Determination of the gas-phase acidities of cysteine-polyalanine peptides using the extended kinetic method

We determined the gas-phase acidities of two cysteine-polyalanine peptides, HSCA3 and HSCA4, using a triple-quadrupole mass spectrometer through application of the extended kinetic method with full entropy analysis. Five halogenated carboxylic acids were used as the reference acids. The negatively charged proton-bound dimers of the deprotonated peptides with the conjugate bases of the reference acids were generated by electrospray ionization. Collision-induced dissociation (CID) experiments were carried out at three collision energies. The enthalpies of deprotonation (Delta(acid)H) of the peptides were derived according to the linear relationship between the logarithms of the CID product ion branching ratios and the differences of the gas-phase acidities. The values were determined to be Delta(acid)H(HSCA3) = 317.3 +/- 2.4 kcal/mol and Delta(acid)H (HSCA4) = 316.2 +/- 3.9 kcal/mol. Large entropy effects (Delta(DeltaS) = 13-16 cal/mol K) were observed for these systems. Combining the enthalpies of deprotonation with the entropy term yielded the apparent gas-phase acidities (Delta(acid)G(app)) of 322.1 +/- 2.4 kcal/mol (HSCA3) and 320.1 +/- 3.9 kcal/mol (HSCA4), in agreement with the results obtained from the CID-bracketing experiments. Compared with that in the isolated cysteine residue, the thiol group in HSCA3,4 has a stronger gas-phase acidity by about 20 kcal/mol. This increased acidity is likely due to the stabilization of the negatively charged thiolate group through internal solvation.     

Synthesis,characterization and DNA binding studies of ruthenium(II) complexes: [Ru(bpy)<Subscript>2</Subscript>(dtmi)]<Superscript>2+</Superscript> and [Ru(bpy)<Subscript>2</Subscript>(dtni)]<Superscript>2+</Superscript>

Two new ruthenium(II) polypyridyl complexes (1) (dtmi = 3-(pyrazin-2-yl)-as-triazino[5,6-f]-5-methoxylisatin) and (2) (dtni = 3-(pyrazin-2-yl)-as-triazino[5,6-f]-5-nitroisatin) have been synthesized and characterized by elemental analysis, FAB-MS, ES-MS and ¹H-n.m.r. The DNA-binding patterns of complexes were investigated by spectroscopic titration, viscosity measurements and thermal denaturation. The results indicate that the complexes (1) and (2) interact with calf thymus DNA (CT-DNA) by intercalative mode. Due to the withdrawing electronic substitutent in the intercalative ligand, ptni, the DNA-binding affinity of the complexes (2) is larger than that complex (1) does.     

New strategies to study the chemical nature of wine oligomeric procyanidins

Tannins represent a key element in red wine flavors, so researchers have made a lot of effort to try to understand the role of their structure in wine taste in recent decades. We report some new routes to achieve a true structure–taste relationship for the major tannins found in wine, which are procyanidins in their monomeric or oligomeric state. All these routes use synthetic standards. New advances in their synthesis and their analyses using chromatographic methods, NMR spectroscopy, and mass spectrometry to obtain more precise information about their chemical structure, including their stereochemistry and their precise degree of polymerization and galloylation, are described.     

A new sesquiterpenoid from <Emphasis Type="Italic">Mallotus apelta</Emphasis>

A new sesquiterpenoid, malloapelin D (1), and a known schizandriside (2) were isolated from the roots of Mallotus apelta. Their structures were elucidated on the basis of spectroscopic methods including IR, MS, and 1D and 2D NMR.     

Thermophysical properties and material modelling of acrylic bone cements used in vertebroplasty

The stabilization of osteoporotic vertebrae with acrylic bone cement, called vertebroplasty, is a common procedure in modern surgery. However, the thermomechanical-chemically coupled material behaviour of curing bone cements makes the application even for experienced surgeons difficult and can lead to potential complications like heat necrosis, leaking bone cement, embolisms and postoperative load shifting. In order to reduce these potential complications, to minimize the risks and to better understand the occurring effects, the thermophysical properties of a commercial acrylic bone cement were investigated in detail using differential scanning calorimetry, volumetric dilatometry and temperature controlled rheometry. More specifically, the reaction kinetics, the specific heat, the thermal conductivity, the thermal expansion, the chemical shrinkage as well as the mechanical behaviour was studied during the reaction process of the bone cement. Furthermore, the explored material behaviour is described by a customized material model that takes into account all observed effects. With the aid of this model the inhomogeneous chemical, thermal and mechanical states that appear during the application and curing of acrylic bone cements, can be studied by finite element treatment.     

Altered Mascot search results by changing the <Emphasis Type="Italic">m</Emphasis>/<Emphasis Type="Italic">z</Emphasis> range of MS/MS spectra: analysis and potential applications

The Mascot search algorithm is one of the most commonly used tools for protein identification. Tandem mass spectrometry data searched against a protein sequence database is utilized for identifying peptides and proteins, each reported with a score. Higher Mascot scores are associated with lower chances of random hits. The process of peak selection performed by the search engine prior to the search is a critical aspect of the process. Here, we show that Mascot divides the MS/MS spectrum into fixed m/z regions for peak selection, starting at the lowest m/z value of the peak list. Therefore, modifying the m/z range of the peak lists by insertion of a dummy peak with low m/z value changes the ensemble of peaks used for searching. As a consequence, Mascot peptide scores and search results are altered significantly and a different subset of the peptides present in the sample is identified after processing. We further show that the effect can be exploited and additional proteins and peptides can be identified by repeating the search with a combined set of differently processed files, even when applying identical false-positive rates.     

