Molecular-level helical stack of a nucleotide-appended oligo(p-phenylenevinylene) directed by supramolecular self-assembly with a complementary oligonucleotide as a template

The nucleotide-appended oligo(p-phenylenevinylene), {bis[2,5-bis(2-methoxyethoxy)-1,4-phenylene]bis(2,1-ethenediyl-1,4-phenylenemethylene)}bis(2'-deoxy-3'-thymidylic acid) (8), has been synthesized, and self-assembly of the single-component 8 and binary self-assembly of 8 with a complementary single-stranded 20-meric oligodeoxyadenylic acid (9) have been examined in aqueous solutions. Atomic force microscopy (AFM), UV-visible (UV-vis), and circular dichroism (CD) measurements revealed that right-handed helical stacks with 6.4- and 5.1-nm diameters self-assemble from the binary components of 8 and 9 as a template depending on the residual stoichiometry of the two components (thymine (T):adenine (A) = 1:1 and T:A = 2:1, respectively). The concentration of 9 was found to strongly influence the CD spectra of 8 in aqueous solutions. Consequently, we concluded that the one side of the thymine moieties in the stacked assemblies of 8 complexes with a single chain of 9. Complementary T-A base pairs thus formed and induced helical stack of the oligo(p-phenylenevinylene)s in the binary self-assembly. In contrast, self-assembly of the single-component 8 and binary self-assembly of 8 with the noncomplementary 20-meric oligothymidylic acid (10) produced no remarkable formation of fibrous structures like helical stacks.     

Two-dimensional 109Ag NMR and random-walk simulation studies of silver dynamics in glassy silver ion conductors

By applying one- and two-dimensional 109Ag NMR, we demonstrate that silver diffusion in silver iodide/silver phosphate glasses is governed by a very broad, continuous distribution of correlation times G(lg tau). As a consequence, over a wide temperature range, the 109Ag NMR spectra can be described by a weighted superposition of a Gaussian and a Lorentzian where these line-shape components result from the slow and the fast silver ions in G(lg tau), respectively. For the 109Ag NMR two-time correlation functions F2(t), measured as a stimulated echo, a very stretched decay to F2SS(t(m)) = 0 is observed. When fitting to a Kohlrausch function, exp[-(t/tau)beta], a stretching parameter beta approximately = 0.2 is found. The temperature dependence of the mean correlation time of silver dynamics is described by an Arrhenius law where the activation energy is consistent with the one reported for the dc conductivity sigma(dc). In addition, it is shown that the effect of complex dynamical processes on NMR multi-time correlation functions can easily be calculated when performing random-walk simulations for schematic models such as the random-barrier model and the random-energy model. Based on these models it is possible to simulate various NMR observables and the mean square displacement, thus revealing the information content of multi-dimensional NMR experiments on solid ion conductors.     

Comparing Y−Ba−Cu−O superconductors by Cu,O and Ba NMR/NQR

This paper discusses various NMR/NQR studies performed by the resonance group at the Physik-Institut on the structurally related compounds YBa₂Cu₃O₇ and YBa₂Cu₄O₈. Employing different isotopes and comparing similar measurements offers the possibility to gain new insights into these compounds. The discussion addresses the following topics: Electric field gradients and their temperature and pressure dependence;T _c enhancement by Ca doping; Cu and O Knight shifts; Ba relaxation. The wide range of applicability of the resonance methods in superconductivity research is demonstrated.     

Octahedral cation partitioning in Mg,Fe2+-olivine. Mössbauer spectroscopic study of synthetic (Mg0.5 Fe2+ 0.5)2SiO4 (Fa50)

The high-temperature partitioning of Fe²⁺ and Mg between the two non-equivalent octahedral M1 and M2 sites in synthetic olivine (Fa₅₀) was studied by Mössbauer spectroscopy. Powder samples have been equilibrated in annealing experiments performed under reducing oxygen fugacity at temperatures between 500 and 800°C followed by rapid quenching in order to prevent redistribution of cations. M-site ordering with Fe²⁺ preferring M1, Mg preferring M2 sites increases continuously with rising equilibrium temperature. K _D values increase from 1.21 at 500°C to 1.48 at 750°C. The results are consistent with both room temperature as well as in situ high temperature single crystal X-ray diffraction experiments of Heinemann et al. [8, 9].     

