Influence of response factors on determining equilibrium association constants of non-covalent complexes by electrospray ionization mass spectrometry

A method for determining the equilibrium association constant of a complexation reaction A + B left harpoon over right harpoon AB by electrospray ionization mass spectrometry is described. The method consists in measuring the relative intensities of the peaks corresponding to A and to AB in equimolar A-B solutions at different concentrations C(0). The results are fitted by a non-linear least-squares procedure, with the two variable parameters being the equilibrium association constant K(a) and a factor R, defined by I(AB)/I(A) = R x [AB]/[A]. The factor R is the ratio between the response factors of AB and A, and corrects for the relative electrospray responses of the complex and the free substrate A, mass discrimination of instrumental origin and/or moderate in-source dissociation. The method is illustrated with the following two systems: complexes between a double-stranded 12-base pair oligonucleotide and minor groove binders, and cyclodextrin complexes with alpha,omega-dicarboxylic acids. For the oligonucleotide complexes, it is found that the response of the complex is not dramatically different to the response of the free oligonucleotide duplex, as the double helix conformation is disturbed by the drug only to a minor extent. In the case of cyclodextrin complexes, these complexes were found to have a much higher response than free cyclodextrin. This may be due to the fact that cyclodextrin is neutral in solution, whereas the complex is charged, but it can also stem from the fact that a significant proportion of the complex is in a non-inclusion geometry. The present method requires the exact determination of the concentrations of the reactants and is applicable to 1 : 1 complexes.     

Multi-turn time-of-flight mass spectrometers with electrostatic sectors

The mass resolution of a time-of-flight (TOF) mass spectrometer is directly proportional to its total flight pathlength. Multi-turn or multi-passage ion optical geometries are necessary to obtain fight distances of sufficient length within reasonable size limitations. We have investigated ion optics for a multi-turn TOF mass spectrometer with electrostatic sectors. The concept of 'perfect' focusing conditions is introduced. Furthermore, a new type of multi-turn TOF mass spectrometer, the MULTUM Linear plus, was developed. It consists of four cylindrical electric sectors and 28 electric quadrupole lenses. It has a vacuum chamber 60 x 70 x 20 cm in size. Mass resolution is demonstrated to increase according to the number of ion cycles. A mass resolution of 350 000 (m/z = 28, FWHM) was achieved after 501.5 cycles. The MULTUM Linear plus analyzer is not simple, however; 28 electric quadrupole lenses are used. In order to reduce the number of ion optical parts, an improved multi-turn TOF mass spectrometer, the MULTUM II, consisting of only four toroidal electric sectors, was also developed. The possibility of tandem mass spectrometric applications using multi-turn TOF mass spectrometers is also discussed.     

Structural investigation of high-valent manganese-salen complexes by UV/Vis,Raman, XANES,and EXAFS spectroscopy

XANES and EXAFS spectroscopic studies at the Mn-K- and Br-K-edge of reaction products of (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride ([(salen)Mn(III)Cl], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) bromide ([(salen)Mn(III)Br], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the Mn(III) complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of approximately 1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as approximately +IV by a comparative study of Mn(III) and Mn(V) reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of IV was deduced from the relative position of the pre-edge 1s-->3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with lambda(max)=648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the Mn(IV) central atom. An oxomanganese(V) or a dimeric manganese(IV) species was not detected.     

Structural Studies of Bis(cyclopentadienyl)molybdenum‐Amino Acid Complexes

The crystal structures of molybdocene‐amino acid compounds of the type [Cp₂Mo^IV(κN, κO‐AA)]⁺Cl^—·xH₂O with AA = D ‐phenylalaninato (x = 1.5), DL ‐leucinato (x = 2) and DL ‐valinato (x = 1) have been determined (Cp = η⁵‐C₅H₅). The compounds feature an almost planar, five‐membered chelate ring of the aminocarboxylate moiety (deprotonated amino acid) with the molybdenum atom. In the phenylalaninato complex π‐stacking between the phenyl rings is found. The complexes were proven kinetically stable at pH < 1 for at least 24 h.     

Photocurrent generation in multilayer self-assembly films fabricated from water-soluble poly(phenylene vinylene)

A novel, water-soluble, cationic PPV derivative poly[(2,5-bis(3-bromotrimethylammoniopropoxy)-phenylene-1,4-divinylene)-alt-1,4-(2,5-bis(2-(2-hydroxyethoxy)ethoxy))phenylene vinylene] (BH-PPV) has been synthesized by a Heck coupling reaction. Multilayered assemblies of the BH-PPV and the sodium salt of hexa(sulfobutyl)fullerenes (C(60)-HS) were fabricated successfully by an alternate deposition technique. The multilayer structures were studied by UV/Vis spectroscopy, small angle X-ray diffraction, and atomic force microscopy. The photoinduced charge transfer property of the self-assembled multilayer film was also measured by a three-electrode cell technique. A steady and rapid cathodic 5.5 microA cm(-2) photocurrent response was measured as the irradiation of the multilayer film was switched on and off. Importantly, the response of on/off cycling is prompt and reproducible. A possible mechanism for the electron-transfer process is proposed.     

