Rapid separation of phenylthiohydantoin amino acids: ambient pressure ion-mobility mass spectrometry (IMMS)

An electrospray ionization (ESI) ambient pressure ion-mobility spectrometer (APIMS) interfaced to an orthogonal reflector time-of-flight mass spectrometer (TOFMS) was evaluated for the first time as a detector for the identification of phenylthiohydantoin (PTH)-derivatized amino acids, the final products in the Edman sequencing process of peptides and proteins. The drift and flight times of the twenty common PTH amino acids were characterized by a well-defined 2-D mobility/mass spectral pattern. The combination of mobility/mass modes of analysis gave rise to a unique trend-line formation for the series of PTH amino acids. In addition, each PTH amino acid had a unique reduced mobility constant K(o), thus enabling the differentiation of all the amino acid derivatives including the PTH-leucine and PTH-isoleucine isomers. More importantly it was shown that it was possible to resolve a complete reference mixture of PTH amino acids in a single experimental run in less than 1 min. Detection limits for the PTH amino acids were found to range from 1.04 to 3.52 ng; indicating that the limits of detection were less than 17.0 pmol for all of the PTH amino acids.     

Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals

Five bis(tert-butylnitroxide) diradicals connected by a silole (7 a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a "Bleaney-Bowers" model that gives values of J = -142.0 cm(-1) for 7 a, -1.8 cm(-1) for 7 b, -1.3 cm(-1) for 7 c, -4.2 cm(-1) for 7 d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with DeltaE(T-S) values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the DeltaE(T-S) and J values (DeltaE(T-S) = -2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a "Karplus-Conroy"-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.     

Study of multiple binding constants of dexamethasone with human serum albumin by capillary electrophoresis–frontal analysis and multivariate regression

Studies into the interactions between drugs and human serum albumin (HSA) are extremely important for drug discovery, since HSA behaves as a carrier for external drugs and internal biological molecules. In this paper, to evaluate the pharmacokinetic and pharmacodynamic properties of dexamethasone (DXM), the interaction between DXM and HSA was studied by capillary electrophoresis–frontal analysis (CE-FA). According to the Klotz equation, four binding sites between DXM and HSA were obtained, and the average binding constant was 1.05 × 10³ M⁻¹. Furthermore, according to multiple equilibrium theory, based on the assumption that there are two types of binding site, the binding constant at one site was calculated to be 3.539 × 10³ M⁻¹, and the average of the other three was 1.234 × 10³ M⁻¹. In addition, to obtain the detailed binding information at each binding site, new equations were deduced by multivariate regression. The four binding constants of DXM and HSA were calculated to be 5.558 × 10¹ M⁻¹, 2.158 × 10⁴ M⁻¹, 7.312 × 10³ M⁻¹ and 2.043 × 10³ M⁻¹, respectively, which is helpful for detailed studies into the interactions between drugs and proteins with multiple binding sites. Figure Electropherograms of DXM sodium phosphate and HAS mixtures for different protein to drug concentration ratios, obtained by CE-FA     

Single-drop microextraction and gas chromatography–mass spectrometry for the determination of volatile aldehydes in fresh cucumbers

Headspace single-drop microextraction (HS-SDME) was used as a rapid and reliable method for the isolation and preconcentration of volatile aldehydes from fresh cucumbers. The utility of this methodology is demonstrated in the determination of (E)-2-nonenal and (E,Z)-2,6-nonadienal. The limit of detection, linearity and repeatability have been determined for 2,6-nonadienal and (E)-2-nonenal. Limits of detection for nonenal and nonadienal were 0.05 and 0.04 mg kg ⁻¹, respectively. The repeatability of extraction was obtained with the RSD values lower than 13%. Concentrations of target aldehydes in fresh cucumbers obtained by means of the HS-SDME method were in the range 9.4–12.5 (nonadienal) and 2.6–3.8 mg kg ⁻¹ (nonenal). The results of the single-drop extraction in combination with gas chromatography show promising potential for the analysis of volatile aldehydes in vegetables. Presented at the 11th International Conference on Chemistry and the Environment, 9-12 September 2007, Torun, Poland.     

