Comparison of the positive and negative ion electrospray mass spectra of some small peptides containing pyroglutamate

The positive ion electrospray mass spectra of [M+H](+) and the negative ion electrospray mass spectra of [M-H](-) ions of selected pyroglutamate containing peptides both provide sequencing data. The negative ion spectra show the normal alpha and beta backbone cleavages in addition to delta and gamma backbone cleavages initiated by the side chains of Glu and Phe residues. For example, the [M-H](-) ion of pGlu Pro Gln Val Phe Val-NH(2) shows delta and gamma peaks at m/z 224 (delta, Gln3), 244 (gamma, Phe4), 451 (delta, Phe4), 471 (gamma, Gln3). Some of the negative ion spectra show unusual grandaughter peaks that originate by alpha and beta, or delta and gamma backbone cleavages of a beta(1) cleavage ion.     

Simultaneous assay of serum vitamin A and vitamin E by high performance liquid chromatography using time-switched UV and fluorimetric detectors

An HPLC method utilizing a UV and a fluorimetric detector linked in series is described. By use of a simple integrator-controlled time-switched relay, analysis of serum vitamin A and E is accomplished on the same chromatogram and at optimum sensitivity for each detector. A single internal standard (retinyl acetate) monitored only by the UV detector permits measurement of both vitamins over a wide linear range. Precision of the assays is satisfactory, both on a within-day and on a day-to-day basis. Recoveries of both vitamins are virtually 100% whilst sensitivity is 2 μg/L (retinol) and 0.05 mg/L (α-tocopherol).     

Small cobaltacarborane clusters in synthesis. Peralkylation, perhalogenation, and macrocycle construction

Methods are described for achieving stepwise replacement of all BH terminal hydrogens in CoC₂B₄ clusters with halogen or alkyl groups, leading to complexes such as Cp*Co(2,3-Et₂C₂B₄X₄) (X=Me or I) and Cp*Co(2,3-Et₂C₂B₄IMe₂[O₃SCF₃]). Building on earlier work on B-functionalization of small metallacarboranes via metal-catalyzed cross-coupling, two- and three-dimensional macrocycles containing four to six metal centers have been constructed systematically from monomeric precursor complexes.     

CONTINUOUS, NONINVASIVE MEASUREMENT OF In Vivo pH IN CONSCIOUS MICE

Extracellular pH was measured continuously and noninvasively in hairless mice using a transcutaneous spectronuorometric technique and a fluorescent pH probe, bis -carboxyethyl carboxyfluorescein (BCECF). The acid form of BCECF was injected intravenously. An optical fiber system excited the florescent probe at 450 and 500 nm alternately, using one branch of a bifurcated, fiber optic bundle. The other branch of the bundle collected the emitted fluorescence which was measured by a photomultiplier tube. The ratio of fluorescence intensities from the two excitation wavelengths was pH dependent. Measurements could be obtained for approximately 60 min following a single injection. Mice exposed to elevated partial pressures of CO₂ demonstrated changes in the fluorescence ratio indicative of a dramatic decrease in extracellular pH. If the exposure was brief, the fluorescence signal recovered within 20 min. The fluorescence intensity ratio was calibrated against aqueous BCECF samples in vitro. Fluorescence pH values determined using the in vitro calibration were compared with measurements made on blood samples taken from seven mice. Extracellular pH in hairless mice was found to be approximately 7.5, within the expected physiological range. The variability of the pH signal derived from the fluorescence signal was approximately 0.05 units.     

Angular aspects of exchange correlation and the fermi hole

The complete (nonreduced) αα probability density functions evaluated from the Hartree–Fock and simple Hartree product wavefunctions have been used to elucidate the angular features of spin correlation and the Fermi hole in the 2³S state of helium and the ground state of beryllium. This approach shows that the local Fermi holes in these two cases are very similar and that the Fermi hole is essentially spherically symmetric when the reference electron is close to the nucleus. As the reference electron is removed to larger radial distances, appreciable polarization of the Fermi hole is observed. The polarization is greater in the direction of the nucleus than away from the nucleus, contrary to the situation in the Coulomb hole of the helium ground state where the polarization is greater away from the nucleus than toward the nucleus. Several other differences between the He 2³S Fermi hole and the He 1¹S Coulomb hole are noted.     

