Aggregation and protein binding of sodium maleate copolymers with varying hydrophobicities

Copolymers of sodium maleate with methyl methacrylate, styrene, or vinyl acetate have been synthesized and studied in aqueous NaCl solutions of various ionic strengths. The polymers are polyelectrolytes with varying hydrophobicities, and their solution properties have been studied using static and dynamic light scattering. Copolymers containing methyl methacrylate or styrene were shown to aggregate in water upon increasing salt concentration. Copolymers of sodium maleate and vinyl acetate do not associate with increasing ionic strength. The binding of bovine serum albumin and cytochrome C to the sodium maleate copolymers was also investigated by light scattering. It was observed that cytochrome C forms complexes with the copolymers containing methyl methacrylate or vinyl acetate whereas albumin does not bind to any of the copolymers studied. Copyright © 2004 John Wiley & Sons, Ltd.     

Why are hexavalent uranium cyanides rare while U–F and U–O bonds are common and short?

Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91–PW91 studies are reported for the series UF ₄X ₂ ( X=H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF ₆ is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these s and p orbital interactions, the electrostatic attraction is important. Evidence for p bonding in the other systems studied was also found. The triatomic pseudohalides as well as fluorine and chlorine are in this sense better ligands than cyanide. The –CN group is a s donor and p acceptor, as uranium itself, and hence is unfit to bond to U(VI). The s-bonded UH ₆ is octahedral.     

New practical synthesis of prazosin

A new four step synthesis of prazosin, 2-[4-(2-furoyl)piperazin-l-yl]-4-amino-6,7-dimethoxyquinazoline, has been described. The method is also adaptable for the preparation of other substituted 4-aminoquinazolines. The yields are good in every step and the reactions are performed with ease. Prazosin hydrochloride of high purity is obtained directly in the last step.     

The elements of Flatland: Hartree-Fock atomic ground states in two dimensions for Z = 1–24

The Hartree–Fock problem in two dimensions (2D) has been solved for 1 ≤ Z ≤ 24 using a Gaussian basis and assuming r^?1 Coulomb interactions. The order of occupation of the one-electron states is like in the 3D case. The 1s shell is found to be particularly small and strongly bound, making the 2D hydrogen a “superhalogen” and the 2D He a “superinert gas.” In contrast to 3D, 4s¹3d² and 4s²3d³ configurations are preferred for the 2D “Sc” and “Cu,” respectively. The six first 2D atoms have stronger and the later ones weaker valence-bonding energies than do their 3D analogs. It is noted that the 2D Dirac energy expression for a hydrogenlike atom for m_j = l + 1/2 agrees with the 3D Klein–Gordon one.     

Issues of microscopic reversibility and an isomeric intermediate in ligand substitution reactions of five-coordinate oxorhenium(V) dithiolate complexes.

Ligand substitution reactions between five-coordinate oxorhenium(V) dithiolates, [CH(3)ReO(SCH(2)C(6)H(4)S)X], or MeReO(mtp)X, and entering ligands Y have been studied; Y is a phosphine and X is a phosphine (usually) or a pyridine. Many of them occur in two distinct stages, and other two-stage reactions merge to a single kinetic term when the successive rate constants are quite different in value. An intermediate can be detected directly by electronic and NMR spectroscopy. Just for phosphines, the range of rate constants is remarkably large; in the first stage, k spans the range 10(-)(4)-10(1) L mol(-)(1) s(-)(1) at 25 degrees C in benzene; in the second, which also shows a first-order dependence on the concentration of the entering ligand, the range is 10(-)(4)-10(3) L mol(-)(1) s(-)(1). Spectroscopic evidence shows that the intermediate has the same composition as the product; the metastable form is designated as MeReO(mtp)Y. The structures of all the isolated products MeReO(mtp)Y have a single stereochemistry: Me and -SCH(2) lie in trans positions, as do Y and -SAr. This structure is believed to be reversed in the transient, Y and -SCH(2) occupying trans positions. Further support for this assignment comes from the (31)P splitting of the (1)H NMR spectrum, where additional coupling indicates unusual four-bond coupling from a W-pattern of the hydrogen and phosphorus atoms. The intermediate does not undergo an intramolecular rearrangement to the final product; instead, it reacts with a ligand of the same type in an intermolecular reaction leading to rearrangement. The activation parameters were determined for selected reactions, and the results support a mechanism with considerable associative character; DeltaS() values are ca. -125 J K(-)(1) mol(-)(1). Because ligand Y must enter the coordination sphere from the vacant coordination position trans to the Re=O group, a means must be devised for the leaving group X to gain that position. To account for the intervention of the isomer while honoring the principle of microscopic reversibility, two mechanisms are proposed. One involves a C(3) ("turnstile") rotation of a specific group of three ligands in the six-coordinate transition state. Turnstile rotation of the groups X, Me, and Y can accomplish the needed transposition; the transition state passes through an approximate trigonal prismatic configuration, giving rise to a different and less stable isomer. The alternative mechanism, which may more easily accommodate data for Y = Me(2)bpy, involves rearrangement of the common octahedral intermediate to a pentagonal pyramid. The arrangement of ligands in the intermediate, governed by their sizes, determines that isomerization accompanies product formation. Following either rearrangement, a second reaction, between MeReO(SCH(2)C(6)H(4)S)Y and Y, then ensues by the same mechanism. The second rearrangement process then generates the more stable isomer of the product. Results are also presented from a study of monomerization of the dimeric rhenium species, [MeReO(mtp)](2), with phosphines(X) of various size and basicity. The results support a mechanism with two intermediates on the pathway to MeReO(mtp)X.     

