Self-reproduction of chirality on α-aminophosphonates: asymmetric synthesis of α-alkylated diethyl pyrrolidin-2-yl-phosphonate

A simple method for the asymmetric synthesis of α-substituted diethyl pyrrolidin-2-yl-phosphonate is described. The chiral oxazolopyrrolidine phosphonate was alkylated diastereospecifically with an alkyl halide. The key intermediate is an α-phosphonate-stabilized carbanion that can be alkylated without loss of optical activity and a single enantiomer of product was formed exclusively in 10-80% yield. The configurational assignment of the products relied on ¹H-¹H NOESY analysis of the alkylated oxazolopyrrolidine phosphonates. This represents an unprecedented case of self-regeneration of stereocenters (SRS) of cyclic aminophosphonates. The enantiomerically pure α-aminophosphonate diethyl-(2S)-(2-methylpyrrolidin-2-yl)-phosphonate, a surrogate of 2-methyl proline, was obtained upon hydrogenolysis of the chiral auxiliary in 83% yield.     

Separation of phenolic compounds and corresponding glucuronides by coupled-column micellar reversed-phase liquid chromatography

Summary An isocratic HPLC technique for separation of phenolic compounds and corresponding glucuronides in urine is developed. Sample pre-treatment, often a tedious and rate-limiting factor, was eliminated by use of a coupled column system. Spiked urine samples were injected directly into a C₄ precolumn and a selected fraction was transferred on-line from the precolumn to a silanized C₁₈ analytical column in the backflush mode. Analyte peak enrichment was attained by employing mobile phases of different elution strengths. The weaker mobile phase (7% v/v acetonitrile) was used to strongly retain the analytes on the precolumn while most of the polar endogenous compounds were washed to waste. Elution and transfer of the trapped analytes from the precolumn to the analytical column was achieved by introducing a stronger mobile phase (20% v/v acetonitrile) with the aid of a switching valve. The use of cetyltrimethylammonium bromide as counter ion and micellar agent in the mobile phase involved a high selectivity for the analytes relative to the urine matrix components and allowed simultaneous analysis of the glucuronides and parent compounds without the need of gradient elution. The system demonstrated a good repeatability on spiked urine samples. Who passed away July 21, 1996     

Determination of impurities in remoxipride by capillary electrophoresis using UV-detection and LIF-detection; principles to handle sample overloading effects

Summary A capillary electrophoresis method for determination of remoxipride-related impurities was developed using a dynamically tetraalkylammonium-coated C₈ capillary. The shape of the overloaded principal peak was regulated by substituting tetraalkylammonium ions of different size for sodium ions, thereby altering the conductivity in the background electrolyte. An adequate separation of the compounds was obtained after addition of methyl-β-cyclodextrin as structural selector in combination with a reversed electroosmotic flow. The detection level for the impurities was 0.05 % (w/w) with the Beckman Pace 2100 using UV-detection. The limit of detection was substantially improved for the fluorescent compound FLA 797 using a LIF-detector equipped with a He-Cd laser as excitation source achieving detection levels below 0.0005 %. Presented at the 6th International Symposium on High Performance Capillary Electrophoresis, Jan. 31–Feb. 4 1994, San Diego, USA.     

Monitoring the DNA binding kinetics of a binuclear ruthenium complex by energy transfer: evidence for slow shuffling

The semirigid binuclear ruthenium complex Delta,Delta-[mu-(11,11'-bidppz)(phen)(4)Ru(2)](4+) has been shown to rearrange slowly from an initial groove-bound nonluminescent state to a final intercalated emissive state by threading one of its bulky Ru(phen)(2) moieties through the DNA base stack. When this complex binds to poly[d(A-T)(2)], a further increase in emission from the complex is observed after completion of the intercalation, assigned to reorganization of the intercalated complex. We here report a study of the threading process in poly[d(A-T)(2)], in which the minor groove binding dye DAPI is used as an energy transfer probe molecule to assess the distribution of ruthenium complex during and also after the actual threading phase. The emission from DAPI is found to change with the same rate as the emission from the ruthenium complex, and furthermore, DAPI does not disturb the binding kinetics of the latter, justifying it as a good probe of both the threading and the reorganization processes. We conclude from the change in the emission from both DAPI and the ruthenium complex with time that DAPI-ruthenium interactions are most pronounced during the process of threading of the complex, suggesting that the complexes are initially threaded slightly anticooperatively and thereafter redistribute along the DNA to reach their thermodynamically most favorable distribution. The final distribution is characterized by a small but significant binding cooperativity, probably as a result of hydrophobic interactions between the complex ions despite their tetravalent positive charges. The mechanism of "shuffling" the complex along the DNA chain is discussed, i.e., whether the ruthenium complex remains threaded (requiring sequential base-pair openings) or if unthreading followed by lateral diffusion within the ionic atmosphere of the DNA and rethreading occurs.     

Nano-scale mapping of mechanical and chemical surface properties of pigment coated surfaces by torsional harmonic atomic force microscopy

In this study, torsional harmonic atomic force microscopy (TH-AFM, HarmoniX mode) was applied for surface mapping of the mechanical properties of pigment-latex coated paper samples. In addition, topographic images and force maps of adhesive tip-sample interactions were captured concurrently. The spatial distribution of latex binder on the composite surface was distinguished with high resolution. The latex was found to dominate the surface chemistry of the composite coating, despite the fact that latex is a minor component in the coating color formulation. The latex resided as a thick layer between the pigments and as a thin layer on the individual pigments. In addition, the tip-sample thermodynamic work of adhesion of the composite materials on the coated surface was compared to the surface energy values obtained by contact angle measurements. A high tip-sample work of adhesion correlated to high surface energy.     

A Laboratory-Scale Vertical Gravity Separator for Emulsion Characterization

A laboratory-scale vertical gravity separator has been built in order to perform characterization on oil and water based emulsions. The separation rig consists of two positive displacement pumps, a feed separator, and a test separator. Probes for sampling are placed at various heights of the test separator and at different points at the pipes. To induce mixing of the oil and water phases, a needle valve is placed downstream of the pumps. The valve, when choked, increases the pressure drop and thus increases the shear of the oil and water mixture pumped through the system. A differential pressure cell is used to monitor the pressure drop over the needle valve when choked. The differential pressure is used as a reference for the energy put into the mixing process. Calculation of the maximum surviving drop size for relevant pressure drops gave sizes in the range of 93 µm for a pressure drop of 0.25 bars and 29 µm for a pressure drop of 4.5 bars. Droplet sizes were determined with a digital video microscope. The results from the microscope analysis gave droplets with sizes beween 2 and 90 µm with mean diameters in the order of 4–12 µm.