Characterisation of sulphoxides by atmospheric pressure ionisation mass spectrometry

An observation that a series of proprietary compounds containing a methyl thiophenyl group all underwent metabolic S-oxidation, and that the product ion spectra of the resulting S-oxides showed methyl radical loss under low-energy atmospheric pressure ionisation tandem mass spectrometry (API-MS/MS) conditions, has led to an investigation of the fragmentation of commercially available sulphoxides. The phenyl methyl sulphoxides studied do lose methyl radicals under MS/MS conditions on triple quadrupole mass spectrometers. In addition, the phenyl sulphoxides, with simple substituents other than a methyl group, also showed a tendency to lose the substituent as a radical. It was concluded that radical loss from these simple sulphoxides was characteristic of S-oxidation of these molecules. Radical losses, such as those reported here, are used in-house to distinguish S-oxidation from N- and C-oxidation in metabolism studies.     

A novel system for purge-and-trap with thermal desorption: Optimization using tomato juice volatile compounds

A system for purge-and -trap with thermal desorption was developed and optimized to moniotor aroma compounds at ambient temperatures. Canned tomato juice volatiles were used as a model system to develop and evaluate the method. Volatile components were first adsorbed on insert-traps packed with Tenax-TA polymer, then thermally desorbed directly inside a gas chromatograph injector. Volatile matgerials occuring in Very low amounts could be entrained and subsequently chrfomatographed, with sensitivity limited by the purity of the sweep gas. Quantitative measurement of tomato juice volatiles was linear with sample size upn to 100 gram samples. The amount of trapped volatiles was proportional to trapping time; howver, low-and intermediate-boilers broke through the trap after one hour while high-boilers continued to be retained. Apurge gas flow rate of 20ml/min gave optimum results mediate-biolers. Optimum recovery of volatile compounds was obtained with a desorption temperature of 200°c for 5 min. Coefficients of variation from triplicate runs were relatively small. The method showed promise for a simple, sensitive, and reproducibel flavor volatiles collection system for the accurate analysis of tomato compounds.     

Scaling in the S and P states of the helium isoelectronic sequence

Mean values of r ₁ ^r and r _r ¹² for the ground and several excited states of the helium isoelectronic sequence are used to demonstrate that a simple scaling which superimposes the distribution function f(r ₁₂) as a function of the atomic number leads to a similar result for the electron density distribution D(r₁). On the basis of a screening interpretation of the scaling parameter , it is concluded that screening is greater in the singlet than the triplet state of a particular configuration, that screening is greater in the P states than the corresponding S states, and that the screening approaches the limiting value of 1 for the highly excited states. The perturbation expansions of Scherr and Knight are used to determine the limiting value of when Z and the relationship between the scaling parameter and the scale factor, chosen so that a trial wave function satisfies the virial theorem, is discussed. A brief discussion of the scaling of the Coulomb hole is presented.     

Optical-fibre sensing of fluoride ions in a flow-stream

The Alizarin Fluorine Blue method for the determination of fluoride has been adapted for use with optical fibres. The reagent was immobilized on a polymer matrix, Amberlite XAD-2, and the reflectance of this reagent phase was measured as a function of fluoride concentration by use of a flow-cell assembly and a bifurcated fibre-optic system. A linear response was obtained for 0.16-0.95mM fluoride at a pH of 4.1, with a response time of approximately 12 min.     

A Non-Born–Oppenheimer Self-consistent Field Method

This communication outlines the development of a novel method for the ab initio computation of molecular systems wherein the Born–Oppenheimer approximation is not invoked. In the current method, a common Hamiltonian is employed to operate on the electrons and nuclei simultaneously. In addition, an inseparable wavefunction is generated to describe the non-Born–Oppenheimer behaviour. The physical implications of the new method are discussed.