Radiation-induced radicals in glucose-1-phosphate. I. Electron paramagnetic resonance and electron nuclear double resonance analysis of in situ X-irradiated single crystals at 77 K

Electron magnetic resonance analysis of radiation-induced defects in dipotassium glucose-1-phosphate dihydrate single crystals in situ X-irradiated and measured at 77 K shows that at least seven different carbon-centered radical species are trapped. Four of these (R1-R4) can be fully or partly characterized in terms of proton hyperfine coupling tensors. The dominant radical (R2) is identified as a C1-centered species, assumedly formed by a scission of the sugar-phosphate junction and the concerted formation of a carbonyl group at the neighboring C2 carbon. This structure is chemically identical to a radical recently identified in irradiated sucrose single crystals. Radical species R1 and R4 most likely are C3- and C6-centered species, respectively, both formed by a net hydrogen abstraction. R3 is suggested to be chemically similar to but geometrically different from R4. Knowledge of the identity of the sugar radicals present at 77 K provides a first step in elucidating the formation mechanism of the phosphoryl radicals previously detected after X-irradiation at 280 K. In paper II, the chemical identity, precise conformation, and possible formation mechanisms of these radical species are investigated by means of DFT calculations and elementary insight into the radiation chemistry of sugar and sugar derivatives is obtained.     

Radiation-induced radicals in glucose-1-phosphate. II. DFT analysis of structures and possible formation mechanisms

Four radiation-induced carbon-centered radicals in dipotassium glucose-1-phosphate dihydrate single crystals are examined with DFT methods, consistently relying on a periodic computational scheme. Starting from a set of plausible radical models, EPR hyperfine coupling tensors are calculated for optimized structures and compared with data obtained from EPR/ENDOR measurements, which are described in part I of this work. In this way, an independent structural identification is made of all the radicals that were observed in the experiments (R1-R4) and tentative reaction schemes are proposed. Also, the first strong evidence for conformational freedom in sugar radicals is established: two species are found to have the same chemical composition but different conformations and consequently different hyperfine coupling tensors. Analysis of the calculated energies for all model compounds suggests that the radiation chemistry of sugars, in general, is kinetically and not necessarily thermodynamically controlled.     

A new type of insulated molecular wire: a rotaxane derived from a metal-capped conjugated tetrayne

The platinum butadiynyl complex trans-(C(6)F(5))(p-tol(3)P)(2)Pt(C≡C)(2)H and a CuI adduct of a 1,10-phenanthroline based 33-membered macrocycle react in the presence of K(2)CO(3) and I(2) or O(2) to give a rotaxane (ca. 9%) in which the macrocycle is threaded by the sp carbon chain of trans,trans-(C(6)F(5))(p-tol(3)P)(2)Pt(C≡C)(4)Pt(Pp-tol(3))(2)(C(6)F(5)). The crystal structure and macrocycle/axle electronic interactions are analyzed in detail.     

On the instability of the Riemann hypothesis for curves over finite fields

We show that it is possible to approximate the zeta-function of a curve over a finite field by meromorphic functions which satisfy the same functional equation and moreover satisfy (respectively do not satisfy) an analog of the Riemann hypothesis. In the other direction, it is possible to approximate holomorphic functions by simple manipulations of such a zeta-function. No number theory is required to understand the theorems and their proofs, for it is known that the zeta-functions of curves over finite fields are very explicit meromorphic functions. We study the approximation properties of these meromorphic functions.     

Linearized Nelson–Siegel and Svensson models for the estimation of spot interest rates

Linearized versions of the Nelson–Siegel (1987) and Svensson (1994) models for the cross-sectional estimation of spot yield curves from samples of coupon bonds are developed and analyzed. It is shown how these models can be made linear in the level, slope and curvature parameters and how prior information about these parameters can be incorporated in the estimation procedure. The performance of the linearized models are assessed in a Monte Carlo setting and with a sample of US government bonds. The results reveal that the linearized models compare favorably to the original models in terms of parameter estimates stability, computing effort and prevalence of local optima.     

Ultra-high-frequency chaos in a time-delay electronic device with band-limited feedback

We report an experimental study of ultra-high-frequency chaotic dynamics generated in a delay-dynamical electronic device. It consists of a transistor-based nonlinearity, commercially-available amplifiers, and a transmission-line for feedback. The feedback is band-limited, allowing tuning of the characteristic time-scales of both the periodic and high-dimensional chaotic oscillations that can be generated with the device. As an example, periodic oscillations ranging from 48 to 913 MHz are demonstrated. We develop a model and use it to compare the experimentally observed Hopf bifurcation of the steady-state to existing theory [Illing and Gauthier, Physica D 210, 180 (2005)]. We find good quantitative agreement of the predicted and the measured bifurcation threshold, bifurcation type and oscillation frequency. Numerical integration of the model yields quasiperiodic and high dimensional chaotic solutions (Lyapunov dimension approximately 13), which match qualitatively the observed device dynamics.     

Measurement of cluster–cluster interaction in liquids by deposition and AFM of silicon clusters onto HOPG surfaces

Experimental studies have been performed on unisize tungsten clusters constructed on a graphite surface by means of the scanning tunneling microscopy. It was found that the geometry of the clusters changes instantaneously from a monatomic-layer tungsten disk to a diatomic-layer structure between the cluster size of 10 and 11. We concluded that this transition is driven by a change in the dominant interaction from the attractive electrostatic interaction between the cluster and the surface to intracluster cohesive metallic interaction.     

Structural rigidity control in arborescent graft polymers

Successive chloromethylation-anionic grafting sequences on polystyrene have led to well-defined hyperbranched macromolecules. Polymers with branching functionalities f > 5000 and molar masses over 10⁷ g/mol were thus prepared while maintaining a ratio of mass- to number-average molar masses M_w/M_n = 1,1-1,3. Three series of styrene polymers were prepared by varying the density of grafting sites along the chain, resulting in controllable stiffening of the molecular structure. Characterization of the branched molecules by size-exclusion chromatography/low-angle laser light scattering and light scattering confirmed a geometric increase in molar mass for successive generations, and a low ratio of mass- to number-average molar masses. Static and dynamic light scattering experiments showed that the molecules behave like hard spheres in dilute solutions. Measurements in the semidilute range, however, showed a progressive structural stiffening effect as the branching density increases.