1,721,094 research outputs found
Determination of enantiomerization barriers of hypericin and pseudohypericin by dynamic high-performance liquid chromatography on immobilized polysaccharide-type chiral stationary phases and off-column racemization experiments
No full textDirect enantiomer separation of hypericin, pseudohypericin, and protohypericin
was accomplished by high-performance liquid chromatography (HPLC) using
immobilized polysaccharide-type chiral stationary phases (CSPs). Enantioselectivities
up to 1.30 were obtained in the polar-organic elution mode whereby for hypericin and
pseudohypericin Chiralpak IC [chiral selector being cellulose tris(3,5-dichlorophenylcarbamate)]
and for protohypericin Chiralpak IA (chiral selector being the 3,5-dimethylphenylcarbamate
of amylose) gave favorable results. Enantiomers were distinguished by
on-line electronic circular dichroism detection. Optimized enantioselective chromatographic
conditions were the basis for determining stereodynamic parameters of the
enantiomer interconversion process of hypericin and pseudohypericin. Rate constants
delivered by computational simulation of dynamic HPLC elution profiles (stochastic
model, consideration of peak tailing) were used to calculate averaged enantiomerization
barriers (DG]
enant) of 97.6–99.6 kJ/mol for both compounds (investigated temperature
range 25–458C). Complementary variable temperature off-column (i.e., in solution) racemization
experiments delivered DG]
enant 5 97.1–98.0 kJ/mol (27–458C) for hypericin and
DG]
enant 5 98.9–101.4 kJ/mol (25–558C) for pseudohypericin. An activation enthalpy of
DH# 5 86.0 kJ/mol and an activation entropy of DS# 5 237.7 J/(K mol) were calculated
from hypericin racemization kinetics in solution, whereas for pseudohypericin these
figures amounted to 74.1 kJ/mol and 282.6 J/(K mol), respectively. Although the natural
phenanthroperylene quinone pigments hypericin and pseudohypericin as well as
their biological precursor protohypericin are chiral and can be separated by enantioselective
HPLC low enantiomerization barriers seem to prevent the occurrence of an
excess of one enantiomer under typical physiological conditions—at least as long as
stereoselective intermolecular interactions with other chiral entities are absen
Design and Evaluation of a New Tweezers-Like Stable Stationary Phase for Hydrophilic Interaction Chromatography-Mass Spectrometry (HILIC-MS)
No full textDynamic high performance liquid chromatography on chiral stationary phases. Low temperature separation of the interconverting enantiomers of diazepam, flunitrazepam, prazepam and tetrazepam
No full textDiazepam and the structurally related 1,4-benzodiazepin-2-ones tetrazepam, prazepam and flunitrazepam are chiral molecules because they adopt a ground state conformation featuring a non-planar seven membered ring devoid of any reflection-symmetry element. The two conformational enantiomers of this class of benzodiazepines interconvert rapidly at room temperature by a simple ring flipping process. Low temperature HPLC on the Whelk-O1 chiral stationary phase allowed us to separate the conformational enantiomers of diazepam and of the related 1,4-benzodiazepin-2-ones, under conditions where the interconversion rate is sufficiently low, compared to the chromatographic separation rate. Diazepam, tetrazepam and prazepam showed temperature dependent dynamic HPLC profiles with interconversion plateaus indicative of on-column enantiomer interconversion (enantiomerization) in the temperature range between −10 °C and −35 °C, whereas for flunitrazepam on-column interconversion was observed at temperatures between −40 °C and −66 °C. Simulation of exchange-deformed HPLC profiles using a computer program based on the stochastic model yielded the apparent rate constants for the on-column enantiomerization and the corresponding free energy activation barriers. At −20 °C the enantiomerization barriers, ΔG≠, for diazepam, prazepam and tetrazepam were determined to be in the range 17.6–18.7 kcal/mol. At −55 °C ΔG≠ for flunitrazepam was determined to be in the 15.6–15.7 kcal/mol range. The experimental dynamic chromatograms and the corresponding interconversion barriers reported in this paper call for a reinterpretation of previously published results on the HPLC behavior of diazepam on chiral stationary phases
On-Column Quantification of Amino Functionalities Bonded to Solid Porous Matrices Packed within High Performance Liquid Chromatography Columns
