The use of a diode laser in an ESPI system

The use of a diode laser in an ESPI system is reported. These lasers have considerable potential advantages over gas lasers in terms of convenience, cost and lifetimes, and may eventually replace the latter in this type of instrument.     

Vibrational spectra and structure of gaseous and solid dimethylboric anhydride

The Raman spectra of gaseous, liquid and solid dimethylboric anhydride (CH₃)₂BOB(CH₃)₂ have been recorded from 10–3500 cm^?1. The IR spectra from 4000–30 cm^?1 have also been recorded. The spectra of the gaseous phase have been interpreted in terms of C₂ symmetry implying a bent B-O-B skeleton with the B(CH₃)₂ groups twisted and consistent with a rather larger barrier to internal rotation about the B-O bonds. The spectra of the crystalline state, however, suggest that the molecular symmetry is altered upon solidification. Isotopic substitution of the oxygen atom by ¹⁸O confirmed that the B-O-B skeleton is linear in the solid state, and the spectra have been interpreted in terms of D_2h molecular symmetry.     

Electron impact ionization mass spectra of lithocholyl amides: Evidence for a C(20) to C(23) rearrangement involving the loss of a C4H9 fragment

Amides of lithocholic acid (3α-hydroxy-5β-cholan-24-oic acid) with 6-aminocaproic acid and 4-aminobutyric acid were prepared and examined by electron impact ionization mass spectrometry. Both these compounds gave an unusual [M ? 57]⁺ fragment. Since the product-ion analysis of [M ? 57]⁺ revealed the presence of fragments corresponding to the intact steroid nucleus in addition to that of the original amino acid (6-aminocaproic acid or 4-aminobutyric acid), we concluded that the integrity of the steroid amide had been retained in this fragment. The absence of this fragment from the product-ion spectrum of [M ? CH₃]⁺ rules out the sequential loss from the molecular ion of 15 + 42 u as the origin of this signal. Mass spectrometry of the 24-¹³C-labelled lithocholylcaproylamide showed the retention of the label in the [M ? 57]⁺ fragment. In contrast, the corresponding compound labelled with deuterium at C(23) showed a significant loss of the label during the formation of this product ion at [M ? 58]⁺. In addition, through a combination of derivatization and tandem mass spectrometry, it was demonstrated that this loss of 57 u represented a rearrangement with the expulsion of a C₄H₉ radical from the side-chain spanning C(20) to C(23) resulting in a truncated steroid-amide fragment. This fragmentation pattern has not been observed in bile acid conjugates with N-α-amino acids.     

Optimization of sequencing conditions using near-infrared lifetime identification methods in capillary gel electrophoresis

We have investigated the sample preparation and electrophoresis conditions necessary to prepare DNA sequencing samples appropriate for use with near-infrared (IR) fluorescent labels with dye identification accomplished via lifetime techniques. It was found that several sample preparation protocols required attention to maximize the fluorescence yields of the labeling dyes, such as thermal cycling conditions, choice of counter ion used for the ethanol precipitation step and also, dye-primer versus dye-terminator chemistries. In addition, several different sieving matrices were investigated for their effects on both the fluorescence properties of the labeling dyes and electrophoretic resolution. Extended times used for the high temperature denaturing of duplexed DNA fragments during cycle sequencing produced cleavage products, in which the covalently attached dye to the sequencing primer was released through attack by dithiothreitol (DTT). Even under optimized thermal cycling conditions, free dye was generated that masked readable data from the sequencing traces. Ethanol precipitation was necessary to remove this free dye with the proper choice of counter ion (sodium). The results using different sieving matrices indicated that linear polyacrylamides (LPAs) were appropriate for any fluorescence measurement, since they could readily be replaced between runs minimizing deleterious memory effects associated with cross-linked polyacrylamide gels. After investigation of several different sieving LPAs, the commercially available POP6 was found to be particularly attractive, since it produced good electrophoretic resolution, single exponential behavior for the near-IR dye series investigated herein, and also, discernible lifetime differences within the dye set. Finally, dye-terminator chemistry was also found to minimize bleeding in the gel matrix produced by large amounts of unextended dye-primer within the gel lane.     