Theoretical study and rate constant calculation for the reactions of SH (SD) with Cl2, Br2, and BrCl

The mechanisms of the SH (SD) radicals with Cl2 (R1), Br2 (R2), and BrCl (R3) are investigated theoretically, and the rate constants are calculated using a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6-311G(d,p) and MPW1K/6-311G(d,p) levels. Higher-level energies are obtained at the approximate QCISD(T)/6-311++G(3df, 2pd) level using the MP2 geometries as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MPW1K geometries. Complexes with energies less than those of the reactants or products are located at the entrance or the exit channels of these reactions, which indicate that the reactions may proceed via an indirect mechanism. The enthalpies of formation for the species XSH/XSD (X = Cl and Br) are evaluated using hydrogenation working reactions method. By canonical variational transition-state theory (CVT), the rate constants of SH and SD radicals with Cl2, Br2, and BrCl are calculated over a wide temperature range of 200-2000 K at the a-QCISD(T)/6-311++G(3df, 2pd)//MP2/6-311G(d, p) level. Good agreement between the calculated and experimental rate constants is obtained in the measured temperature range. Our calculations show that for SH (SD) + BrCl reaction bromine abstraction (R3a or R3a') leading to the formation of BrSH (BrSD) + Cl in a barrierless process dominants the reaction with the branching ratios for channels 3a and 3a' of 99% at 298 K, which is quite different from the experimental result of k3a'/k3' = 54 +/- 10%. Negative activation energies are found at the higher level for the SH + Br2 and SH + BrCl (Br-abstraction) reactions; as a result, the rate constants show a slightly negative temperature dependence, which is consistent with the determination in the literature. The kinetic isotope effects for the three reactions are "inverse". The values of kH/kD are 0.88, 0.91, and 0.69 at room temperature, respectively, and they increase as the temperature increases.     

Silica nanoparticles for separation of biologically active amines by capillary electrophoresis with laser-induced native fluorescence detection

This paper describes the analysis of biologically active amines by capillary electrophoresis (CE) in conjunction with laser-induced native fluorescence detection. In order to simultaneously analyze amines and acids as well as to achieve high sensitivity, 10 mM formic acid solutions (pH < 4.0) containing silica nanoparticles (SiNPs) were chosen as the background electrolytes. With increasing SiNP concentration, the migration times for seven analytes decrease as a result of increase in electroosmotic flow (EOF) and decrease in their electrophoretic mobilities against EOF. A small EOF generated at pH 3.0 reveals adsorption of SiNPs on the deactivated capillary wall. The decreases in electrophoretic mobilities with increasing SiNP concentration up to 0.3x indicate the interactions between the analytes and the SiNPs. Having a great sensitivity (the limits of detection at a signal-to-noise ratio (S/N) = 3 of 0.09 nM for tryptamine (TA)), high efficiency, and excellent reproducibility (less than 2.4% of the migration times), this developed method has been applied to the analysis of urinal samples with the concentrations of 0.50 +/- 0.02 microM, 0.49 +/- 0.04 microM, and 74 +/- 2 microM for TA, 5-hydroxytryptamine, and tryptophan, respectively. The successful examples demonstrated in this study open up a possibility of using functional nanoparticles for the separation of different analytes by CE.     

Identification (GC and GC-MS) of unsaturated acetates in Elasmopalpus lignosellus and their biological activity (GC-EAD and EAG)

Two insect colonies of Elasmopalpus lignosellus were reared in our laboratory, the first being initiated from pupae obtained from a cornfield in the region of Sete Lagoas, Minas Gerais and the second from a cornfield in the region of Goiania, Goiás. From the two colonies, two extracts were prepared from the pheromone glands of virgin E. lignosellus females. The extract obtained from the first colony was designated as extract 1 while the extract obtained from the second colony was designated as extract 2. Extract 1 was analyzed by gas chromatography-mass spectrometry (GC-MS) with (Z)-9-hexadecenyl acetate [(Z)-9-HDA] and (Z)-11-hexadecenyl acetate [(Z)-11-HDA] being identified and confirmed by the formation of DMDS derivatives. In addition, a third acetate, which could be either (E)-8-hexadecenyl acetate [(E)-8-HDA] or (E)-9-hexadecenyl acetate [(E)-9-HDA] was detected by GC-MS. Extract 2 was analyzed by gas chromatography (GC) and gas chromatography-electroannetography (GC-EAD) revealing the presence of (Z)-11-HDA and (Z)-9-TDA. In addition, the same compounds elicited a response with the E. lignosellus male antenna obtained from the second insect colony. Electroantennography (EAG) screening with the male E. lignosellus antenna (obtained from the second insect colony) was conducted with the 23 possible tetradecenyl acetates (TDA) and 22 hexadecenyl acetates (HDA) as standards. Out of the 23 TDA isomers evaluated, only (Z)-9-TDA elicited a response and out of the 22 HDA [(Z) and (E) isomers gamma2 to delta13] evaluated only (Z)-11-HDA elicited a response. The acetate compositions of two extracts obtained from insects originating from the two states (Minas Gerais and Goiás) of Brazil were different from one another as well as from that obtained from insects in Tifton, GA, USA. The bioactivity data (GC-EAD) of the extract 2 differed from those reported for the Tifton, GA, USA population. These data suggest polymorphism in relation to the insect populations found in Brazil and in the USA. The possibility of the existence of an E. lignosellus sub-species cannot be ruled out.     

Oxidative addition of the ethane C-C bond to Pd. An ab initio benchmark and DFT validation study

We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol.     

Supramolecular approaches to generate libraries of chelating bidentate ligands for homogeneous catalysis

The process of catalyst discovery and development relying on combinatorial methods has suffered so far from the difficult access to structurally diverse and large libraries of ligands, in particular the structurally more complex class of bidentate ligands. A completely new approach to streamline the difficult ligand synthesis process is to use structurally less complex monodentate ligands that self-assemble in the coordination sphere of a metal center through noncovalent attractive ligand-ligand interactions to generate bidentate, chelating ligands. When complementary attractive ligand-ligand interactions are employed, it is even possible to generate libraries of defined chelate-ligand catalysts by simply mixing two different monomeric ligands. This Minireview summarizes the first approaches and results in this new field of combinatorial homogeneous catalysis.