Prediction of the effects of imperfect construction of a QMS filter

Modelling techniques have previously predicted the observed behaviour of perfectly manufactured QMS mass filters. These methods are extended to examination of the behaviour of imperfect QMS filters; this examination considers the behaviour of QMS filters with one rod displaced radially inward as this is a simple manufacturing defect that arises when a rod does not fit correctly into the mounting.The results demonstrate the well known, but poorly documented property, that exchanging the connections of a poorly performing QMS device sometimes improves performance. This is shown to arise because only a displacement of a y-rod produces a large effect. The results also show that displacement of a y-rod may produce a spurious additional peak known as a precursor. More detailed investigation suggests that precursors are not separate peaks but are formed because a section of the main peak is removed causing an apparent double peak.Finally results confirm that adjustment of the voltage applied to a displaced rod can be used to significantly improve the QMS behaviour. A small change by a fraction alpha in the position of a single rod may be compensated by a change of 2 x alpha in the voltage applied to that rod.     

On the reality of residual entropies of glasses and disordered crystals

We show that the hypothesis that the configurational entropy of a liquid disappears when it is kinetically frozen into a single glass state implies a spontaneous decrease of entropy. We show further that this is not an innocuous exception to the second law that requires a slight modification of that law, but rather implies directly the possibility of an uncompensated conversion of heat to work. We also note that the number of microstates visited in the course of a measurement does not determine the entropy, but rather that this number is always an inconceivably small fraction of the accessible microstates. Various experimental and computational tests of the possible vanishing of configurational entropy on kinetic arrest (e.g., from studies of glass vapor pressure and solubilities, and the coexistence curve between a disordered crystal and the liquid), with the exception of one questionable case, are consistent with the view that it does not vanish. We then show that this result is actually required by the second law. These considerations apply equally to the residual entropy of disordered crystals.     

Concentration dependence of the structure of aqueous solutions of lutetium nitrate according to X-ray diffraction

Aqueous solutions of lutetium nitrate over a wide range of concentrations are studied by X-ray diffraction under standard temperature and pressure. The low-angle peaks in experimental scattering intensity curves are interpreted. It is shown that the structure of these solutions can be of two types. It is found that a saturated solution and solutions concentrated to a molar ratio of 1: 20 have a quasi-crystalline structure resulting from interionic interactions. It is determined that dilute solutions form a water-like structure characterized by a tetrahedral network of hydrogen bonds between the water molecules. It is found that low-angle peaks also appear in the intensity curves of dilute solutions; this proves that the so-called “long-range” order is preserved in these solutions. It is revealed that in all the studied systems, the contributions to the total scattering pattern that are responsible for the occurrence of pre-peaks are intercationic interactions.     

Dependence of the Rate of Electrodeposition of Metals at Mono- and Multielectrodes on Voltage in the Case of an Exaltation Mechanism of Mass Transport

Dependences of the rate of copper deposition on mono- and multielectrodes out of dilute solutions of acetic acid at voltages of 20–250 V are compared. It is established that the same rate of deposition in the case of a multielectrode (25 electrodes with a diameter of 0.1 mm) takes place at a voltage that is lower by 2.7 times than that in the case of a monoelectrode with a diameter of 0.5 mm and the same area. The same results are obtained during electrodeposition of cadmium and thallium. In order to explain this effect, a two-layered model of potential distribution is suggested. In accordance with this model, the resistance of electrolyte in the vicinity of a multielectrode is equal to the sum of the resistance of the layer nearest to the electrode (where it is determined by the sum of conductances to each microelectrode) and the external layer, where the potential distribution is close to hemispherical. It is shown that calculations in accordance with this model allow one to give an estimate of the effect's scale that agrees with experiment and to predict its possible limits following an increase in the number of electrodes that make up a multielectrode.     

Effect of netropsin on one-electron oxidation of duplex DNA

The effect of netropsin on the oxidative reactions of duplex DNA was examined. One-electron oxidation of DNA creates a radical cation that migrates through duplex DNA and reacts primarily at GG steps. Netropsin is a dication that specifically binds primarily by hydrogen bonding in the minor groove at sites that have four or more contiguous A.T base pairs. We showed that the oxidation potential of netropsin is less than that of any of the four nucleobases. We find that netropsin quenches the oxidative reactions of DNA independent of whether it is specifically bound. Within the Perrin model of static quenching, a netropsin within a rather large fixed volume around the DNA is an effective quencher.     