The preparation and resolution of 2-(2-pyridyl)- and 2-(2-pyrazinyl)-Quinazolinap and their application in palladium-catalysed allylic substitution

The preparation and resolution of two new axially chiral quinazoline-containing phosphinamine ligands, 2-(2-pyridyl)-Quinazolinap and 2-(2-pyrazinyl)-Quinazolinap, is described. The ligands were synthesised in good yield over eight steps and included two Pd-catalysed reactions, a Suzuki coupling to form the biaryl linkage and the introduction of the diphenylphosphino group, as the key transformations. The racemic ligands were resolved via the fractional crystallisation of diastereomeric palladacycles derived from (+)-di-μ-chlorobis{(R)-dimethyl[1-(1-naphthyl)ethyl]aminato-C₂,N}dipalladium (II) X-ray crystal structures of the (S,R)-2-pyridyl- and (S,R)-2-pyrazinyl-palladacycles are included. Displacement of the resolving agent by reaction with 1,2-bis(diphenylphosphino)ethane gave enantiopure 2-(2-pyridyl)-Quinazolinap and 2-(2-pyrazinyl)-Quinazolinap, new atropisomeric phosphinamine ligands for asymmetric catalysis. These ligands were applied in the palladium-catalysed allylic substitution of 1,3-diphenylprop-2-enyl acetate resulting in moderate conversions and enantioselectivities of up to 81%.     

Solubilization chromatography: Ethers,carboxylic acids,and hydrocarbons

Synthetic mixtures of aliphatic and aromatic ethers, satmated fatty acids, and substituted benzenes and naphthalenes were separated by elution through columns of ion-exchange resins with aqueous solutions of acetic acid as eluent An attempt was made to use a resin of low capacity for the separation of such mixtures, and the results of this attempt are used to help elucidate the theory of solubilization chromatography     

High-performance liquid chromatographic analysis of basic drugs on silica columns using non-aqueous ionic eluents. I. Factors influencing retention, peak shape and detector response

The use of silica columns together with non-aqueous ionic eluents provides a stable yet flexible system for the high-performance liquid chromatographic analysis of basic drugs. At constant ionic strength, eluent pH influences retention via ionisation of surface silanols and protonation of basic analytes, pKa values indicating the pH of maximum retention. At constant pH, retention is proportional to the reciprocal of the eluent ionic strength for fully protonated analytes and quaternary ammonium compounds. The addition of water up to 10% (v/v) has little effect on retention if the protonation of the analytes is unaffected. Thus, it is likely that retention is mediated primarily via cation exchange with surface silanols. However, additional factors must play a part with compounds such as morphine which give tailing peaks at acidic or neutral eluent pHs.     

Mass spectra of n-butylboronate ester derivatives of the major metabolite of prostaglandin F

The EI mass spectra of six n-butylboronate ester derivatives of the major metabolite of prostaglandins F_1α and F_2α (PGF-M) are presented and discussed. Proposed ion assignments and fragmentation pathways are based on substituent shifts, on data from a deuterium-labeled methoxime derivative and on the analysis of collision-induced dissociation spectra of selected ions. Fragment ions suitable for identification and quantification of PGF-M in a biological matrix and diagnostically valuable ions for structure recognition are proposed.     

Preparation and characterization of nanoparticles formed by chitosan-caseinate interactions

Intermacromolecular complexation between chitosan and sodium caseinate in aqueous solutions was studied as a function of pH (3–6.5), using absorbance measurements (at 600 nm), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The chitosan–caseinate complexes formed were stable and soluble in the pH range 4.8–6.0. In this pH range, the biopolymers had opposite charges. At higher concentrations of chitosan (0.15 wt%), the soluble complexes associated to form larger particles. DLS data showed that, between pH 4.8 and 6.0, the particles formed by the complexation of chitosan and caseinate had sizes between 250 and 350 nm and these nanoparticles were visualized using negative staining TEM. Above pH 6.0, the nanoparticles associated to form larger particles, causing phase separation. Addition of NaCl increased the particle size. The pH dependence of the zeta potential of the mixture solutions was appreciably different from that of the pure protein and pure chitosan solutions.     

Asymmetric synthesis of an N-acylpyrrolidine for inhibition of HCV polymerase

A practical asymmetric synthesis of a highly substituted N-acylpyrrolidine on multi-kilogram scale is described. The key step in the construction of the three stereocenters is a [3+2] cycloaddition of methyl acrylate and an imino ester prepared from l-leucine t-butyl ester hydrochloride and 2-thiazolecarboxaldehyde. The cycloaddition features novel asymmetric catalysis via a complex of silver acetate and a cinchona alkaloid, particularly hydroquinine, with complete diastereomeric control and up to 87% enantiomeric control. The alkaloid serves as a ligand as well as a base for the formation of the azomethine ylide or 1,3-dipole. Experiments have shown that the hydroxyl group of hydroquinine is a critical element for the enantioselectivities observed. The cycloaddition methodology is also applicable to methylvinyl ketone, providing access to either alpha- or beta-epimers of 4-acetylpyrrolidine depending on the reaction conditions utilized. The synthesis also highlights an efficient N-acylation, selective O- versus N-methylation, and a unique ester reduction with NaBH4-MeOH catalyzed by NaB(OAc)3H that not only achieves excellent chemoselectivity but also avoids formation of the undesired but thermodynamically favored epimer. The highly functionalized target is synthesized in seven linear steps from l-leucine t-butyl ester hydrochloride with all three isolated intermediates being highly crystalline.