Comparison between the isocratic and gradient retention behaviour of polypeptides in reversed-phase liquid chromatographic environments.

The isocratic and gradient elution behaviour of beta-endorphin and glucagon, two polypeptides known to exist in amphipathic alpha-helical conformations in lipophilic environments, have been examined under reversed-phase high-performance liquid chromatographic (RP-HPLC) conditions with low pH, aquo-acetonitrile mobile phases. The effects of changes in the volume fraction, psi, of the organic solvent modifier and temperature, T, on the magnitudes of the S and log k(o) values of these two polypeptides, obtained from the plots of logarithmic capacity factor (log k') vs. psi using isocratic elution conditions have been determined. These data have then been compared to the corresponding S and log k(o) values, obtained from the plots of logarithmic median capacity factor (log k) versus the median volume fraction of the organic solvent modifier (psi) derived from the linear gradient elution data, using the same n-butyl silica sorbent and related aquo-acetonitrile mobile phase conditions. As apparent from these studies, substantial differences occur in the temperature-dependent trends and magnitudes of the corresponding S and S values, or the log k(o) and log k(o) values, when these parameters are derived from experimental data acquired by these two different elution methods. Moreover, when gradient elution data for beta-endorphin and glucagon are utilised, the extrapolated values of the intercept and slope of the plots of log k vs. 1/T (corresponding to an apparent change in the median enthalpy of association, deltaH(o)assoc, or an apparent change in the median entropy of association, deltaS(o)assoc) substantially deviated from the values obtained for the thermodynamic parameters, deltaH(o)assoc and deltaS(o)assoc, derived from the log k' vs. 1/T plots using the corresponding isocratic data. These findings thus have important implications for biophysical and thermodynamic investigations when gradient elution data are employed to assess the molecular basis of the interaction of polypeptides with non-polar ligates.     

Reaction of hydrogen gas with C60 at elevated pressure and temperature: hydrogenation and cage fragmentation

Products of the reaction of C(60) with H(2) gas have been monitored by high-resolution atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS), X-ray diffraction, and IR spectroscopy as a function of hydrogenation period. Samples were synthesized at 673 K and 120 bar hydrogen pressure for hydrogenation periods between 300 and 5000 min, resulting in the formation of hydrofullerene mixtures with hydrogen content ranging from 1.6 to 5.3 wt %. Highly reduced C(60)H(x) (x > 36-40) and products of their fragmentation were identified in these samples by APPI FT-ICR MS. A sharp change in structure was observed for samples with at least 5.0 wt % of hydrogen. Low-mass (300-500 Da) hydrogenation products not observed by prior field desorption (FD) FT-ICR MS were detected by APPI FT-ICR MS and their elemental compositions obtained for the first time. Synthetic and analytical fragmentation pathways are discussed.     

Estimating bone conduction transfer functions using otoacoustic emissions

A technique for estimating the nonparametric bone conduction transfer function using distortion product otoacoustic emissions (DPOAEs) is presented. Individual transfer functions were obtained using DPOAEs recorded from a single ear of five normal-hearing adults. Repeatability of the technique was investigated by performing measurements on at least three dates. Functions were reasonably repeatable, and were unique to each individual as expected from subjective measurements. Input force and DPOAE measurements were made for each individual, and a model of the auditory periphery representative of an average person was employed. The technique is objective and requires only passive cooperation, but robust DPOAEs are needed and the measurement time can be onerous for a wide frequency band or fine frequency resolution. With appropriate adjustments to the model of the auditory periphery, the method could be applied with animal models.     

DYNAMIC STRESS ANALYSIS OF GAS TURBINE ROTOR AIRFOILS USING THERMOELASTIC TECHNIQUES

The SPATE system provided a means for experimental ther-moelastic analysis of high frequency modes of vibration of a complicated turbine impeller which did not facilitate analysis with conventional methods due to low stress response and high modal density.
Testing also demonstrated a unique experimental stress analysis technique which was applied to a blade and vane airfoil and exhibited excellent stress distribution correlation with an established experimental technique (brittle lacquer) and with analytical stress distributions predicted by NASTRAN.
Full field dynamic stress analysis of gas turbine compressor airfoils accomplished using thermoelastic techniques provided an efficient method of determining optimum strain gage locations for monitoring rig and engine vibratory stress.
Significant cost savings could be realized using this technique from: (1) reduction in the amount of instrumentation required to characterize airfoil dynamic behavior and (2) increased vibratory response mode coverage thereby preventing the re-running of engine test programs to measure vibratory stress response levels of unexpected resonances encountered during initial engine testing.
Care is required when using SPATE results for vibratory modes resulting in high bi-axial stress distributions since the system output indicated stress magnitudes based on the sum of the principal stresses which could be significantly higher than either of the individual principal stresses.     

