Electrospray ionization mass spectrometry and partial least squares discriminant analysis applied to the quality control of olive oil

Direct infusion electrospray ionization mass spectrometry in the positive ion mode [ESI(+)‐MS] is used to obtain fingerprints of aqueous–methanolic extracts of two types of olive oils, extra virgin (EV) and ordinary (OR), as well as of samples of EV olive oil adulterated by the addition of OR olive oil and other edible oils: corn (CO), sunflower (SF), soybean (SO) and canola (CA). The MS data is treated by the partial least squares discriminant analysis (PLS‐DA) protocol aiming at discriminating the above‐mentioned classes formed by the genuine olive oils, EV (1) and OR (2), as well as the EV adulterated samples, i.e. EV/SO (3), EV/CO (4), EV/SF (5), EV/CA (6) and EV/OR (7). The PLS‐DA model employed is built with 190 and 70 samples for the training and test sets, respectively. For all classes (1–7), EV and OR olive oils as well as the adulterated samples (in a proportion varying from 0.5 to 20.0% w/w) are properly classified. The developed methodology required no ions identification and demonstrated to be fast, as each measurement lasted about 3 min including the extraction step and MS analysis, and reliable, because high sensitivities (rate of true positives) and specificities (rate of true negatives) were achieved. Finally, it can be envisaged that this approach has potential to be applied in quality control of EV olive oils. Copyright © 2013 John Wiley & Sons, Ltd.     

Enzymatic synthesis of medium chain monoglycerides in a solvent-free system

The synthesis of monocaprin, monolaurin, and monomyristin in a solvent-free system was conducted by mixing a commercial immobilized lipase with the organic reactants (glycerol and fatty acids) in a 20-mL batch reactor with constant stirring. The effects of temperature, fatty acid/glycerol molar ratio, and enzyme concentration on the reaction conversion were determined. The addition of molecular sieves in the assays of monomyristin synthesis was also evaluated. The reactions were carried out for 5 to 6 h and the nonpolar phase was analyzed by gas chromatography. The best results in terms of selectivity and conversion (defined as the percentage of fatty acid consumed) were achieved when the stoichiometric amount of reagents (molar ratio=1) and 9% (w/w) commercial enzyme were used and the reaction was performed at 60°C. The addition of molecular sieves did not improve the synthesis of monomyristin. Conversions as high as 80%, with monoglycerides being the major products, were attained. After 5 h of reaction, the concentration of monoglyceride was about twice that of diglyceride, and only trace amounts of triglyceride were found. The results illustrate the technical possibility of producing medium chain monoglycerides in a solvent-free medium using a simple batch reactor.     

13C NMR determination of the microstructure of polypropylene obtained with the DADNi(NCS)2/methylaluminoxane catalyst system

A complete ¹³C NMR characterization of a polymer synthesized with a new Ni‐diimine complex [DADNi(NCS)₂, where DAD = 2,6‐iPr? C₆H₃? N?C(Me)? C(Me)? N? 2,6‐iPr? C₆H₃] activated by methylaluminoxane by homopolymerization of propylene is presented. The amorphous material was made up mainly of blocks of syndiotactic polypropylene and ethylene–propylene copolymer. Some degree of propylene inversion (<1.2 mol %) and of long isobutyl and 2‐methyl hexyl branching (<1 mol %) were assigned and quantified. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2171–2178, 2004     

Homochiral oligopeptides generated by induced "mirror symmetry breaking" lattice-controlled polymerizations in racemic crystals of phenylalanine N-carboxyanhydride

The formation of diastereoisomeric libraries of oligopeptides through the heterogeneous polymerization of racemic crystals of phenylalanine N-carboxyanhydride (PheNCA) is reported. The diastereoisomeric compositions of the oligopeptides formed on polymerization of (R,S) crystals incorporating the deuterium-tagged S enantiomer were determined by MALDI-TOF mass spectrometry. The racemic mixtures of the oligopeptides longer than pentamers are represented primarily by diastereoisomers of homochiral sequence and with peptides containing only one heterochiral repeating unit. A mechanism comprising the following three sequential steps to account for this unusual observation is proposed: 1) formation of dimers and trimers at a partially damaged liquid/solid interface, 2) chain propagation that takes place within the bulk of the crystal through a lattice-controlled "zipper-like" mechanism between homochiral molecules arranged in a head-to-tail motif to yield crystalline antiparallel beta-sheets of alternating oligopeptide chains of homochiral sequence of opposite handedness, and 3) enantiomeric cross-inhibition that results in chain termination. Induced desymmetrization of the racemic mixtures of the formed peptides was achieved by the polymerization of the mixed quasi-racemic crystals of (R)-PheNCA, ((S)-PheNCA), and (S)-ThieNCA (3-(2-thienyl)-alanine N-carboxyanhydride) of various compositions. These experiments resulted in the formation of nonracemic libraries of oligopeptides composed of homochiral chains of (R)-Phe and copolymers of randomly distributed (S)-Phe and (S)-Thie sequences. From these findings, we propose a stochastic model for the generation of libraries of nonracemic mixtures of oligopeptides from the polymerization of host (R,S)-PheNCA with racemic mixtures of other guest NCA amino acids dissolved in limited quantities in the crystal.     

