration dependent aggregation of this complex has also been observed. Fluorescence analysis reveals that complexes have similar binding affinities as native ZAG, for their proposed ligands like arachidonic acid, polyethylene glycol and synthetic peptide. This is the first report on purification and characterization of a naturally occurring complex of ZAG-PIP in human seminal fluid.     

Determination of order parameter of YMn0.5Fe0.5O3 epitaxial thin films by anomalous X-ray scattering technique

The crystal structure of an orthorhombic YMn_0.5Fe_0.5O₃ (010) (YMFO) epitaxial films on YAlO₃(010) substrate was studied using X-ray diffraction, X-ray absorption spectroscopy, and anomalous X-ray diffraction techniques. Due to the utmost similar scattering factors of Mn and Fe atoms, it is hard to distinguish them at specific sites of the unit cell from the variations in the diffraction peak intensity. Therefore, anomalous X-ray scattering was used to determine the order or disorder structure of YMFO films. To estimate the order parameter of the YMFO film, the incident X-ray energies have been scanned around the Mn K-edge and Fe K-edge, resulting in enhanced diffraction intensities of the forbidden YMFO (010) peak by 15–20 times, respectively. This in turn revealed that YMFO films have a partially ordered structure of about 40 ± 10% in the epitaxially grown thin film.     

Development and validation of a high-performance thin-layer chromatography method for the estimation of bromfenac in ophthalmic solution

A novel, simple, precise, specific, accurate high-performance thin-layer chromatography (HPTLC) method was developed and validated for the estimation of bromfenac in ophthalmic solution. Diclofenac sodium was used as an internal standard (IS) because of its structural resemblance with bromfenac to develop a more accurate and precise method. Silica gel 60 F₂₅₄ HPTLC plates were used to separate bromfenac from the formulation with a mobile phase consisting of toluene-ethyl acetate-glacial acetic acid (65:35:0.2, V/V). Densitometric scanning was performed at 274 nm after the HPTLC plates were air-dried. Well-resolved bands and good peak shapes were obtained for both bromfenac and diclofenac sodium, with retention factor (R_F) values of 0.28 and 0.44, respectively. The proposed method was validated as per International Council for Harmonisation Q2 (R1) guidelines for specificity, precision, robustness, accuracy, and recovery. The drug shows linearity in the concentration range of 60‒270 ng/band and the correlation coefficient was found to be 0.999. The mean percent recovery of bromfenac was found to be 100.7%. The limit of detection and limit of quantification values for bromfenac were found to be 7.4 ng/band and 22.5 ng/band, respectively. The method was found to be novel since no HPTLC methods have yet been reported for the estimation of bromfenac. The developed method was successfully applied for the quantitative analysis of the drug in the ophthalmic formulation.

     

Ring opening of the cyclobutane in a thymine dimer radical anion

The reactions of hydrated electrons (e(aq) (-)) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t(1/2)<35 ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. gamma-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T(-)* is unable to transfer an electron to the thymine dimer 2.     

Asymmetric aerobic oxidation of alpha-hydroxy acid derivatives by C4-symmetric, vanadate-centered, tetrakisvanadyl(V) clusters derived from N-salicylidene-alpha-aminocarboxylates

A series of chiral vanadyl(V) methoxides bearing 3-t-butyl-5-substituted N-salicylene-L-valinate and L-t-leucinate as chiral auxiliaries has been prepared. In all cases except the 3,5-di-t-butyl analogue, they exist as monomers both in solution and in the single crystal state. In the case of the 3,5-di-t-butyl analogue, the architectural nature of the vanadyl(V) complex highly depends on the base used during the complex formation event. A pentanuclear C4-symmetric complex was formed when potassium salts were employed instead of the corresponding sodium salts. A central vanadate(V) unit serves to grip four identical chiral monomeric vanadyl(V) units together, by which a potassium ion sits on top of the four flanking units through carbonyl coordinations and serves to hold the whole cluster by cooperation with the central vanadate(V) unit. In comparison with the corresponding monomeric vanadyl(V) methoxide complex, the cluster complex was utilized to facilitate the asymmetric aerobic oxidations of various racemic alpha-hydroxyesters, -amides, and -thioesters with excellent selectivity factors (krel 40 to >500).     

Accurate and efficient corrections for missing dispersion interactions in molecular simulations

In simulations, molecular dispersion interactions are frequently neglected beyond a cutoff of around 1 nm. In some cases, analytical corrections appropriate for isotropic systems are applied to the pressure and/or the potential energy. Here, we show that in systems containing macromolecules, either of these approaches introduce statistically significant errors in some observed properties; for example, the choice of cutoff can affect computed free energies of ligand binding to proteins by 1 to 2 kcal/mol. We review current methods for eliminating this cutoff-dependent behavior of the dispersion energy and identify some situations where they fail. We introduce two new formalisms, appropriate for binding free energy calculations, which overcome these failings, requiring minimal computational effort beyond the time required to run the original simulation. When these cutoff approximations are applied, which can be done after all simulations are completed, results are consistent across simulations run with different cutoffs. In many situations, simulations can be run with even shorter cutoffs than typically used, resulting in increased computational efficiency.