Numerical simulation of the effect of solvent viscosity on the motions of a beta-peptide heptamer

This report examines the effect of a decrease in solvent viscosity on the simulated folding behaviour of a beta-peptide heptamer in methanol. Simulations of the molecular dynamics of the heptamer H-beta3-HVal-beta3-HAla-beta3-HLeu-(S,S)-beta3-HAla(alphaMe)-beta3-HVal-beta3-HAla-beta3-HLeu-OH in methanol, with an explicit representation of the methanol molecules, were performed for 80 ns at various solvent viscosities. The simulations indicate that at a solvent viscosity of one third of that of methanol, only the dynamic aspects of the folding process are altered, and that the rate of folding is increased. At a viscosity of one tenth of that of methanol, insufficient statistics are obtained within the 80 ns period. We suggest that 80 ns is an insufficient time to reach conformational equilibrium at very low viscosity because the dependence of the folding rate of a beta-peptide on solvent viscosity has two regimes; a result that was observed in another computational study for alpha-peptides.     

Kinetics and mechanism of the polymerization of styrene–acrylonitrile in the presence of polybutadiene

The mechanism of free radical grafting of styrene–acrylonitrile with polybutadiene was studied. By assuming a copolymerization mechanism an equation that related the amount of homopolymer formed and its molecular weight to a reactivity ratio and the charged ratio of monomer to polydiene was derived. Polymerizations that contained a variety of ratios of monomer to polybutadiene, two different catalysts, and variable amounts of a mercaptan modifier were studied. The weight fraction of homopolymer and its molecular weight were determined by high-speed GPC. The results were analyzed by the new equation and all showed a constant value of the reactivity ratio, which strongly suggests that the mechanism of the grafting reaction is copolymerization. Evidence that suggests that none of the grafting is a result of a hydrogen abstraction mechanism is also presented.     

Features of polytherms of the viscosity of Fe–B melts

It is established that the formation of a viscous film on the surface of a melt is possible during the viscosimetry of Fe–B melts under normal conditions. It is noted that the film has a considerable effect on the results from measurements and can lead to the appearance of additional features on polyterms. Thermodynamic analysis of the composition of surface layers shows that a viscous film contains mainly boron oxide B₂O₃. It is concluded that the evaporation of B₂O₃ from the surface of the alloy produces additional anomalies on the polytherm of viscosity.     

Aminolyses of aryl diphenylphosphinates and diphenylphosphinothioates: effect of modification of electrophilic center from P=O to P=S

A kinetic study is reported for aminolysis of aryl diphenylphosphinothioates (2a-i). The phosphinothioates 2a-i are less reactive than aryl diphenylphosphinates (1a-i), the oxygen analogues of 2a-i, regardless of the basicity of the leaving aryloxides or the attacking amines. The Yukawa-Tsuno plot for the reactions of 2b-i with piperidine exhibits good linearity with a small r value (r=0.28), indicating that the leaving group departs at the rate-determining step with a small degree of bond fission. Reactions of 2,4-dinitrophenyl diphenylphosphinothioate (2a) with alicyclic secondary amines result in a good linear Br?nsted-type plot with betanuc=0.52, implying that the reactions proceed through a concerted mechanism. The betanuc value determined for the reactions of 2a is slightly larger than that reported for the corresponding reactions of 2,4-dinitrophenyl diphenylphosphinate (1a, i.e., betanuc=0.38), suggesting that reactions of 2a proceed through a tighter transition state (TS) than that of 1a. The reaction of 2a with piperidine exhibits a ca. 0.4 kcal/mol more favorable enthalpy of activation (DeltaH) than that of 1a. On the contrary, the entropy of activation at 25.0 degrees C (TDeltaS) is ca. 1.5 kcal/mol more unfavorable for the reaction of 2a than for that of 1a. This result supports the proposal that the reaction of 2a proceeds through a tighter TS than that of 1a and explains why 2a-i are less reactive than 1a-i.     