Ratio-based RTS determination in weight-restricted DEA models

This paper provides computationally efficient approaches for determining to which returns to scale (RTS) class a unit belongs in weight-restricted Data Envelopment Analysis (DEA) models. A non-traditional computational algorithm is introduced. The suggested approach is based on the calculation of certain ratios within the data set and offers obvious computational advantages over the traditional approaches involving the solution of standard DEA models. Some theorems and algorithms are given. Computational advantages of the provided results are discussed and one of the algorithms is illustrated using real world data.     

Measuring position and momentum together

We describe an operational scheme for determining both the position and momentum distributions in a large class of quantum states, together with an experimental implementation.     

Effect of molecular size on the parity-non-conserving contributions to the nuclear magnetic resonance shielding constant

We present density-functional theory studies on the effects of molecular size on the parity-violating contribution to the nuclear magnetic shielding constant. We focus on models with different backbone and side chain lengths, as well as the details of geometry optimization for certain helical polysilylenes and investigate the parity-violating contribution to the shielding constant of the ²⁹Si nucleus of the backbone. Our calculations show that the molecular geometry has a large influence on the magnitude of the parity-violating shielding contribution, a result that is in line with the previous studies on much smaller molecules. In addition, we find convergence in the magnitude of the PV effect with respect to system size, when using geometries that preserve the helical Si backbone structure optimized for the largest of the present systems. This can be interpreted in terms of the non-size-extensive nature of the parity-violating operator influencing the leading-order effect on nuclear magnetic shielding, as opposed to the size-extensive interaction affecting the energy difference between enantiomers. Our molecules are truncated models of large polysilylene systems, for which a difference in the ²⁹Si chemical shift between enantiomers has been observed to be 0.06 ppm (Fujiki in Macromol Rapid Commun 22, 669–674, 2001). As expected based on earlier first principles studies of small molecules, we do not find support for the difference to be of the parity-violating origin. Instead, the predicted parity-violation-induced splitting of the ²⁹Si resonance is found to converge at values around 10^?8 ppm with increasingly large Si backbone.     

Experimental and theoretical treatment of hydrogen splitting and storage in boron-nitrogen systems

Hydrogen gas serves as a reducing agent and hydrogen atom source in numerous industrially important chemical processes and also has a great potential as a clean power source for fuel cells. In this respect, the reversible storage of hydrogen and the development of new metal-free hydrogenation catalysts are important tasks. Here, we review the recent literature, primarily on cases where the split H₂ forms an N-H?H-B dihydrogen bond. In these systems dihydrogen interaction was found to be the key actor in the hydrogen liberating process. Accordingly, the intramolecular ansa-aminoboranes (where B and N atoms are situated within each other’s range) can reversibly activate hydrogen. Moreover, the theoretical studies of the hydrogen splitting by bulky Lewis acid-Lewis base systems are discussed.