Pressure-induced internal redox reaction of Cs2[PdI4].I2, Cs2[PdBr4].I2, and Cs2[PdCl4].I2

The pressure-induced redox reaction within the system Cs2[Pd2+I4].I2/Cs2[Pd4+I6] was investigated by means of powder X-ray diffraction. Analogous high-pressure X-ray diffraction experiments were performed on the isostructural compounds Cs2[PdX4].I2 (X = Cl, Br). Additionally, the phase transition of Cs2[PdBr4].I2 to Cs2[PdBr4I2] was characterized by means of Raman scattering experiments as well as theoretical calculations based on density functional theory. On the basis of experimentally determined crystal structure data, a pathway for the topology of the redox reactions was developed and outlined.     

Differentiation of Regioisomeric Aromatic Ketocarboxylic Acids by Positive Mode Atmospheric Pressure Chemical Ionization Collision-Activated Dissociation Tandem Mass Spectrometry in a Linear Quadrupole Ion Trap Mass Spectrometer

Positive-mode atmospheric pressure chemical ionization tandem mass spectrometry (APCI-MS ⁿ ) was tested for the differentiation of regioisomeric aromatic ketocarboxylic acids. Each analyte forms exclusively an abundant protonated molecule upon ionization via positive-mode APCI in a commercial linear quadrupole ion trap (LQIT) mass spectrometer. Energy-resolved collision-activated dissociation (CAD) experiments carried out on the protonated analytes revealed fragmentation patterns that varied based on the location of the functional groups. Unambiguous differentiation between the regioisomers was achieved in each case by observing different fragmentation patterns, different relative abundances of ion-molecule reaction products, or different relative abundances of fragment ions formed at different collision energies. The mechanisms of some of the reactions were examined by H/D exchange reactions and molecular orbital calculations.     

Observation of heterogeneous gene products by FT-ICR MS

The ability of FT-ICR MS to resolve isotopic variants of intact proteins for each of the charge states formed by electrospray ionization offers a sensitive, rapid method for detecting "low mass" heterogeneity, where this is defined as the presence of structural variants differing in mass by 2 Da or less. Such heterogeneity may reflect biological or chemical modifications of structure or may result from the coexpression of related proteins from a multi-gene family. In the analytical approach described here, comparisons are made between observed isotopic distributions and those expected for predicted protein sequences. Close agreement is demonstrated for a homogeneous model protein, and the utility of the method has been evaluated in the study of mouse major urinary proteins (MUPs), a group of closely related sequences. Divergence of the experimental isotopic distribution from distributions predicted for known MUP sequences can be explained, in quantitative terms, by the coexpression of closely related sequences. This approach provides a facile method for the assessment of protein homogeneity and for the detection of structural variants, without recourse to proteolytic digestion and analysis of the resulting products.     

Packing density of slurry-packed capillaries at low aspect ratios

The average interparticle voidage or porosity (epsilon(inter)) in cylindrical capillaries is studied in dependence on the column diameter (d(c)) to particle diameter (d(p)) ratio for 5 < d(c)/d(p) < 50. Using optimized slurry and packing solvents, high pressure and ultrasonication, 5 mum-sized porous C18-silica particles were slurry-packed into fused-silica capillaries having ids from 30 to 250 mum. Packing densities are assessed by a polystyrene standard which is size-excluded from the intraparticle pore space of the packings. For d(c)/d(p) > 35 densely packed beds are realized (epsilon(inter) = 0.36-0.37), while for decreasing aspect ratios an exponential increase in epsilon(inter )is observed reaching epsilon(inter ) approximately 0.47 at d(c)/d(p) = 5. This behaviour is ascribed to a combination of the geometrical wall effect operating in the direct vicinity of the column wall, caused by the inability of the particles to form a dense packing against the hard surface of the column wall, and particle characteristics like the size distribution, shape and surface roughness. Results are compared with the literature data to address also the importance of absolute particle size in studying structure-transport relations in packed beds in dependence on the aspect ratio d(c)/d(p).     

Effects of methyl groups on the geometry and conformational equilibrium of 1,3-dioxanes

The geometries and energies of polymethyl-1,3-dioxanes were studied by molecular mechanics calculations. Buttressing effects of the Me groups are discussed. The chair/twist conformational equilibrium of 1,3-dioxanes having two syn-axial Me groups in the chair were calculated, and a twist form (the 1,4-twist) is found to be more stable than the chair only for 9 and 14, chair and 2,5-twist form are of comparable energies for 10 and 13, and the chair is considerably favored in 11 and 12. The chair/1,4-twist energy difference of 1 was calculated to be only 16.4 kJ mol^?1. Ring inversion of 1 goes through a transition state with C-C-C-O coplanar with a calculated activation enthalpy of 28.3 kJ mol^?1.