Search results for "liquid"
showing 10 items of 4351 documents
Separation of unmodified α-amino acid enantiomers by reverse phase HPLC
1980
A novel chiral phase system is presented for the resolution of unmodified α-amino acid racemates which is composed of a reverse phase packing coated with N-alkyl-L-hydroxyproline (where alkyl is n-C7H15-, n-C10H21- and n-C16H33-) and a hydro-organic eluent containing copper(II)acetate. The factors controlling retention and enantioselectivity such as concentration of Cu(II)ion and pH of the eluent, addition of NH4Ac to the eluent, type and content of organic solvent of hydro-organic eluent and column temperature were examined. The extremely high enantioselectivity observed (α up to 16) is assumed to be caused by a three site sorbate-sorbent interaction involving bidentate coordination of two…
Correlation between hydrophobicity of amino acids and retention data in reversed-phase liquid chromatography with micellar eluents
1995
Hydrophobic character is usually expressed in terms of the partition coefficient in 1-octanol-water (log PO/W). However, measurement of this coefficient is often problematic. Retention in micellar liquid chromatography is mainly due to hydrophobic interactions and can also be used as an index of hydrophobicity. A hydrophobicity scale was established with retention data foro-phthalaldehyde (OPA)-N-acetyl-L-cysteine (NAC) amino acid derivatives, using the glycine derivative as reference. Since the OPA-NAC derivatives only differ in the nature of R1 in the amino acid (R1CH(COOH)NH2), in the absence of electrostatic interactions the hydrophobic character of the substituent was responsible for r…
Fluorenylmethoxycarbonyl-N-methylamino Acids Synthesized in a Flow Tube-in-Tube Reactor with a Liquid-Liquid Semipermeable Membrane
2013
Both steps of the N-methylation of 9-fluorenylmethoxycarbonyl (Fmoc) amino acids were carried out in a microstructured tube-in-tube reactor equipped with a semipermeable Teflon® AF 2400 membrane as the inner tubing. In the first step, gaseous formaldehyde was passed through the inner membrane to effect the acid-catalyzed conversion of the Fmoc amino acids into the corresponding N-Fmoc oxazolidinones. In the second step, liquid–liquid transfer of trifluoroacetic acid was used for the first time in such a reactor for the reductive opening of these oxazolidinones to give Fmoc N-methylamino acids in high yields.
Chromatographic quantitation of the hydrophobicity of ionic compounds by the use of micellar mobile phases
1998
Abstract Many biologically active compounds of interest in structure–activity relationships are ionic at physiological pH. However, ionic organic compounds are only weakly or not retained in conventional RPLC which impedes the chromatographic estimation of their hydrophobicity and the development of quantitative retention–activity relationship studies. The use of micellar mobile phases allows the retention of ionic compounds. Hydrophobic and electrostatic forces govern the retention of ionic compounds in micellar liquid chromatography. In this paper three different retention models log k–log P for ionic compounds are tested (P=partition coefficient). The retention model (log k=a log P+bα+c)…
Effect of the hydrophobic spacer in bonded [Cu(l-hydroxyprolyl)alkyl]+ silicas on retention and enantioselectivity of α-amino acids in high-performan…
1983
Abstract The following chiral ligands were bonded to silica: [Cu( l -hydroxyprolyl)methyl) + on LiChrosorb Si 60 (1), [Cu( l -hydroxyprolyl)methyl] + on LiChrosorb Si 100 (2) and [Cu( l -hydroxyprolyl) n -octyl] + on LiChrosorb Si 100 (3). The packings contained residual iodomethyl- and ω-iodooctyl groups at the surface. Studies on packing 1 and 2 under comparable conditions in eluents containing 10 -4 M copper acetate showed a higher retention on 1 than on 2 but a much better enantioselectivity in the latter case. The retention of enantiomers on all packings examined was found to be governed by the eluent pH and methanol content as well as by the concentration of ammonium acetate. The vari…
Elucidation of structure and microheterogeneity of the polypeptide antibiotics paracelsin and trichotoxin A-50 by fast atom bombardment mass spectrom…
1984
Specific and sensitive procedures have been developed which enabled the structure elucidation of the polypeptide antibiotics (peptaibols), paracelsin isolated from Trichoderma reesei, and of trichotoxin A-50 from Trichoderma viride, by fast atom bombardment and field desorption mass spectrometry. Both peptides were found to exhibit a pronounced microheterogeneity by single and multiple exchange of amino acids. Separation by analytical and semipreparative high-performance liquid chromatography (HPLC) on octadecylsilyl-bonded, reversed-phase columns afforded a series of sequence analogues for each polypeptide. Unequivocal molecular weight and sequence identifications were obtained by positive…
Application of molecularly imprinted polymers in supercritical fluid chromatography
2000
Molecularly imprinted polymers (MIPs), for the templates free base racemic propranolol and the L-enantiomer of phenylalanine anilide (L-PA), were investigated as stationary phases in supercritical fluid chromatography (SFC). Large retention differences were observed on the propranolol MIP for both the template molecule and the structural analogue metoprolol compared to that observed on the corresponding blank polymer. Mobile phase composition and solute concentration were found to affect this retention behaviour. The phenylalanine anilide MIP (L-PA MIP) was found to be enantioselective in SFC with stronger retention observed for the template enantiomer. Throughout the study, characteristic …
The second component of human complement: Detection of two hemolytic forms in plasma by pH Variation
1988
The second component of human complement (C2) in pseudoglobulin prepared from normal plasma eluted as a single peak at high conductivity (30 mS) and pH 4.5 from the cationic exchangers S-Sepharose or Mono S in the Fast Protein Liquid Chromatography (FPLC) System. The C2 was stable at pH 4.5 and 0 degrees C if enzyme inhibitors were used and the pH was raised to 6.0 after elution from the columns. After rechromatography on Mono S in the FPLC System at the median isoelectric point of 5.5 or pH 6.0, the C2 eluted as two distinct hemolytic forms: the first peaked at 16 mS, the second at 30 mS. The two forms of C2 did not correlate with the allotypic variant of C2 in individual, normal human pla…
Environmental analysis of chlorinated aromatic thioethers, sulphoxides and sulphones
1993
Abstract Chlorinated aromatic thioethers discussed here are polychlorinated dibenzothiophenes, thianthrenes and diphenylsulphides. Relatively little is known about their occurrence, behaviour and fate in the environment. Polychlorinated dibenzothiophenes and diphenylsulphides have recently been found to be formed in waste combustion and analysed in pulp mill effluents. Chlorinated sulphoxides and sulphones are usually metabolites or oxidation products of different chlorinated aromatic compounds. Different gas chromatographic-mass spectrometric techniques are used in the analysis of the chlorinated thioethers. The sulphoxides and sulphones, because of their higher polarity, can be isolated f…
Single-pump bi-dimensional LC applied to the characterization of derivatized fatty alcohol ethoxylates.
2010
A column selection valve and a 6-port 2-position injection valve were combined to implement bi-dimensional LC in a system driven by a single pump. The system was applied to the separation of linear fatty alcohol ethoxylates with 12-18 carbon atoms in the hydrocarbon moiety, previous derivatization with diphenic anhydride. Separation on the first separation dimension (C(1)) was carried out with a propyl-diol column and an acetonitrile/aqueous ammonium acetate gradient. Fatty alcohol ethoxylate hydrocarbon series (oligomers with the same number of carbon atoms in the alkyl chain but differing in the number of ethylene oxide units) were resolved without distinguishing the oligomers within the …