Lipid composition-dependent incorporation of multiple membrane proteins into liposomes

Membrane proteins from bacteria Pasteurella multocida were used as a model for studying its incorporation into liposomes. An important step to achieve efficient high yield protein incorporation in proteoliposomes is the study of the more suitable lipid composition. To this end, we compared the amount of total protein, reconstituted by co-solubilization methods, into liposomes of phospholipids with different polar head groups and acyl chain lengths. The liposomes and proteoliposomes were characterised by isopycnic centrifugation in sucrose gradient and by dynamic light scattering. Experimental and theoretical results were compared considering the effects exerted through the hydrocarbon chain length, volume, and optimal cross-sectional area of the phospholipid (combined in the geometrical critical packing parameter, lipid–protein matching), critical spontaneous radius of curvature of the bilayer vesicle, phase transition temperature of the lipid and ratio of lipid–protein molecules present in the vesicles. The highest incorporation of multiple proteins was found with dipalmitoylphosphatidylcholine (DPPC), reaching a yield of 93% compared to the lower relative amounts incorporated in proteoliposomes of the other lipids. The incorporation of multiple proteins induces a proportional enhancement of vesicular dimension, since DPPC–proteoliposomes have an average diameter of 1850 Å, compared to the 1430 Å for pure DPPC vesicles.     

Luminescence lifetime distributions analysis in heterogeneous systems by the use of Excel's Solver

A detailed study of the luminescence decay curves of pyrene included within p-tert-butylcalix[4]arene cavities and benzophenone into silicalite channels is reported. A new methodology for a lifetime distribution analysis of the decay curve of probes onto heterogeneous surfaces is presented, which allows for asymmetric distributions and uses Voigt profiles (Gaussian and Lorentzian mixture) instead of pure Gaussian or Lorentzian distributions. Our approach uses a very simple and widely available tool for fitting, the Microsoft Excel Solver. In the case of the pyrene/tert-butylcalix[4]arene sample, the room temperature luminescence detected in the microsecond time scale was not only the phosphorescence of pyrene but also monomer delayed fluorescence, crystal phosphorescence, and excimer delayed fluorescence. In the benzophenone/silicalite case, three emissive forms of benzophenone could be assigned, one of benzophenone included into the silicalite circular zigzag channels, another for emplacement into the elliptical straight channels and finally when benzophenone is placed at the crossing points of those silicalite channels, where smaller spatial restrictions for benzophenone exist.     

119Sn-Mössbauer spectroscopic studies on organoheterobimetallic half sandwich ruthenium–tin containing derivatives

A series of heterobimetallic complexes containing Ru–Sn bonds, of general formula [RuCp(L)₂SnX₂Y] [L = PPh₃, 1/2 dppe = 1,2-bis(diphenylphosphino)ethane; X, Y = F, Cl and Br], was prepared and studied by elemental analysis, ¹¹⁹Sn-Mössbauer spectroscopy, i.r. and ¹H-, ¹³C-, ¹⁹F-, ³¹P- and ¹¹⁹Sn-n.m.r. spectroscopy. ¹¹⁹Sn-Mössbauer studies allowed determination of the coordination number of the Sn^II center as well as the group electronegativity of the organometallic fragment [RuCp(L)₂]⁺. These results, supported by multinuclear n.m.r. data suggested that the electronic charge distribution between the Ru^II and Sn^II centers are strongly dependant upon the nature of L and the electronegativity of X and Y.     

Non-linear correlations between formal potential and Hammett parameters of substituted iron phthalocyanines and catalytic activity for the electro-oxidation of hydrazine

The activity of the different iron phthalocyanines was examined using the complexes adsorbed on graphite electrodes. The effect of the Fe(II)/(I) formal potential of iron phthalocyanines on the their catalytic activity for the electro-oxidation of hydrazine was investigated. A plot of log k (rate constant at constant potential) versus the Fe(II)/(I) formal potential gives a volcano curve. The rate of the reaction increases with the driving force of the catalyst (measured as its formal potential) and then decreases for higher driving forces. A similar graph is obtained with a plot of log k versus the sum of the Hammett parameters of the substituents on the periphery of the phthalocyanine ligand. A maximum activity is obtained for a complexes having an M(II)/(I) redox potential close to –0.6 V which agrees with previous studies conducted with phthalocyanines of different metals and with cobalt phthalocyanines bearing different substituents.Dedicated to Prof. Wolf Vielstich on the occasion of his 80th birthday in recognition of his numerous contributions to interfacial electrochemistry.