Determination of nitrate in pickling baths with a nitrate-selective electrode and a standard addition procedure

A reverse addition method for the determination of nitrate in pickling baths is described. Calibration solutions of nitric acid were added to the standard solution so that the slope and intercept of the electrode response function could be determined. The standard deviation of the method was better than 1 %. Nitrate in pickling baths containing hydrofluoric and nitric acids was measured and compared to measured hydrogen ion concentrations as a function of running time for the bath. The effects of iron(III) and hydrofluoric acid on the results were studied.     

Enzyme reactors in unsegmented flow injection analysis

The design of immobilized-enzyme reactors for use in flow injection analysis is discussed. The reactors should be optimized for a short residence time and a very high (> 99.9%) conversion of substrate to products. Selection of carrier and immobilization method is important in order to increase the amount of active enzyme per unit volume. The effeciency of the reactor can be increased by decreasing the particle size in packed-bed reactors and the radius of open tubular reactors. The maximum inherent rate constant that can be obtained under optimal conditions is estimated for a number of enzymes of analytical interest; it is shown that with high rate constants and small particle diamters, residence times less than seconds can be obtained. Some applications of immobilized-enzyme reactors in flow systems are reviewed.     

A New Helicopodand: Molecular Recognition of Dicarboxylic Acids with High Diastereoselectivity

The helicopodand (PM)- 2 is prepared following the photocyclodehydrogenation route to helicenes (Scheme). At the ends of a [7]helicene backbone, this acyclic receptor (‘podand’) possesses a H-bonding recognition site shaped by two convergent N-(pyridin-2-yl)carboxamide (CONH(py)) units. In the crystal of diethyl [7]helicene-2,17-dicarboxylate ((PM)- 3 ), a direct synthetic precursor of 2 , molecules of the same chirality form stacks, and two stacks of opposite chirality are interlocked in a pair having average face-to-face aromatic contacts of 3.82 Å between benzene rings of different enantiomers (Fig. 2). In contrast, two conformations are observed in the crystal structure of 2 , one with both CONH(py) residues pointing with their H-bonding centers NH/N away from the binding site (‘out-out’) and a second (‘in-out’) with one of the two CONH(py) residues pointing towards the binding site (‘in’; Fig. 4). While no H-bonding network propagates throughout the crystal, enantiomers of 2 in the different conformations ‘out-out’ and ‘in-out’ form H-bonded pairs that are further stabilized by a H-bond to one molecule of CHCl₃. In the productive ‘in-in’ conformation, 2 forms stable 1:1 complexes with α,ω-dicarboxylic acids in CHCl₃, and a diastereoselectivity in complexation of Δ(ΔG°) = 1.4 kcal mol^?1 is measured for two substrates differing only in the (E)/(Z)-configuration at their double bond (see Table 2). A comprehensive force-field molecular-modeling study suggests that only the (E)-derivative possesses the correct geometry for a ditopic four-fold H-bonding interaction between its two COOH residues and the two CONH(py) groups in 2 (Fig. 5). With N,N′-bis [(benzyloxy)carbonyl]-L -cystine, the formation of diastereoisomeric complexes with (PM)- 2 is observed (Fig. 7).     

Titration of mixtures of acids of nearly equal strengths

Accurate and precise experimental methods that were developed for the purpose of determining stability constants in aqueous solutions are employed in titrations of acids which have nearly equal strengths. A computer program capable of handling five acids simultaneously has been designed to evaluate the titration data. The results show that it is possible to determine two acids which have stability constants differing by only about one logarithmic unit with an accuracy better than 1%. If the conditions are favourable, it is even possible to analyse a mixture of two acids with about 2% accuracy when the constants differ by as little as 0.2 logarithmic units.     

Systematic errors in the total energy of molecular wave functions calculated within the PRDDO approximations

We compare calculated total energies for 150 open-chain molecules using ab initio methodology and the PRDDO approximations. The bulk of the errors implicit in the PRDDO approximations are apparently of a one-center nature, i.e., they are due to the number and type of atoms in the molecule, and not the details of the molecular geometry. Atomic correction factors are developed which reduce the errors in the calculated total energy of PRDDO wave functions by a factor of eight relative to the ab initio reference calculations. PRDDO calculations on ring and cage compounds are shown to have additional systematic errors in the total energy.     

