Nucleosides—XCV: Total synthesis of pentopyranamine D,the nucleoside moiety of blasticidin H

1-(4-Amino-3,4-dideoxy-β-d-ribo hexopyranosyluronic acid)cytosine was synthesized and its identity with the natural product pentopyranamine D was established.     

Morphology and mechanical properties of surfactant aggregates at water-silica interfaces: molecular dynamics simulations

Dilute and concentrated surfactant systems at the solid-liquid interface are examined using classical molecular dynamics simulations. Particular emphasis is placed on understanding how surfactants aggregate and form the micellar structure, how micelles change shape at high concentrations in aqueous media and in the presence of hydrophilic surfaces, and at what force this micellar structure breaks apart during indentation of micelle-covered surfaces with a proximal probe microscope tip. The specific system of interest is C12TAB (n-dodecyltrimethylammonium bromide) surfactant in an aqueous medium that is modeled with empirical potentials. The simulations predict that the micelle structure in water is compact and either spherical or elliptical in shape. In the presence of a hydrophilic surface of silica, the structure evolves into a flat elliptical shape, in agreement with experimental findings. The simulated indentation of the micelle/silica system causes the micelle to break apart at an indentation force of about 1 nN and form a surfactant monolayer. The predicted force curve is in excellent agreement with experimental measurements.     

Response of SI cortex to ipsilateral,contralateral and bilateral flutter stimulation in the cat

Background  

While SII cortex is considered to be the first cortical stage of the pathway that integrates tactile information arising from both sides of the body, SI cortex is generally not considered as a region in which neuronal response is modulated by simultaneous stimulation of bilateral (and mirror-image) skin sites.     

D-type fiber biosensor based on surface-plasmon resonance technology and heterodyne interferometry

A D-type fiber biosensor based on surface-plasmon resonance (SPR) technology and heterodyne interferometry is presented. The sensing device is a single-mode optical fiber in which half the core is polished away and a thin-film layer of gold is deposited. We measure the phase-difference variations instead of the light intensity as in traditional SPR techniques. Therefore the accuracy and resolution of our method are very high. Its sensitivity can reach 2 x 10(-6) refractive-index units. The sensor has some merits, e.g., tunable high sensitivity, small size, lower cost, smaller sample volume, and suitability for in vivo testing. This novel method of a D-type fiber biosensor based on SPR technology and heterodyne interferometry is valuable for chemical, biological, and biochemical sensing, and the novel method of D-type fiber biosensing is a feasible means of study.     

Metal-metal interactions in weakly coupled mixed-valence E- and Z-diferrocenylethylene complexes

To study metal-to-metal interactions in mixed-valence states of two weakly coupling ferrocenyl groups assembled in E or Z conformation on an ethylenic double bond, E-1,2-dimethyldiferrocenylethylene (1), Z-1,2-dimethyldi-ferrocenylethylene (2), and 1,2-diferrocenylcyclohexene (3) were synthesized and structurally characterized. Crystals of 1 are triclinic, P1, with a = 7.494(9) A, b = 10.801(3) A, c = 11.971(2) A, alpha = 102.17(2) degrees, beta = 106.12(9) degrees, gamma = 90.42(2) degrees, V = 907.8 A3, and Z = 2. Crystals of 2 are monoclinic, P2(1)/c, with a = 13.601(8) A, b = 11.104(4) A, c = 13.732(1) A, beta = 114.26(7) degrees, V = 1890.8(3) A3, and Z = 4. Crystals of 3 are orthorhombic, P2(1)2(1)2(1), with a = 5.766(2) A, b = 13.090(1) A, c = 26.695(2) A, V = 2014.9(3) A3, and Z = 4. Intervalence transition spectra (IT) and electrochemical data have been determined and compared with those of diferrocenyl-benzene (para, ortho, and meta). The comproportionation constants in nitrobenzene at 25 degrees C were found to be 490 and 813 for 1 and 3, respectively. That of 2 was not measured because of the fact that 2+ isomerizes rapidly in all solvents tested, yielding nearly a racemic mixture of E and Z conformers. This finding helps to clear the paradoxical phenomenon between experimental results of mixed-valence complexes of E- and Z-1,2-bis(1'-ethyl-1-ferrocenyl)-1,2-dimethylethylene and theories. The stability of the mixed-valence species was discussed in terms of resonance delocalization, Coulomb repulsion energy, inductive effect, magnetic interaction, structural factors, and statistical factor. According to our analysis based on the Hush formalism, the contribution due to Coulomb repulsion energy dominates the overall stability of the mixed-valence state in 1+, 2+, and 3+. Stabilization that arises from resonance delocalization is only minor and contributes less than 4% to the overall stability, even in 3+ where linked Cp rings and the ethylenic plane are coplanar. In calculating the resonance contribution, crystallographic Fe-Fe distances of 7.44 A (1) and 6.68 A (3) were used for 1+, and 3+, respectively.     

