Electrospray mass spectrometric study of anionic complexes of enkephalins with Cu(II): regioselective distinction of Leu/Ile at the C-terminus induced by metal reduction

The yield of metallation of methionine-enkephalin and leucine-enkephalin isomers by copper(II) chloride was investigated by electrospray ionization ion trap mass spectrometry (ESI-ITMS) in negative ionization mode. Binary ([(M-3H)+Cu(II)](-)) and ternary ([(M-3H)+Cu(II)Cl](-)) complexes were observed. Soft and hard desolvation conditions (by changing the declustering voltage) were applied to study their influence on the metallation yield and on the observed deprotonated and metallated species. Structures of the binary complexes with defined charge locations are proposed, based on the observed in-source fragmentations. It was demonstrated that the in-source fragmentations under hard desolvation conditions could differentiate the Leu/Ile isomers if located at the C-terminal position but not at the N-terminal position. This behavior was also observed using a triple quadrupole analyzer. This facile distinction, due to a different radical loss from the [(M-3Hbond;CO(2))+Cu(II)](-) species (loss of [C(3)H(7)](.) for YGGFL and [C(2)H(5)](.) for YGGFI), was facilitated by the reduction of the oxidation state of Cu(II). This in-source differentiation of YGGFI and YGGFL was also implemented in LC/ESI-MS analysis by post-column addition of the copper salt with a syringe pump.     

Towards a microscopic theory of the metal-insulator transition

A perturbative analysis up to second order in the interaction strength is carried out for the single-particle density of states in a disordered Fermi system. The screening also is evaluated up to the first order. We discuss the implications of these calculations on the structure of the fixed-point equations for the metal-insulator transition.     

Ion/molecule reactions with dimethyl ether and dimethyl-d6 ether: differentiation of four isomers contained in patchouli essential oil

In this study, we show that it is possible to differentiate four sesquiterpene isomers (C(15)H(24)) preliminarily separated by gas chromatography/mass spectrometry (GC/MS). Dimethyl ether is evaluated as a selective ionization reagent and the relative abundance of adducts formed with this reagent gas under positive chemical ionization conditions are compared and adduct ions are characterized using collision-induced dissociation. The mechanisms have been confirmed by achieving the same experiments with deuterated dimethyl ether. Copyright 2000 John Wiley & Sons, Ltd.     

Sequencing of a branched peptide using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Chemical degradation methods combined with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and post-source decay (PSD)-MALDI reflex TOF mass spectrometry (MS) were used to determine the sequence of a peptide branched on to a known peptide backbone. This study was applied to a branched peptide model (derivative of substance P). The branched peptide mimics a digest of a membrane receptor on to which a derivative of substance P was photochemically linked. Chemical degradation based on N-terminal ladder sequencing in combination with MALDI-TOF-MS gave only partial sequence information. Although single PSD mass spectra still remain difficult to interpret unambiguously, PSD-MALDI-TOF-MS was combined with on-target acetylation and H -- D exchange to give a better and successful approach to the unambiguous determination of the complete amino acid side-chain sequence. This study shows the capability of MALDI-TOF-MS to help in characterizing ligand-receptor interactions.     

Evaluation of atmospheric pressure ionization interfaces for quantitative measurement of sulfonamides in honey using isotope dilution liquid chromatography coupled with tandem mass spectrometry techniques

A comparison was made between electrospray, atmospheric pressure chemical and atmospheric pressure photospray ionizations to evaluate the MS/MS responses of standard sulfonamides and honey spiked samples. The sample preparation entails an acidic hydrolysis followed by a liquid/liquid extraction. Full method validation was realised by LC-APPI-MS/MS. Decision limit and detection capability were calculated for each analyte (at 50 microg/kg) and ranged between 53.6 and 56.9 and 57.5 and 63.2 microg/kg, respectively. Limits of detection and of quantification ranged, respectively, at 0.4-4.5 and 1.2-15.0 microg/kg. Precursor ion scan experiments of m/z 92 were also carried out as a survey experiment, linked with an enhanced product ion scan experiment to potentially identified additional sulfonamides via a library search.     

Low temperature synthesis of hexagonal ZnO nanorods and their hydrogen sensing properties

The growth of hexagonal ZnO nanorods was demonstrated by low temperature chemical synthesis approach. X-ray diffraction (XRD) analysis revealed a wurtzite hexagonal structure of the ZnO nanorods. The optical properties were measured by UV-vis spectrophotometer at room temperature. X-ray photoelectron spectroscopy (XPS) confirmed high purity of the ZnO nanorods. The hydrogen sensor made of the ZnO nanorods showed reversible response. The hydrogen gas tests were carried out in presence of ambient air and the influence of operation temperature on the hydrogen gas sensing property of ZnO nanorods was also investigated.     

Possible conformational change within the desolvated and cationized sBBI/trypsin non-covalent complex during the collision-induced dissociation process

Electrospray ionization mass spectrometry (ESI-MS) has become an analytical technique widely used for the investigation of non-covalent protein-protein and protein-ligand complexes due to the soft desolvation conditions that preserve the stoichiometry of the interacting partners. Dissociation studies of solvated or desolvated complexes (in the source and in the collision cell, respectively) allow access to information on protein conformation and localization of the metal ions involved in protein structure stabilization and biological activity. The complex of bovine trypsin and small soybean Bowman-Birk inhibitor (sBBI) was studied by ESI-MS to determine changes occurring within the complex during its transfer from droplets to the gas phase independently of the ion polarity. Under collision-induced dissociation (CID) conditions, unexpected binding of the Ca(2+) ion (cofactor of native trypsin) to the inhibitor molecule was observed within the desolvated sBBI/trypsin/Ca(2+) complex (with a 1:1:1 stoichiometry). This formal gas-phase migration of the calcium ion from trypsin to the inhibitor may be related to conformational rearrangements in the solvent-free and likely collapsed complex. However, under conditions leading to the increase in complex charge state, the appearance of the cationized trypsin molecule was detected during complex dissociation, thus reflecting different pathways of the evolution of complex conformation.