Use of pressure drop profiles to assess the accuracy of Total Pore Blocking measurements of the external porosity of chromatographic columns

By comparing the pressure drop in a column where the meso-pores of the particles have been blocked using the Total Pore Blocking (TPB) method to measure the interstitial volume of the column with that in the same column when the particle meso-pores are fully open, it could be demonstrated in a very sensitive way that the interstitial volume is completely devoid of any significant amount of remaining pore blocking agent in the final phase of a TPB experiment. Monitoring the pressure signal until it returns to its original value can hence be used as a reliable indicator that all blocking agent has been removed from the interstitial void at the end of the flushing period. As a consequence, any small molecular weight dead volume marker that is employed in this phase can explore the full interstitial volume, so that the value of the latter can be measured without being underestimated by the fact that some fractions of the interstitial void would still be occupied by the blocking agent.     

The effect of plasticizer on the ageing of Metolose films

The film-forming material, Metolose, was studied by positron annihilation spectroscopy. It is known that above a given concentration the plasticizer (PEG) forms a separate phase. The controlled ageing of the films showed that there is a difference between the ageing processes of the monophase and that of the separated phase films. The ageing consists of a fast and a slow step in both cases. Our PAS measurements showed that this slow process is hindered in the phase-separated samples.     

Amplification of chirality of the majority-rules type in helical supramolecular polymers: the impact of the presence of achiral monomers

We present a theoretical treatment describing the conformational state of helical supramolecular polymers that consist of three types of monomer: right-handed and left-handed chiral monomers and achiral ones. We find that chirality amplification of the majority-rules type, that is, a disproportionately large shift in the helix screw sense due to a small enantiomeric excess, can occur in these polymers. The strength of the chirality amplification depends on the free-energy penalty of a helix reversal along the self-assembled chain and on that of a mismatch between the conformation of a bond and the preferred conformation of the preceding monomer. It turns out that the impact of achiral monomers also depends on these two parameters. For high values of these free energies, the net helicity does not change much from the situation where no achiral material is present. However, if the free-energy penalties are not both large, the impact of the achiral monomers on the conformational state of the aggregates can be quite substantial.     

Enhanced shape-selective recognition of anion guests through complexation-induced organization of porphyrin hosts

We present a fortuitous discovery of enhanced shape-selective recognition of anion guests that stems from a complexation-induced conformational change in porphyrin hosts upon anion binding. Porphyrin hosts reported here exist in a conformation that is not favorable to guest binding. Anions that bind strongly are those that can induce a conformational change in the host to allow guest binding. Furthermore, guests that mimic the shape of the newly formed pocket bind the strongest.     

Antioxidant effects of photodegradation product of nifedipine

Recently, increasing evidence suggests that the antihypertensive drug nifedipine acts as a protective agent for endothelial cells, and that the activity is unrelated to its calcium channel blocking. Nifedipine is unstable under light and reportedly decomposes to a stable nitrosonifedipine (NO-NIF). NO-NIF has no antihypertensive effect, and it has been recognized as a contaminant of nifedipine. The present study for the first time demonstrated that NO-NIF changed to a NO-NIF radical in a time-dependent manner when it interacted with human umbilical vein endothelial cells (HUVECs). The electron paramagnetic resonance (EPR) signal of NO-NIF radicals in HUVECs showed an asymmetric pattern suggesting that the radicals were located in the membrane. The NO-NIF radicals had radical scavenging activity for 1,1-diphenyl-2-picrylhydrazyl, whereas neither NO-NIF nor nifedipine did. In addition, the NO-NIF radical more effectively quenched lipid peroxides than NO-NIF or nifedipine. Furthermore, NO-NIF attenuated the superoxide-derived free radicals in HUVECs stimulated with LY83583, and suppressed iron-nitrilotriacetic acid (Fe-NTA)-induced cytotoxicity in rat pheochromocytoma (PC12) cells. Our findings suggest that NO-NIF is a candidate for a new class of antioxidative drugs that protect cells against oxidative stress.     

