Volume contents

Volume Contents

Volume contents     

Vitamin E chemistry. Studies into initial oxidation intermediates of alpha-tocopherol: disproving the involvement of 5a-C-centered "chromanol methide" radicals

Contrary to concepts handed down in the literature from the early days of vitamin E research, one-electron oxidation of vitamin E does not involve 5a-C-centered radicals. A combined approach of analytical techniques, in particular electron paramagnetic resonance spectroscopy (EPR), organic synthesis of special derivatives, isotopic labeling, kinetic studies, and computational chemistry was used to re-evaluate the one-electron and two-electron oxidation chemistry of alpha-tocopherol (alpha-toc). EPR in combination with 5a-13C-labeled compounds provided no indication of the involvement of 5a-C-centered radicals. Oxidation of special tocopherol derivatives were used to disprove the occurrence of 5a-C-centered one-electron intermediates. Additionally it was shown that those vitamin E reactions that were commonly evoked to plead for the involvement of C-centered tocopheryl radicals actually proceeded via heterolytic, i.e., non-radical, intermediates. The results will help to clear widely spread misunderstandings about the chemistry of vitamin E and will have mechanistic implications for the synthesis of tocopherol-based supramolecular structures and 5a-substituted alpha-tocopherol derivatives.     

On segregation during crystal growth from a solution zone

The distribution in a mixed crystal A_1−xB_xC grown from an either (A, B)-rich or C-rich solution zone (e.g. travelling heater method or related techniques) is to be described by Pfann's zone melting function which depends on the segregation coefficient and a characteristic length. Using pseudobinary mole fractions x the characteristic length of the distribution function is not the actual length of the liquid solution zone but an effective zone length is to be regarded which may be considerably smaller or much bigger for C-rich or (A, B)-rich solutions zones, respectively.     

Holonomic quantum computation in decoherence-free subspaces

We show how to realize, by means of non-Abelian quantum holonomies, a set of universal quantum gates acting on decoherence-free subspaces and subsystems. In this manner we bring together the quantum coherence stabilization virtues of decoherence-free subspaces and the fault tolerance of all-geometric holonomic control. We discuss the implementation of this scheme in the context of quantum information processing using trapped ions and quantum dots.     

Etch studies on GaPO4 single crystals

Etch figures on gallium orthophosphate (GaPO₄) crystals are presented for the {0 0 1}, {0 1 0}, , and {1 0 1} faces. Primarily they are used in detecting possibly occurring domains of twinning. Generally, the etch figures of GaPO₄ show similarities as well as differences compared to those of low-quartz modifications of SiO₂ and AlPO₄.     

Synthesis of alpha-trifluoromethylated nitrogen heterocycles

The syntheses of various alpha-trifluoromethylated nitrogen heterocycles have been achieved from readily available alpha-(trifluoromethyl)homoallylamine through a ring-closing metathesis.     

Analysis of the morphology of hectorite by use of small-angle X-ray scattering

An analysis of the three-dimensional correlation function of small-angle scattering is applied for the direct determination of stereological parameters of hectorite samples. Beside characteristic lengths and volume fractions the specific order distances are given. The samples can be described by homogeneous particles of different order magnitudes up to the size of the secondary particles, which were estimated to have maximum dimensions of about 450 nm. Beside traditional stereological formulas, including the interpretation of the derivatives of the small-angle correlation function, the so-called transformed correlation function is applied in order to detect distinctive characteristic lengths. Received: 2 December 1999 Accepted: 9 March 2000     

Novel tocopheryl compounds XXV: synthesis and comparison of the para-quinones of all four homologous tocopherol model compounds and their 3,4-dehydro derivatives

Four tocopherol model compounds, the chroman-6-ols (1-4) having the typical substitution pattern of α-, β-, γ-, and δ-tocopherol (vitamin E), were oxidized to the corresponding para-quinones (5-8), and dehydrogenated to the 2H-chromen-6-ols (17-20) involving initial acetyl protection of the phenolic OH and deprotection as the last step. The chromenols were also converted into the para-quinones (21-24), which existed in the bicyclic hemiketal form, in contrast to the chromanol-derived, monocyclic quinones 5-8, the ketalization behavior agreeing well with computations on the DFT level.     

A graded Gersten-Witt complex for schemes with a dualizing complex and the Chow group

We construct for any scheme X with a dualizing complex I_• a Gersten-Witt complex and show that the differential of this complex respects the filtration by the powers of the fundamental ideal. To prove this we introduce second residue maps for one-dimensional local domains which have a dualizing complex. This residue maps generalize the classical second residue morphisms for discrete valuation rings. For the cohomology of the quotient complexes of this filtration we prove , where μ_I is the codimension function of the dualizing complex I_• and denotes the Chow group of μ_I-codimension p-cycles modulo rational equivalence.