Measurement of single electrons and implications for charm production in Au+Au collisions at square root[s(NN)] = 130 GeV

Transverse momentum spectra of electrons from Au+Au collisions at square root[s(NN)] = 130 GeV have been measured at midrapidity by the PHENIX experiment at the Relativistic Heavy Ion Collider. The spectra show an excess above the background from photon conversions and light hadron decays. The electron signal is consistent with that expected from semileptonic decays of charm. The yield of the electron signal dN(e)/dy for p(T) > 0.8 GeV/c is 0.025+/-0.004(stat)+/-0.010(syst) in central collisions, and the corresponding charm cross section is 380+/-60(stat)+/-200(syst) microb per binary nucleon-nucleon collision.     

A Simple Method for Determining the Critical Micellar Concentration of a Surfactant

This paper will present a simple method for critical micellar concentration (cmc) determination based on light scattering using a turbidimeter. The method does not require the optical clarification of the surfactant solution. The surfactant solutions were prepared from distilled water after boiling. Distinct cmc values were observed for polyoxyethylene mono n-dodecylether, sodium taurodeoxycholate and sodium dodecyl sulfate. The cmc values obtained using the turbidity method were compared with cmc values obtained by the surface tension method and with data given in the literature. Values obtained by our simple method have comparable accuracy with data obtained from more elaborate experiments.     

In situ generation of the silica shell layer--key factor to the simple high yield synthesis of silver nanowires

a-Silica encapsulated silver nanowires (diameter of 25 +/- 5 nm, average length of 10 mum) have been synthesized by reacting (Me3Si)4Si with AgNO3 in nearly quantitative yield. Formation of the a-silica shell layer (1-3 nm) in situ appears to be one of the most important factors in this simple process.     

Polyvalent interactions in unnatural recognition processes

The synthesis of two cluster compounds, one containing six secondary dialkylammonium ion centers and the other possessing six benzo-m-phenylene[25]crown-8 (BMP25C8) macrocycles, both appended to hexakis(thiophenyl)benzene cores, is described. The binding of these clusters with complementary mono- and divalent ligands is investigated with NMR spectroscopy to probe polyvalency in these unnatural recognition systems. The ability of the two different families of clusters to bind complementary monovalent ligands is compared with that of the monovalent receptor pair, namely the dibenzylammonium ion and BMP25C8. This comparison is made possible by determining an average association constant (K(AVE)) for the binding of each recognition site on the cluster with the corresponding monovalent ligand. We have found that the clustering of recognition sites together in one molecule is detrimental to their individual abilities to bind monovalent ligands. In the case of the polyvalent interaction between the hexakisBMP25C8 cluster and divalent dialkylammonium ions, an association constant, K(POLY), was calculated from the value of K(AVE) determined for the complexation of the individual component recognition sites. This polyvalent interaction is significantly stronger than that associated with the averaged monovalent interactions.