Monolayer cuprous chloride dispersed on pillared clays for olefin-paraffin separations by π-complexation

New adsorbents containing cuprous chloride dispersed on pillared interlayered clays (PILC) have been prepared and studied for olefin-paraffin separations. High surface-area PILC's were synthesized with different metal oxide (Al₂O₃, Fe₂O₃, TiO₂ and ZrO₂) as the intercalating pillars. Cuprous chloride was dispersed in a submonolayer form on these PILC's. Pure-component isotherms were measured for C₂H₄, C₂H₆, C₃H₆ and C₃H₈ at 25°C and 60°C. All sorbents exhibited high C₂H₄/C₂H₆ and C₃H₆/C₃H₈ ratios with significantly high amounts of olefins adsorbed. The best sorbent was CuCl/TiO₂-PILC which showed a C₂H₄/C₂H₆ ratio of 5.3 and C₃H₆/C₃H₈=2.9 at 25°C. In all cases, olefins adsorbed by -complexation with Cu(I) ion, reflected by heats of adsorption in the range 10.7–13.7 kcal/mol, as compared to 4.8–6.9 kcal/mol for the physical adsorption of the paraffins. The -complexation was fully reversible, limited only by the rates of pore diffusion. Diffusion of C₂'s was rapid while for C₃'s the diffusion reached 60% completion in approximately 6 min. Comparing these results with those of CuCl/-Al₂O₃, the olefin/paraffin adsorption ratios were not as high as those of the later. However, the olefin isotherms on the PILC-supported CuCl displayed the desirable feature of having a steeper portion above the knee of the isotherm (the knee occurred at below 0.1 atm). This was a useful feature for separation because it yielded a larger working capacity. The steeper isotherm was attributed to a higher degree of energy heterogeneity as the PILC contained both surfaces of pillars and clay layers as opposed to only -Al₂O₃.     

Syntheses and biological evaluation of vinblastine congeners

Sixty-two congeners of vinblastine (VLB), primarily with modifications of the piperidine ring in the carbomethoxycleavamine moiety of the binary alkaloid, were synthesized and evaluated for cytotoxicity against murine L1210 leukemia and RCC-2 rat colon cancer cells, and for their ability to inhibit polymerization of microtubular protein at < 10(-6) M, and for induction of spiralization of microtubular protein, and for microtubular disassembly at 10(-4) M concentrations. An ID50 range of >10(7) M concentrations was found for L1210 inhibition by these compounds, with the most active 1000x as potent as vinblastine.     

Pi-accepting-pincer rhodium complexes: an unusual coordination mode of PCP-type systems

The novel pi-accepting, pincer-type ligand, dipyrrolylphoshinoxylene (DPyPX), is introduced. This ligand has the strongest pi-accepting phosphines used so far in the PCP family of ligands and this results in some unusual coordination chemistry. The rhodium(I) complex, [(DPyPX)Rh(CO)(PR3)] (4, R=Ph, Et, pyrrolyl) is prepared by treating the relevant [(DPyPX)Rh(PR3)] (3) complex with CO and is remarkably resistant to loss of either ligand. X-ray crystallographic analysis of complex 4 b (R=Et) reveals an unusual cisoid coordination of the PCP phosphine ligands. These observations are supported by density functional theory (DFT) calculations.     

Affinity partitioning of enzymes using dextran-bound procion yellow HE-3G. Influence of dye-ligand density

The dye Procion Yellow HE-3G was bound to dextran of molecular weight 70,000 and the partitioning of this dye-polymer within an aqueous two-phase system containing dextran and poly(ethylene glycol) was studied as function of ligand density, polymer concentration, type of salt, concentration of salt and concentration of dye-dextran. Even moderate dye:dextran molar ratios (5-8) make the partitioning strongly salt-dependent. The dye-dextran can be directed to either the upper or the lower phase with partition coefficients from 0.02 to 28 by using salts. The dye-dextran in the two-phase system affects the partitioning of dye-binding enzymes (lactate dehydrogenase, glucose-6-phosphate dehydrogenase, 3-phosphoglycerate kinase) towards the dye-containing phase. Measurements of competition with nucleotide binding show an increased affinity of the dye for the enzyme with increasing ligand:dextran ratio. Theoretical considerations indicate that 1-2 dextran molecules are attached per enzyme molecule when affinity partitioning is fully developed.     

Novel synthesis of castanospermine and 1-epicastanospermine

[reaction: see text] Polyhydroxylated indolizidines have potential for treatment of HIV, hepatitis C and HSV infection, multiple sclerosis, angiogenesis, cancer, and diabetes. A new synthetic approach to the title compounds from a 5-C-methoxypyranosyl azide has been developed. The route incorporates the aldol reaction and a novel catalytic reductive amination cascade to generate the indolizidine ring.