No full textStationary phases ( SP s ) based on silica matrices functionalized with amino groups linked to their surface through alkyl chains of various length have found remarkable success in performing HILIC separations, showing really effective resolution towards a wide typology of compounds of biological interest, such as carbohydrates, nucleosides, purine and pyrimidine bases. Recently, we developed an operationally simple procedure, named DNBA‐M , non-destructive for the analysed SP , designed to quantify the density of basic groups (typically amino groups) chemically bonded to the surface of porous solids. In the present study the DNBA‐M procedure has been suitably modified to allow the quantification of any typology of amino groups present on silica matrices packed into High Performance Liquid Chromatography (HPLC) columns. The new approach, named OC‐DNBA‐M , has been successfully validated through analysis of two HPLC columns packed with aminopropyl-silica matrices. Afterwards, it was also demonstrated as the OC‐DNBA‐M procedure may allow the effective and in-depth analysis of the structural composition characterizing SP s packed inside HPLC columns, in which amino-groups have been differently and only partially involved in following ureidic functionalizations. It was also proved how the analysed columns can be readily re-employed for the chromatographic applications for which they have been designed, without appreciable deterioration of the respective discrimination abilities
Introducing enantioselective UHPLC in the chiral science separation scenario: ultra-fast separations on a new sub-2 μm Whelk-O1 stationary phase
No full textA new chiral stationary phase for UHPLC applications was prepared by covalent attachment of the Whelk-O1 selector to spherical high-surface-area 1.7-μm porous silica particles. Columns of varying dimensions (50-150 mm length and 3.0 or 4.6 mm internal diameter) were packed and characterized in terms of permeability, efficiency, retention and enantioselectivity.1 Van Deemter plots, performed on a 100 x 4.6 mm I.D. column, generated Hmin values of 3.53 μm for 1,3-dinitrobenzene, at an interstitial mobile phase linear velocity (μinter) of 5.07 mm/s, and Hmin of 4.26 and 4.17 μm for the two enantiomers of acenaphthenol, at μinter of 4.85 mm/s and 4.24 mm/s, respectively. Resolution of 21 enantiomeric pairs including alcohols, epoxides, sulfoxides, phosphine oxides, benzodiazepines and 2-aryloxyproprionic esters used as herbicides, were obtained with significant advantages in terms of efficiency (with N/m values as high as 270 000) and analysis time (in the seconds-time-scale with trans-stilbene oxide being resolved in only 10 s).2 The excellent efficiency and selectivity, conjugated with the very high-throughput and the ultra-fast analysis time, prove the potentials of the eUHPLC Whelk-O1 columns in the development of enantioselective UHPLC methods and represent a powerful tool in organic chemistry
Recognition mechanism of aromatic derivatives resolved by argentation chromatography: The driving role played by substituent groups
No full textArgentation chromatography is widely used nowadays as a powerful tool to separate complex mixtures of analytes containing unsaturated and/or aromatic fragments. Here we present the results of chromatographic and computational studies on a silver-thiolate stationary phase, in which the silver metal is covalently bonded to mercaptopropyl silica particles. The exceptionally high selectivity displayed by this organometallic moiety prompted us to deeply investigate its molecular recognition properties. The interactions of the silver atom with a series of benzene derivatives was investigated to gain information on the mechanism by which the different ring substituents modulate retention factors and selectivity. The experimental trend was fully rationalized by means of quantum-mechanical Density Functional Theory (DFT) calculations, which allowed us to elucidate the chromatographic results in the light of unusual and unexpected substituent effects
Teicoplanin based CSPs: eluent composition and temperature effects on retention and enantioselectivity
No full textTetrasubstituted cyclopentadienones as suitable enantiopure ligands with axial chirality.
No full textA series of thermally stable atropisomeric phencyclone ligands (ΔG‡
rac > 35 kcal mol−1), bearing two chiral
axes, has been successfully synthesized, taking into account the results of DFT calculations on model
systems. The absolute configurations of the novel atropisomers have been assigned using TD-DFT simulation
of ECD spectra. Atropisomeric phencyclones herein presented pave the way towards new ruthenium-
based enantioselective hydrogenation catalyst
Static vs. dynamic electrostatic repulsion reversed phase liquid chromatography: solutions for pharmaceutical and biopharmaceutical basic compounds
Full text linkMany efforts have been made to separate basic compounds, which are challenging to resolve in reversed phase liquid chromatography. In this process, they are strongly retained and the peak shape undergoes significant distortion. The principal origin of this has been identified with the non-negligible interaction with residual deprotonated silanols. Consequently, all solutions that efficiently shield silanols are being sought. This review is an upgrade on the use of the electrostatic repulsion reversed phase (ERRP) approach: retention of bases, in protonated form, can be achieved by modulating the charge repulsion caused by the presence of positive charges in the chromatographic system. This study successfully (i) introduced fixed positive charges in the structure of stationary phases, (ii) used cationic and hydrophobic additives in the mobile phase, and (iii) used the ERRP-like approach employed at the preparative level for peptide purification
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