Topography of the free-energy landscape probed via mechanical unfolding of proteins

Single-molecule experiments in which proteins are unfolded by applying mechanical stretching forces generally force unfolding to proceed along a reaction coordinate that is different from that in chemical or thermal denaturation. Here we simulate the mechanical unfolding and refolding of a minimalist off-lattice model of the protein ubiquitin to explore in detail the slice of the multidimensional free-energy landscape that is accessible via mechanical pulling experiments. We find that while the free-energy profile along typical "chemical" reaction coordinates may exhibit two minima, corresponding to the native and denatured states, the free energy G(z) is typically a monotonic function of the mechanical coordinate z equal to the protein extension. Application of a stretching force along z tilts the free-energy landscape resulting in a bistable (or multistable) free energy G(z)-fz probed in mechanical unfolding experiments. We construct a two-dimensional free-energy surface as a function of both chemical and mechanical reaction coordinates and examine the coupling between the two. We further study the refolding trajectories after the protein has been prestretched by a large force, as well as the mechanical unfolding trajectories in the presence of a large stretching force. We demonstrate that the stretching forces required to destabilize the native state thermodynamically are larger than those expected on the basis of previous experimental estimates of G(z). This finding is consistent with the recent experimental studies, indicating that proteins may refold even in the presence of a substantial stretching force. Finally, we show that for certain temperatures the free energy of a polyprotein chain consisting of multiple domains is a linear function of the chain extension. We propose that the recently observed "slow phase" in the refolding of proteins under mechanical tension may be viewed as downhill diffusion in such a linear potential.     

Emergence of solution-phase homochirality via crystal engineering of amino acids

The evolution of homochirality from a prebiotic environment has long intrigued scientists. Here we report how highly enantioenriched solutions may be produced by manipulation of amino acid phase behavior, a concept that has far-reaching implications for prebiotic chemistry. We demonstrate that the eutectic composition of aqueous mixtures of L and D amino acids may be tuned by the addition of achiral dicarboxylic acids that cocrystallize with chiral amino acids. We find that, in several cases, these systems yield new eutectic compositions of 98% ee or higher. This work suggests a forerunner of modern crystal engineering that provides a general and facile mechanism for the evolution of homochirality as well as a conceptual advance for the separation of enantiomers of molecules forming racemic compounds.     

New aspects of the photo-oxidation and photo-stabilization of polymers

The mechanism of the photo-oxidation of an ethylene-propylene random copolymer (EPM) has been compared with that of a styrene-butadiene-styrene block copolymer (SBS). The former is considered as a model for polyolefins that undergo chain scission on irradiation and the latter as a model for unsaturated rubbers that become cross-linked. A hindered amine light stabilizer (HALS) that is a better inhibitor of the absorption of oxygen in SBS than in EPM is, however, less efficient to prevent formation of carbonyl groups and change of the mechanical properties. These results are discussed and an explanation is proposed.     

Dynamics and stability of dispersions of polyelectrolyte-filled multilayer microcapsules

The authors report dynamic and coagulation properties of a dispersion of polyelectrolyte multilayer microcapsules filled with solutions of a strong polyelectrolyte. Microcapsules are shown to take a charge of the sign of encapsulated polyions and are characterized by a nonuniform distribution of inner polyions, which indicates a semipermeability of the shell and a leakage of counterions. The capsule self-diffusion coefficient in the vicinity of the similarly charged wall is measured using a particle tracking procedure from confocal images of the dispersion. The diffusion of capsules in the force field suggests that the effective interaction potential contains an electrostatic barrier, so that we deal with the same types of interaction forces as for solid particles. The theoretical estimates of the authors show that when microcapsules are in close proximity, their interaction should even be quantitatively the same as that of colloids with the same surface potential. However, due to the mobility of inner polyions they might repel stronger at large distances. The authors thus conclude that the encapsulation of charged polymers is an important factor in determining the adhesion and interaction properties of multilayer microcapsules.     

Vibrational spectroscopic study of the contents of a chest excavated from the wreck of the HMS Pandora

The FT-IR and Raman spectroscopic analysis of a red powder found in a chest from an officer's cabin during the excavation of the wreck of the 18th Century frigate HMS Pandora have confirmed that the pigment is cinnabar, mercury(II) sulphide. Weaker signals in the Raman spectrum are assignable to a proteinaceous material, such as collagen, typical of a degraded vellum or parchment. Comparison of the Raman spectra with that of a pigmented seal from a 1786 Lieutenant's commission demonstrated that the beeswax component of the seal was not observable.