Enantioselective analysis of venlafaxine and its active metabolites: A review on the separation methodologies

Venlafaxine (VFX) is a serotonin and norepinephrine reuptake inhibitor chiral drug used in therapy as an antidepressant in the form of a racemate consisting of R‐ and S‐VFX. The two enantiomers of VFX exhibit different pharmacological activities: R‐VFX inhibits both norepinephrine and serotonin synaptic reuptake, whereas S‐VFX inhibits only the serotonin one. R‐ and S‐VFX are metabolized in the liver to the respective R‐ and S‐O‐desmethylvenlafaxine (ODVFX), R‐ and S‐N‐desmethylvenlafaxine (NDVFX), and R‐ and S‐N,O‐didesmethylvenlafaxine (NODVFX). The pharmacological profile of ODVFX is close to that of VFX, whereas the other two chiral metabolites (NDVFX and NODVFX) have lower affinity for the receptor sites. The pharmacokinetics of the VFX enantiomers appear stereoselective, including the metabolism process. In the past 20 years, several studies describing the enantioselective analysis of R‐ and S‐VFX in pharmaceutical formulations and its chiral metabolites in biological matrices were published. These methods encompass liquid chromatography coupled with UV detection, mass spectrometry, or tandem mass spectrometry, and capillary electrophoresis. This paper reviews the published methods used for the determination of the individual enantiomers of VFX and its chiral metabolites in different matrices.     

p-tert-Butylcalix[4]arene complexes of molybdenum and tungsten: reactivity of the calixarene methylene C-H bond and the facile migration of the metal around the phenolic rim of the calixarene

p-tert-Butylcalix[4]arene, [CalixBut(OH)4], reacts with Mo(PMe3)6 and W(PMe3)4(eta2-CH2PMe2)H to yield compounds of composition {[CalixBut(OH)2(O)2]M(PMe3)3H2} which exhibit unprecedented use of a C-H bond of a calixarene methylene group as a binding functionality in the form of agostic and alkyl hydride derivatives. Thus, X-ray diffraction studies demonstrate that, in the solid state, the molybdenum complex [CalixBut(OH)2(O)2]Mo(PMe3)3H2 exists as an agostic derivative with a Mo...H-C interaction, whereas the tungsten complex exists as a metallated trihydride [Calix-HBut(OH)2(O)2]W(PMe3)3H3. Solution 1H NMR spectroscopic studies, however, provide evidence that [Calix-HBut(OH)2(O)2]W(PMe3)3H3 is in equilibrium with its agostic isomer [CalixBut(OH)2(O)2]W(PMe3)3H2. Dynamic NMR spectroscopy also indicates that the [M(PMe3)3H2] fragments of both the molybdenum and tungsten complexes [CalixBut(OH)2(O)2]M(PMe3)3H2 migrate rapidly around the phenolic rim of the calixarene on the NMR time scale, an observation that is in accord with incorporation of deuterium into the methylene endo positions upon treatment of the isomeric mixture of [CalixBut(OH)2(O)2]W(PMe3)3H2 and [Calix-HBut(OH)2(O)2]W(PMe3)3H3 with D2. Treatment of {[CalixBut(OH)2(O)2]W(PMe3)3H2} with Ph2C2 gives the alkylidene complex [CalixBut(O)4]W=C(Ph)Ar [Ar = PhCC(Ph)CH2Ph].     

Hollow-fiber flow field-flow fractionation: a gentle separation method for mass spectrometry of native proteins

Low-impact ionization sources like electrospray ionization (ESI) and matrix-assisted, laser desorption/ionization (MALDI) equipped with time-of-flight (TOF) mass analyzers provide intact protein analysis over a very wide molar mass range. ESI/TOFMS provides also indications on the higher-order structure of intact proteins and non-covalent protein complexes. However, direct analysis of intact proteins mixtures in real samples shows limited success, mainly because spectra become very complex to interpret. This is also due to sample contaminants, and to the mechanism of competitive ionization in ESI or MALDI. Rapid and efficient sample clean-up and separation methods can significantly enhance the power of TOFMS for intact protein analysis. However, if protein native conditions want to be maintained, the methods should affect neither the three-dimensional structure nor the non-covalent chemistry of the proteins. Reversed-phase (RP) HPLC, size-exclusion chromatography (SEC), and capillary zone electrophoresis (CZE) are on-line or off-line coupled to ESI/TOFMS or MALDI/TOFMS. In fact, these separation methods often show limitations when applied to the analysis of native proteins. Organic modifiers and saline buffers are required in the case of RP HPLC or CZE. They can induce protein degradation or affect ionization when MS is performed after separation. High voltages used in CZE can contribute to alter proteins from their native form. In the case of high molar mass proteins, SEC is scarcely selective, and barely able to detect protein aggregates. Sample entanglement/adsorption on the stationary phase can also occur.     

The Mitsunobu Reaction: An Alternative Synthesis of 1-(Primary Alkyl)benzotriazoles

1-(Primary alkyl)benzotriazoles are obtained in convenient yields by treating the corresponding alcohol with triphenylphosphine, N-bromosuccinimide and benzotriazole. Under these conditions, the alcohols gave exclusively the corresponding 1-alkyl-benzotriazoles. Allyl and propargyl alcohols also reacted regiospecifically in a typical Sn2 manner, and no products derived from prototropic rearrangement were found.     

