Hydrogen/deuterium exchange on aromatic rings during atmospheric pressure chemical ionization mass spectrometry

It has been demonstrated that substituted indoles fully labelled with deuterium on the aromatic ring can undergo substantial exchange back to partial and even fully protonated forms during atmospheric pressure chemical ionisation (APCI) liquid chromatography/mass spectrometry (LC/MS). The degree of this exchange was strongly dependent on the absolute quantity of analyte, the APCI desolvation temperature, the nature of the mobile phase, the mobile phase flow rate and the instrument used. Hydrogen/deuterium (H/D) exchange on several other aromatic ring systems during APCI LC/MS was either undetectable (nitrobenzene, aniline) or extremely small (acetanilide) compared to the effect observed for substituted indoles. This observation has major implications for quantitative assays using deuterium‐labelled internal standards and for the detection of deuterium‐labelled products from isotopically labelled feeding experiments where there is a risk of back exchange to the protonated form during the analysis. Copyright © 2010 John Wiley & Sons, Ltd.     

Diastereoselective synthesis of substituted prolines via 5-endo-trig cyclisations of aza-[2,3]-Wittig sigmatropic rearrangement products

The major diastereoisomers of aza-[2,3]-Wittig sigmatropic rearrangement products from α-amino acid derivatives are susceptible to a rare nucleophilic 5-endo-trig cyclisations of an amine onto a non-conjugated vinylsilane in high yield and complete diastereocontrol. Five examples are presented, with cyclisation yields between 35 and 87%. A rationale for the stereoselectivity of the cyclisation is forwarded based upon the steric control factors that have been documented for the aza-[2,3]-Wittig sigmatropic rearrangement. A discussion of the mechanism in the context of the reaction conditions is also presented. Oxidation of the silyl group to a hydroxyl group and complete removal were demonstrated for synthetic utility.     

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.     

Doubly diastereoselective conjugate addition of enantiopure lithium amides to enantiopure N-enoyl oxazolidin-2-ones: a mechanistic probe

The doubly diastereoselective conjugate addition of the antipodes of lithium N-benzyl-N-(α-methylbenzyl)amide to a range of enantiopure N-enoyl oxazolidin-2-ones has been used as a mechanistic probe to determine that the reactive conformation is the anti-s-cis form. The β-amino carbonyl products resulting from these conjugate addition reactions are useful templates for further elaboration into an α,β,α-pseudotripeptide.     

Homo-timeric structural model of human microsomal prostaglandin E synthase-1 and characterization of its substrate/inhibitor binding interactions

Inducible, microsomal prostaglandin E synthase 1 (mPGES-1), the terminal enzyme in the prostaglandin (PG) biosynthetic pathway, constitutes a promising therapeutic target for the development of new anti-inflammatory drugs. To elucidate structure–function relationships and to enable structure-based design, an mPGES-1 homology model was developed using the three-dimensional structure of the closest homologue of the MAPEG family (Membrane Associated Proteins in Eicosanoid and Glutathione metabolism), mGST-1. The ensuing model of mPGES-1 is a homo-trimer, with each monomer consisting of four membrane-spanning segments. Extensive structure refinement revealed an inter-monomer salt bridge (K26-E77) as well as inter-helical interactions within each monomer, including polar hydrogen bonds (e.g. T78-R110-T129) and hydrophobic π-stacking (F82-F103-F106), all contributing to the overall stability of the homo-trimer of mPGES-1. Catalytic co-factor glutathione (GSH) was docked into the mPGES-1 model by flexible optimization of both the ligand and the protein conformations, starting from the initial location ascertained from the mGST-1 structure. Possible binding site for the substrate, prostaglandin H₂ (PGH₂), was identified by systematically probing the refined molecular structure of mPGES-1. A binding model was generated by induced fit docking of PGH₂ in the presence of GSH. The homology model prescribes three potential inhibitor binding sites per mPGES-1 trimer. This was further confirmed experimentally by equilibrium dialysis study which generated a binding stoichiometric ratio of approximately three inhibitor molecules to three mPGES-1 monomers. The structural model that we have derived could serve as a useful tool for structure-guided design of inhibitors for this emergently important therapeutic target.     

Enhancement of cyclosporine aqueous solubility using α- and hydroxypropyl β-cyclodextrin mixtures

Cyclodextrins (CDs) are cyclic oligosaccharides that form inclusion complexes with lipophilic molecules through their hydrophobic central cavity. In this study, the effect of α-CD, hydroxylpropyl-β-CD (HP-β-CD) and mixtures of these two CDs on the aqueous solubility of cyclosporine A (CyA) was investigated. Infrared spectroscopy and thermal analysis were used to confirm CyA-CD complex formation. CyA aqueous solubility was increased by 10 and 80 fold in the presence of α-CD and HP β-CD, respectively. The phase-solubility profile for HP-β-CD was linear while that for α-CD had positive deviation from linearity. In the presence of constant concentration of α-CD (15% w/v), aqueous solubility of CyA was further increased upon addition of HP-β-CD up to a concentration of 20% w/v. At higher HP-β-CD concentrations, aqueous solubility of CyA was observed to decrease. Addition of sodium acetate (up to 5% w/v) to aqueous solutions containing 20% w/v HP-β-CD and increasing concentrations of α-CD resulted in a significant reduction in CyA solubility. Complex formation between CyA and both α-CD and HP-β-CD was confirmed by differential scanning calorimetry (DSC). No significant changes were observed in the IR spectra of either CyA or CD following complex formation suggesting chemical interaction between CyA and the CD was unlikely. Phase-solubility studies showed that α-CD had a much greater effect on the solubility of CyA than HP-β-CD. Addition of HP-β-CD to aqueous solutions of α-CD affected the solubility of CyA in these systems. A mixture of 15% w/v α-CD and 20% w/v HP-β-CD was optimal for increasing aqueous solubility of CyA.     

Novel human lens metabolites from normal and cataractous human lenses

4-(2-Aminophenyl)-4-oxobutanoic acid, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid and glutathionyl-kynurenine have been identified as novel metabolites in normal and cataractous human lenses following total synthesis and comparison with authentic human lens samples. Their structures are consistent with those derived from the major human lens UV filters kynurenine and 3-hydroxykynurenine, and it is proposed that these compounds also play a role as UV filters. These metabolites were isolated in pmol/mg levels (dry mass) in lenses. 4-(2-Amino-3-hydroxyphenyl)-4-oxobutanoic acid and glutathionyl-kynurenine were found to be unstable at physiological pH. Other potential metabolites, glutathionyl-3-hydroxykynurenine, kynurenine yellow and 3-hydroxykynurenine yellow, were not detected in either normal or cataractous lenses.