A counter-intuitive approach to calculating non-exchangeable 2H isotopic composition of hair: treating the molar exchange fraction fE as a process-related rather than compound-specific variable

Hair is a keratinous tissue that incorporates hydrogen from material that an animal consumes but it is metabolically inert following synthesis. The stable hydrogen isotope composition of hair has been used in ecological studies to track migrations of mammals as well as for forensic and archaeological purposes to determine the provenance of human remains or the recent geographic life trajectory of living people. Measurement of the total hydrogen isotopic composition of a hair sample yields a composite value comprised of both metabolically informative, non-exchangeable hydrogen and exchangeable hydrogen, with the latter reflecting ambient or sample preparation conditions. Neither of these attributes is directly measurable, and the non-exchangeable hydrogen composition is obtained by estimation using a commonly applied mathematical expression incorporating sample measurements obtained from two distinct equilibration procedures. This commonly used approach treats the fraction of exchangeable hydrogen as a mixing ratio, with a minimal procedural fractionation factor assumed to be close or equal to 1. Instead, we propose to use full molar ratios to derive an expression for the non-exchangeable hydrogen composition explicitly as a function of both the procedural fractionation factor α and the molar hydrogen exchange fraction f(E). We apply these derivations in a longitudinal study of a hair sample and demonstrate that the molar hydrogen exchange fraction f(E) should, like the procedural fractionation factor α, be treated as a process-dependent parameter, i.e. a reaction-specific constant. This is a counter-intuitive notion given that maximum theoretical values for the molar hydrogen exchange fraction f(E) can be calculated that are arguably protein-type specific and, as such, f(E) could be regarded as a compound-specific constant. We also make some additional suggestions for future approaches to determine the non-exchangeable hydrogen composition of hair and the use of standards.     

Enantioselective alpha-hydroxylation of 2-arylacetic acid derivatives and buspirone catalyzed by engineered cytochrome P450 BM-3

Here we report that an engineered microbial cytochrome P450 BM-3 (CYP102A subfamily) efficiently catalyzes the alpha-hydroxylation of phenylacetic acid esters. This P450 BM-3 variant also produces the authentic human metabolite of buspirone, R-6-hydroxybuspirone, with 99.5% ee.     

Growth and studies of Hg1−xCdxTe based low dimensional structures

The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8×8 k·p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained.Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV.By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg_1−xMn_xTe QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current.Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.     

Determination of the g-factor of n-type HgTe/Hg1−xCdxTe single quantum wells

The effective g-factor of modulation doped n-type HgTe single quantum wells, SQWs, has been determined by the coincidence method in tilted magnetic fields to lie between 15 and 35. For symmetrically doped samples the effective g-factor has been found to be constant for different filling factors; however, for asymmetric SQWs, a large increase with increasing filling factor has been observed. This can be ascribed to a combination of Zeeman spin splitting and Rashba spin–orbit splitting. Reasonable agreement has been achieved between theoretical calculations based on the 8×8 k · p method and experimental results.     

Diatoms as living photonic crystals

We present an analysis of the optical structure of a representative diatom, Coscinodiscus granii. The silica cell wall can be regarded as a photonic crystal slab waveguide with moderate refractive-index contrast. In a cell, at least two different patterns are found: a hexagonal array of pores with a large lattice constant in the valve, and a square array of holes with a small lattice constant in the girdle. It is demonstrated that light can be coupled into the waveguide and that there are some photonic resonances in the visible spectral range, which have been determined by band-structure calculations. PACS 42.70.Qs; 87.17.-d     

Differential magnetoreflection spectroscopy of doped and undoped II–VI semiconductor quantum wells

A method has been developed for recording and analyzing the differential magnetoreflection (magnetotransmission) spectra of semiconductor structures with quantum wells. The method was used to determine the exciton g-factor in semimagnetic CdTe/(Cd, Mn)Te heterostructures with quantum wells. In nonmagnetic structures with quantum wells containing a two-dimensional electron gas, the excitonic damping depends on the spin state of the exciton. This effect is explained by the exchange contribution to exciton-electron scattering. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 44–49 (10 January 1997)     

Shubnikov-de Haas oscillations in p-type inversion layers on n-type silicon

Surface Shubnikov-de Haas oscillations have been measured in p-type channels of (110) silicon field effect transitors between 1.4 and 4.2 K in magnetic fields up to 10 Tesla. Two electric subbands were revealed and the effective masses of the holes in both bands could be determined. For one subband the dependence of the mass on the surface electric field was investigated. At low gate voltages the g-factor of the holes was measured from the spin splitting of the Landau-levels.     

Magnetic field dependence of the valley splitting in n-type inverted silicon mosfet surfaces

The influence of a magnetic field dependence of the valley splitting in n-type inverted (100) silicon MOSFET surfaces is observed for the first time from an abrupt phase shift for the position of the Shubnikov - de Haas extrema in the osciallatory magnetoresistance at constant gate voltages. The results allow a quantitative determination of the valley splitting.     

Surface quantum oscillations in n-type (100) silicon inversion layers on sapphire

Measurements of Shubnikov-de Haas oscillations in (100) Si/($\text{1}$012)Al₂O₃ MOSFET's were performed in magnetic fields up to 10 Tesla for different tilt angles between the magnetic field direction and the surface normal. The experimental results show that the lowest electric subband in this system is twofold degenerate and is formed by the “heavy” cyclotron mass valleys. This can be explained by a large lateral stress present in the SOS (silicon on sapphire) system.