Formation of {Cu6[S2P(OC2H5)2]6} on Cu2S surfaces from aqueous solutions of the KS2P(OC2H5)2 collector: scanning electron microscopy and solid-state 31P cross-polarization/magic angle spinning and static 65Cu NMR studies

The interactions of synthetic chalcocite surfaces with diethyldithiophosphate, potassium salt, K[S2P(OC2H5)2], were studied by means of 31P cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy and scanning electron microscopy (SEM). To identify the species formed on the Cu2S surfaces, a polycrystalline {CuI6[S2P(OC2H5)2]6} cluster was synthesized and analyzed by SEM, powder X-ray diffraction techniques and solid-state 31P CP/MAS NMR and static 65Cu NMR spectroscopy. 31P chemical shift anisotropy (CSA) parameters, delta(cs) and eta(cs), were estimated and used for assigning the bridging type of diethyldithiophosphate ligands in the {CuI6[S2P(OC2H5)2]6} cluster. The latter data were compared to 31P CSA parameters estimated from the spinning sideband patterns in 31P NMR spectra of the collector-treated mineral surfaces: formation of polycrystalline {CuI6[S2P(OC2H5)2]6} on the Cu2S surfaces is suggested. The second-order quadrupolar line shape of 65Cu was simulated, and the NMR interaction parameters, CQ and etaQ, for the copper(I) diethyldithiophosphate cluster were obtained.     

Molecular view of water dynamics near model peptides

Incoherent quasi-elastic neutron scattering (QENS) has been used to measure the dynamics of water molecules in solutions of a model protein backbone, N-acetyl-glycine-methylamide (NAGMA), as a function of concentration, for comparison with results for water dynamics in aqueous solutions of the N-acetyl-leucine-methylamide (NALMA) hydrophobic peptide at comparable concentrations. From the analysis of the elastic incoherent structure factor, we find significant fractions of elastic intensity at high and low concentrations for both solutes, which corresponds to a greater population of protons with rotational time scales outside the experimental resolution (>13 ps). The higher-concentration solutions show a component of the elastic fraction that we propose is due to water motions that are strongly coupled to the solute motions, while for low-concentration solutions an additional component is activated due to dynamic coupling between inner and outer hydration layers. An important difference between the solute types at the highest concentration studied is found from stretched exponential fits to their experimental intermediate scattering functions, showing more pronounced anomalous diffusion signatures for NALMA, including a smaller stretched exponent beta and a longer structural relaxation time tau than those found for NAGMA. The more normal water diffusion exhibited near the hydrophilic NAGMA provides experimental support for an explanation of the origin of the anomalous diffusion behavior of NALMA as arising from frustrated interactions between water molecules when a chemical interface is formed upon addition of a hydrophobic side chain, inducing spatial heterogeneity in the hydration dynamics in the two types of regions of the NALMA peptide. We place our QENS measurements on model biological solutes in the context of other spectroscopic techniques and provide both confirming as well as complementary dynamic information that attempts to give a unifying molecular view of hydration dynamics signatures near peptides and proteins.     

The dimeric and tetrameric octarepeat fragments of prion protein behave differently to its monomeric unit

Potentiometric and spectroscopic data have shown that octarepeat dimer and tetramer are much more effective ligands for Cu(II) ions than simple octapeptide. Thus, the whole N-terminal segment of prion protein due to cooperative effects, could be more effective in binding of Cu(II) than simple peptides containing a His residue. The gain of the Cu(II) binding by longer octarepeat peptides derives from the involvement of up to four imidazoles in the coordination of the first Cu(II) ion. This type of binding increases the order of the peptide structure, which allows successive metal ions for easier coordination.     

Spectroscopic and structural characterization of pure and FeCl<Subscript>3</Subscript>-containing tri-<Emphasis Type="Italic">n</Emphasis>-butyl phosphate

The spectroscopic properties and liquid structure of pure tri-n-butyl phosphate (TBP) and FeCl₃/TBP solutions have been investigated by Uv–Vis and Raman spectroscopies, X-ray diffraction and conductometry. Uv–Vis and Raman spectra, supported by conductometric measurements, consistently indicate that the solubilized salt is present mostly as TBP _n [FeCl_3???n ] ⁿ⁺ and FeCl₄ ^? complex ions due to specific interaction with the TBP phosphate group. Thanks to this interaction, a high amount of salt (up to 13 % w/w) can be dissolved despite the relatively low dielectric constant of TBP. The X-ray diffractogram of pure TBP has been interpreted in terms of three main contributions which can be attributed to spatial pair correlations between atoms of interacting TBP molecules. In the presence of increasing FeCl₃ amounts, it has been observed a progressive structuring effect, exerted by the dissolved salt, on the layers of opportunely oriented TBP molecules due to the formation of the complex ionic species. By simple treatment with NaBH₄, the synthesis of Fe nanoparticles has been achieved. The absence of water, the easiness of preparation, the high amount of salt which can be suspended and the peculiar physico-chemical properties of such systems are all elements worth of note for the fields of nanoparticle synthesis and for specialized technological applications.