Geometrical spin dephasing in quantum dots

We study spin-orbit mediated relaxation and dephasing of electron spins in quantum dots. We show that higher order contributions provide a relaxation mechanism that dominates for low magnetic fields and is of geometrical origin. In the low-field limit relaxation is dominated by coupling to electron-hole excitations and possibly 1/f noise rather than phonons.     

Mass spectra of dihydroxy stereoisomers of 3-methoxy-1,3,5(10)-estratrienes

Electron impact mass spectral data for each of the four isomeric 16,17-, 15,17- and 14,17-diols of 3-methoxy-1,3,5(10)-estratriene and the 15,17-diols of 3-methoxy-14β-1,3,5(10)-estratriene are reported. The mass spectra of the diols show very similar fragmentation patterns except for differences in the relative abundances of particular ions. The different [M ? H₂O]⁺˙/[M] ⁺˙ and [M ? 2H₂O] ⁺˙ [M] ⁺˙ ratios can be used for distinguishing between the four isomeric 3-methoxy-1,3,5(10)-estratriene-14,17-diols as well as between the four isomeric 3-methoxy-14β-1,3,5(10)-estratriene-15,17-diols. No significant differences could be detected in the spectra of the epimeric 16,17-and 15,17-diols of 3-methoxy-1,3,5(10)-estratriene.     

Some Comments on Fulminic Acid (Knallsäure) and a Praise to Heinrich Wieland and his Work on Organic Derivatives of Nitrogen

The structure, vibrational spectrum, the polymerization and [3+2] cycoaddition reactions of fulminic acid are discussed. In the mass spectra of the solid deposit from gaseous fulminic acid (prepared from sodium fulminate and sulfuric acid) the mass of the adduct of HCNO with hydrogen cyanide was detected, which may be formed by Huisgen 1,3‐dipolar cycloaddition. As examples for Heinrich Wieland's experimental mastership the polymerization and his syntheses of fulminic acid and of explosive dinitrosomethanide [HC(NO)₂]^– are emphasized; the structure of the latter has recently been disclosed by X‐ray structure determination.     

Synthese und Kristallstrukturen von Chlororhenaten(III) mit den zweiwertigen Kationen Ethylendiammonium und Piperazinium:(EnH2)2(Pipzh2)[Re3Cl12]2· 6H2O,und (PipzH2) [Re3Cl12]CI·H2O und (Pipzh2) [Re3Cl11(H2O)]·3H2O

Synthesis and Crystal Structures of Chlororhenates(III) with the Divalent Cations Ethylenediammonium and Piperazinium: (EnH₂)₂(PipzH₂) [Re₃Cl₁₂]₂·6H₂O, (EnH₂) (PipzH₂) [Re₃Cl₁₂]Cl· H₂O, and (PipzH₂) [Re₃Cl₁₁(H₂O)] · 3H₂O The deep red salt (EnH₂)₂(PipzH₂)[Re₃CI₁₂] · 6 H₂O ( 1 ), (EnH₂)(PipzH₂)[Re₃Cl₁₂]CI · H₂O ( 2 ), and (PipzH₂)[Re₃Cl₁₁(H₂O)] · 3H₂O ( 3 ) crystallize upon evaporation from hydrochloride acid solutions of ReCl₃ on addition of ethylenediammonium chloride (EnH₂Cl₂) and/or piperazinium chloride (PipzH₂Cl₂). The crystal structures have been determined from four-circle diffractometer data. 1: monoclinic; a = 1889.63(11), b = 1615.82(8), c = 790.28(4)pm; β = 101.354(5)°; Z = 2; P2₁/n; R = 0.119, R_w = 0.070. 2: triclinic; a = 1330.35(4), b = 1051.14(5), c = 1165.32(6)pm; α = 122.308(4), β = 102.412(3), γ = 92.226(4)°; Z = 2, P1 ; R = 0.092, R_w = 0.059. 3: orthorhombic; a = 971.43(4), b = 1619.51(7), c = 1478.87(6)pm; Z = 4; Pbcm; R = 0.034, R_w = 0.032.     