Adsorption of arsenic (V) from a water solution onto a surfactant-modified zeolite

In this study a surfactant-modified zeolite (SMZ) was prepared by adsorbing the cationic surfactant hexadecyltrimethylammonium (HDTMA) bromide on a clinoptilolite. The adsorption of the surfactant modified the surface properties of the clinoptilolite and enhanced the anionic capacity of the SMZ. The adsorption equilibrium data of As(V) from the water solution on the SMZ were obtained in a batch adsorber, and the Langmuir isotherm matched the data reasonably well. The As(V) adsorption capacity of the SMZ was 12.5 times greater than that of the clinoptilolite. The adsorption of As(V) on SMZ was mainly due to the interactions between the anionic sites of the SMZ and the As(V) anions in water solution. The adsorption capacity of the SMZ was dependent on the solution pH. The adsorption capacity was increased and decreased by augmenting the pH from 5 to 7 and from 7 to 12, respectively. This unusual behavior was due to the fact that the affinity of the As(V) for the SMZ was dependent on the As(V) species that were present in solution. The adsorption capacity of the SMZ was slightly favored by decreasing the temperature from 25 to 15 °C. The heat of adsorption was estimated to be ΔH _ads=−46.82 KJ/mol, indicating that the adsorption was exothermic and the As(V) was chemisorbed on the SMZ.     

Different Behaviors of Glutathione in Aqueous and DMSO Solutions: Molecular Dynamics Simulation and NMR Experimental Study

An all-atom molecular simulation and NMR experiments have been carried out to investigate the interactions and conformations of glutathione (GSH) in aqueous and DMSO solutions. The simulations started, from different initial conformations, are characterized by intramolecular distance, radius of gyration, root-mean-square deviation, and solvent-accessible surface. Interestingly, different behaviors are found in the two different solutions. GSH is highly flexible in an aqueous solution with transitions to the extended, semifolded, and folded states. However, once GSH reaches the folded state in DMSO, it remains there and becomes difficult to break down. The NMR results show agreement with the MD simulations. The water molecule is small. It is also a good proton donor and a good proton acceptor. Water molecules can easily break down the “folded” conformation. In DMSO solution, the stronger hydrogen bonds and the hydrophobic interactions are more important, which can make the GSH in the folded state stable. Variations in the distribution of conformations and the hydrogen-bonding network may play an important role in its function under physiological conditions.     

In defense of adiabatic ϕ/ψ mapping for cellobiose and other disaccharides

Thorough conformational study of cellobiose requires consideration of numerous arrangements of the exocyclic groups. Therefore, it is customary to prepare a number of structures with different arrangements of hydroxyl and hydroxymethyl groups. These “starting geometries” are then given different values of the glycosidic linkage torsion angles ϕ and ψ. At each increment of ϕ and ψ, the energy is calculated. Usually, the final product is an “adiabatic” contour plot of the lowest energy at each ϕ/ψ point after considering all of the starting geometries. The present paper advocates for adiabatic maps despite the statement by Schnupf and Momany (preceding paper) that adiabatic maps are not of interest because they contain sparse details about the structures at each minimum. Similar information is computed by their method and adiabatic mapping, and comparable details can be provided from adiabatic studies. Although Schnupf and Momany presented maps from calculations in vacuum and in water that considered all of their calculated energies, they favored the presentation of two to four maps for each of 36 individual minima, each with its own zero of relative energy. However, previous work showed that more structures are needed to provide the lowest energies at each point in ϕ/ψ space. Following their preferred strategy would result in even more maps when the added structures are considered. The need to map individual minima can be avoided by starting calculations with the same exocyclic orientations at each ϕ/ψ point instead of using the preceding optimized structure to start the next energy minimization. Using the same orientations at each point allows periodic maps that depict barriers between minima.     

Evaluation of adiabatic runaway reaction of methyl ethyl ketone peroxide by DSC and VSP2

Methyl ethyl ketone peroxide (MEKPO) is generally applied to manufacturing in the polymerization processes. Due to thermal instability and high exothermic behaviors of MEKPO, if any operation is undertaken recklessly or some environmental effect is produced suddenly during the processes, fires and explosions may inevitably occur. In this study, thermal analysis was evaluated for MEKPO by differential scanning calorimetry (DSC) test. Vent sizing package 2 (VSP2) was used to analyze the thermal hazard of MEKPO under various stirring rates in a batch reactor. Thermokinetic and safety parameters, including exothermic onset temperature (T ₀), maximum temperature (T _max), maximum pressure (P _max), self-heating rate (dT dt ⁻¹), pressure rise rate (dP dt ⁻¹), and so on, were discovered to identify the safe handling situation. The stirring rates of reactor were confirmed to affect runaway and thermal hazard characteristics in the batch reactor. If the stirring rate was out of control, it could soon cause a thermal hazard in the reactor.