A host-guest optical sensor for aliphatic amines based on lipophilic cyclodextrin

A host-guest optical sensor for the determination of aliphatic amines as exemplified by octylamine is proposed. It is based on the reversible fluorescence enhancement of heptakis(2,6-di-O-isobutyl)-β-cyclodextrin(DOB-β-CD) hosting tetraphenylporphyrin (TPP) immobilized in poly(vinyl chloride) (PVC) membrane by aliphatic amine extracted from aqueous phase into membrane phase. The optimum membrane contained 1.15 wt % TPP, 6.15 wt % DOB-β-CD as sensing reagent and other membrane materials. The fluorescence enhancement of the membrane resulted from the formation of a stable three-component complex among DOB-β-CD, TPP, and aliphatic amines. With the optimum conditions described, the fluorescence response of the sensor to octylamine shows a good correlation with the theoretically derived equation in the range 1.0 × 10^–6 to 8.0 × 10^–4 mol/L. The response characteristics including reversibility, response time, reproducibility and lifetime and selectivity of this optical device are also discussed in detail. This sensor has also been applied for the determination of octylamine in water samples containing interferents with satisfactory recovery. Received: 21 November 1999 / Revised: 10 January 2000 / Accepted: 15 January 2000     

Kinetic study of the protonations and substitutions of different cobalt(III)–nta complexes

The behaviour of the Cobalt(III)–nta (nta = nitrilotriacetate) system in an acidic medium was investigated. The acid dissociation constant, pK _a1, of [(nta)(H₂O)Co(-OH)Co(H₂O)(nta)]^– was determined as 3.09(3) and the pK _a of the cis-[Co(nta)(H₂O)₂]/[Co(nta)(H₂O)(OH)]^– equilibrium was determined as 6.71(1). cis-[Co(nta)(H₂O)₂] undergoes ring-opening upon acidification below pH = 2.0. The formation of [Co( ³-nta)(H₂O)₃]⁺ was also studied. The substitutions between cis-[Co(nta)(H₂O)₂] and NCS^– ions were investigated in the pH = 2–7 ranges. [Co(nta) (H₂O)(OH)]^– reacts ca. 70 times faster at 24.7 °C with NCS^– ions than cis-[Co(nta)(H₂O)₂], indicating a cis-labilising effect of the OH^– ligand.     

Synthesis,protonation and substitution behaviour of K3[ReO2(CN)4]

Summary The preparation and characterization of salts of the [ReO₂(CN)₄]^3–, [ReO(OH)(CN)₄]^2–, [ReO(H₂O)(CN)₄]^–, [Re₂O₃(CN)₈]^4– and [ReO(NCS)(CN)₄]^2– species are described. The nature of the protonation reactions of [ReO₂(CN)₄]^3– was established by the successful isolation of these salts.     

Atmospheric pressure photoionization proton transfer for complex organic mixtures investigated by fourier transform ion cyclotron resonance mass spectrometry

To further clarify the role of dopant solvent in proton transfer in atmospheric pressure photoionization (APPI), we employ ultrahigh-resolution FT-ICR mass analysis to identify M(+*), [M + H](+), [M - H](-), and [M + D](+) species in toluene or perdeuterotoluene for an equimolar mixture of five pyrrolic and pyridinic nitrogen heterocyclic model compounds, as well as for a complex organic mixture (Canadian Athabasca bitumen middle distillate). In the petroleum sample, the protons in the [M + H](+) species originate primarily from other components of the mixture itself, rather than from the toluene dopant. In contrast to electrospray ionization, in which basic (e.g., pyridinic) species protonate to form [M + H](+) positive ions and acidic (e.g., pyrrolic) species deprotonate to form [M - H](-) negative ions, APPI generates ions from both basic and acidic species in a single positive-ion mass spectrum. Ultrahigh-resolution mass analysis (in this work, m/Deltam(50%) = 500,000, in which Deltam(50%) is the mass spectral peak full width at half-maximum peak height) is needed to distinguish various close mass doublets: (13)C versus (12)CH (4.5 mDa), (13)CH versus (12)CD (2.9 mDa), and H(2) versus D (1.5 mDa).