NMR studies of double proton transfer in hydrogen bonded cyclic N,N'-diarylformamidine dimers: conformational control, kinetic HH/HD/DD isotope effects and tunneling

Using dynamic NMR spectroscopy, the kinetics of the degenerate double proton transfer in cyclic dimers of polycrystalline (15)N,(15)N'-di-(4-bromophenyl)-formamidine (DBrFA) have been studied including the kinetic HH/HD/DD isotope effects in a wide temperature range. This transfer is controlled by intermolecular interactions, which in turn are controlled by the molecular conformation and hence the molecular structure. At low temperatures, rate constants were determined by line shape analysis of (15)N NMR spectra obtained using cross-polarization (CP) and magic angle spinning (MAS). At higher temperatures, in the microsecond time scale, rate constants and kinetic isotope effects were obtained by a combination of longitudinal (15)N and (2)H relaxation measurements. (15)N CPMAS line shape analysis was also employed to study the non-degenerate double proton transfer of polycrystalline (15)N,(15)N'-diphenyl-formamidine (DPFA). The kinetic results are in excellent agreement with the kinetics of DPFA and (15)N,(15)N'-di-(4-fluorophenyl)-formamidine (DFFA) studied previously for solutions in tetrahydrofuran. Two large HH/HD and HD/DD isotope effects are observed in the whole temperature range which indicates a concerted double proton transfer mechanism in the domain of the reaction energy surface. The Arrhenius curves are non-linear indicating a tunneling mechanism. Arrhenius curve simulations were performed using the Bell-Limbach tunneling model. The role of the phenyl group conformation and hydrogen bond compression on the barrier of the proton transfer is discussed.     

A chemical approach to the pharmaceutical optimization of an anti-HIV protein

Chemical protein synthesis is important for dissecting the molecular basis of protein function. Here we advance its scope by demonstrating the significant improvement of the multifaceted pharmaceutical profile of small proteins exclusively via a chemical-based approach. The focus of this work centered on CCL-5 (RANTES) derivatives with potent anti-HIV activity. The overall chemical strategy involved a combination of coded and noncoded amino acid mutagenesis, peptide backbone engineering, and site-specific polymer attachment. The ability to alter specific protein residues, as well as precise control of the position and type of polymer attachment, allows for the exploration of specific molecular designs and resulted in novel CCL-5 analogues with significant differences in their respective biochemical and pharmaceutical properties. Using this approach, the complex-interplay of variables contributing to the noncovalent self-association (aggregation) state, CCR-5 specificity, in vivo elimination half-life, and anti-HIV activity of CCL-5-based protein analogues could be empirically evaluated via total chemical synthesis. This work has led to the identification of potent (sub-nanomolar) anti-HIV proteins with significantly improved pharmaceutical profiles, and illustrates the increasing value of protein chemical synthesis in contemporary therapeutic discovery. These antiviral molecules provide a novel mechanism of action for the development of a new generation of anti-HIV therapeutics which are still desperately needed.     

NMR provides checklist of generic properties for atomic-scale models of periodic mesoporous silicas

MCM-41 and SBA-15 silicas were studied by (29)Si solid-state NMR and (15)N NMR in the presence of (15)N-pyridine with the aim to formulate generic structural parameters that may be used as a checklist for atomic-scale structural models of this class of ordered mesoporous materials. High-quality MCM-41 silica constitutes quasi-ideal arrays of uniform-size pores with thin pore walls, while SBA-15 silica has thicker pore walls with framework and surface defects. The numbers of silanol (Q(3)) and silicate (Q(4)) groups were found to be in the ratio of about 1:3 for MCM-41 and about 1:4 for our SBA-15 materials. Combined with the earlier finding that the density of surface silanol groups is about three per nm(2) in MCM-41 (Shenderovich, et al. J. Phys. Chem. B 2003, 107, 11924) this allows us to discriminate between different atomic-scale models of these materials. Neither tridymite nor edingtonite meet both of these requirements. On the basis of the hexagonal pore shape model, the experimental Q(3):Q(4) ratio yields a wall thickness of about 0.95 nm for MCM-41 silica, corresponding to the width of ca. four silica tetrahedra. The arrangement of Q(3) groups at the silica surfaces was analyzed using postsynthesis surface functionalization. It was found that the number of covalent bonds to the surface formed by the functional reagents is affected by the surface morphology. It is concluded that for high-quality MCM-41 silicas the distance between neighboring surface silanol groups is greater than 0.5 nm. As a result, di- and tripodical reagents like (CH(3))(2)Si(OH)(2) and CH(3)Si(OH)(3) can form only one covalent bond to the surface. The residual hydroxyl groups of surface-bonded functional reagents either remain free or interact with other reagent molecules. Accordingly, the number of surface silanol groups at a given MCM-41 or SBA-15 silica may not decrease but increase after treatment with CH(3)Si(OH)(3) reagent. On the other hand, nearly all surface silanol groups could be functionalized when HN(Si(CH(3))(3))(2) was used.