Triple-quantum dynamics in multiple-spin systems undergoing magic-angle spinning: application to 13C homonuclear correlation spectroscopy

We analyze the multiple-quantum dynamics governed by a new homonuclear recoupling strategy effecting an average dipolar Hamiltonian comprising three-spin triple-quantum operators (e.g., S(p)+S(q)+S(r)+) under magic-angle spinning conditions. Analytical expressions are presented for polarization transfer processes in systems of three and four coupled spins-1/2 subject to triple-quantum filtration (3QF), and high-order multiple-quantum excitation is investigated numerically in moderately large clusters, comprising up to seven spins. This recoupling approach gives highly efficient excitation of triple-quantum coherences: ideally, up to 67% of the initial polarization may be recovered by 3QF in three-spin systems in polycrystalline powders. Two homonuclear 2D correlation strategies are demonstrated experimentally on powders of uniformly 13C-labeled alanine and tyrosine: the first correlates the single-quantum spectrum in the first dimension with the corresponding 3QF spectrum along the other. The second protocol correlates triple-quantum coherences with their corresponding single-quantum coherences within triplets of coupled spins.     

Transfer of pi-conjugated columnar stacks from solution to surfaces

Three hydrogen-bonded oligo(p-phenylenevinylene)s, OPV3, OPV4, and OPV5, that differ in conjugation length have been synthesized and fully characterized. All three compounds contain chiral side chains, long aliphatic chains, and a ureido-s-triazine hydrogen bonding unit. (1)H NMR and photophysical measurements show that the OPV oligomers grow hierarchically in an apolar solvent; initially, dimers are formed by hydrogen bonds that subsequently develop into stacks by pi-pi interactions of the phenylenevinylene backbone with induced helicity via the chiral side chains. SANS measurements show that rigid cylindrical objects are formed. Stacks of OPV4 have a persistence length of 150 nm and a diameter of 6 nm. OPV3 shows rigid columnar domains of 60 nm with a diameter of 5 nm. Temperature and concentration variable measurements show that the stability of the stacks increases with the conjugation length as a result of more favorable pi-pi interactions. The transfer of the single cylinders from solution to a solid support as isolated objects is only possible when specific concentrations and specific solid supports are used as investigated by AFM. At higher concentrations, an intertwined network is formed, while, at low concentration, ill-defined globular objects are observed. Only in the case of inert substrates (graphite and silicium oxide) single fibers are visible. In the case of the repulsive surfaces (mica and glass), clustering of the stacks occurs, while, at attractive surfaces (gold), the stacks are destroyed.     

Lebensdauern und Oszillatorstärken im Sr I- und Ca I-Spektrum

Radiative lifetimes of some highly excited levels in Sr I were measured by zero-field level crossing technique. These levels have been populated using optical excitation starting from the metastable 4d 5s ¹ D ₂ or 5s5p ³ P _2,1,0 states. The high population of these metastable levels necessary for the experiments was obtained by a discharge in the pure Sr vapour burning in the atomic beam oven. The following lifetimes have been determined (in units of 10^?8 sec):τ(5s 6s³ S ₁)=1.09±0.11,τ(5s 5d ³D₁)=1.67±0.10,τ(5s4f¹ F ₃)=3.43±0.28,τ(4d5p ¹ D ₂)=2.19±0.16,τ(5p ^{2 3}P₁)=0.88±0.12,τ(5p ^{2 3} P ₂)=0.78 _?0.10 ^+0.26 . These results are compared with lifetimes derived from oscillator strengths given in the literature, and the reliability of different oscillator strengths tables is discussed. A corresponding discussion is given for radiative lifetimes of some levels in Ca I published previously. Good agreement with data derived from arc emission oscillator strengths has been found. Ca lifetimes are fairly well consistent with oscillator strengths calculated with semiempirical scaled Thomas-Fermi-wave functions.