Iso-specific Ziegler-Natta polymerization of α-olefins with a single-component organoyttrium catalyst

The synthesis of a sterically tailored ligand array (M)_2((C_5H_2-2-Si(CH_3)_3-4-C(CH_3)_3)S_2i(CH_3)_2]("M_2Bp") (M = Li, 16; K, 19) is described. Transmetallation of Li_2Bp with YCl_3(THF)_3 affords exclusively the C_2 symmetric product rac-[BpY(µ_2-Cl)_2Li(THF)_2], 20. A X-ray crystal structure of 20 has been determined; triclinic, P1, a= 13.110 (8), b = 17.163 (15), c = 20.623 (14) Å, α = 104.02 (7), β = 99.38 (5), γ = 100.24 (6)° , Z = 4, R = 0.056. Transmetallation of K_2Bp with YCl_3(THF)_3 affords the halide free complex rac-BpYCl, 23. The corresponding rac-BpLaCl, 28, is prepared in an anlogous manner. In all cases the achiral meso isomer is not obtained since only for the racemic isomers are the unfavorable steric interactions between the Si(CH3)_3 groups in the narrow portion of the [Cp-M'-Cp] wedge avoided. Alkylation of 20 or 23 with LiCH(Si(CH_3)_3)_2 affords rac-BpYCH(Si(CH_3)_3)_2, 26 in good yield. Alkylation of 28 with LiCH(Si(CH_3)_3)_2 affords rac-BpLaCH(Si(CH_3)_3)_2 29. Hydrogenation of 26 cleanly affords the bridging hydride species [BpY(µ_2-H)]_2, 27, as the homochiral (R,R) and (S,S) dimeric pairs. 26 is an efficient initiator for the polymerization of ethylene to high molecular weight linear polyethylene. 27 catalyzes the polymerization of propylene (25% v/v in methylcyclohexane) and neat samples of 1-butene, 1-pentene, 1-hexene to moderately high molecular weight polymers: polypropylene (M_n = 4,200, PDI 2.32, T_m 157 °C); poly-1-butene (M_n = 8,500, PDI 3.44, T_m 105 °C); poly-1-pentene (M_n = 20,000, PDI 1.99, T_m 73 °C); poly-1-hexene (M_n = 24,000, PDI 1.75, T_m < 25 °C). ^(13)C NMR spectra at the pentad analysis level indicates that the degree of isotacticity is 99% mmmm for all polymer samples. 27 is the first single component iso-specific α-olefin polymerization catalyst. The presumed origins of the high isospecificity are presented.

Single-cell analysis of the physiology of mechanosensation in bacteria

Escherichia coli is one of the best studied living organisms and a model system for many biophysical investigations. Despite countless discoveries of the details of its physiology, we still lack a holistic understanding of how these bacteria react to changes in their environment. One of the most important examples is their response to osmotic shock. One of the mechanistic elements protecting cell integrity upon exposure to sudden changes of osmolarity is the presence of mechanosensitive channels in the cell membrane. These channels are believed to act as tension release valves protecting the inner membrane from rupturing. This thesis presents an experimental study of various aspects of mechanosensation in bacteria. We examine cell survival after osmotic shock and how the number of MscL (Mechanosensitive channel of Large conductance) channels expressed in a cell influences its physiology. We developed an assay that allows real-time monitoring of the rate of the osmotic challenge and direct observation of cell morphology during and after the exposure to osmolarity change. The work described in this thesis introduces tools that can be used to quantitatively determine at the single-cell level the number of expressed proteins (in this case MscL channels) as a function of, e.g., growth conditions. The improvement in our quantitative description of mechanosensation in bacteria allows us to address many, so far unsolved, problems, like the minimal number of channels needed for survival, and can begin to paint a clearer picture of why there are so many distinct types of mechanosensitive channels.

High and stable conversion efficiency obtaining in single-stage multi-crystal optical parametric chirped pulse amplification system

An optical parametric chirped-pulse amplification system is demonstrated to provide 32.9% pump-to-signal conversion efficiency . Special techniques are used to make the signal and pump pulses match with each other in both spectral and temporal domains. The broadband 9.5-mJ pulses are produced at the repetition rate of 1 Hz with the gain of over 1.9 x 10(8). The output energy fluctuation of 7.8% is achieved for the saturated amplification process against the pump fluctuation of 10%.

Interacting single atoms with nanophotonics for chip-integrated quantum networks

Underlying matter and light are their building blocks of tiny atoms and photons. The ability to control and utilize matter-light interactions down to the elementary single atom and photon level at the nano-scale opens up exciting studies at the frontiers of science with applications in medicine, energy, and information technology. Of these, an intriguing front is the development of quantum networks where N >> 1 single-atom nodes are coherently linked by single photons, forming a collective quantum entity potentially capable of performing quantum computations and simulations. Here, a promising approach is to use optical cavities within the setting of cavity quantum electrodynamics (QED). However, since its first realization in 1992 by Kimble et al., current proof-of-principle experiments have involved just one or two conventional cavities. To move beyond to N >> 1 nodes, in this thesis we investigate a platform born from the marriage of cavity QED and nanophotonics, where single atoms at ~100 nm near the surfaces of lithographically fabricated dielectric photonic devices can strongly interact with single photons, on a chip. Particularly, we experimentally investigate three main types of devices: microtoroidal optical cavities, optical nanofibers, and nanophotonic crystal based structures. With a microtoroidal cavity, we realized a robust and efficient photon router where single photons are extracted from an incident coherent state of light and redirected to a separate output with high efficiency. We achieved strong single atom-photon coupling with atoms located ~100 nm near the surface of a microtoroid, which revealed important aspects in the atom dynamics and QED of these systems including atom-surface interaction effects. We present a method to achieve state-insensitive atom trapping near optical nanofibers, critical in nanophotonic systems where electromagnetic fields are tightly confined. We developed a system that fabricates high quality nanofibers with high controllability, with which we experimentally demonstrate a state-insensitive atom trap. We present initial investigations on nanophotonic crystal based structures as a platform for strong atom-photon interactions. The experimental advances and theoretical investigations carried out in this thesis provide a framework for and open the door to strong single atom-photon interactions using nanophotonics for chip-integrated quantum networks.

Repeat-until-success distributed quantum computation by using single-photon interference at a beam splitter

A repeat-until-success (RUS) measurement-based scheme for the implementation of the distributed quantum computation by using single-photon interference at a 50:50 beam splitter is proposed. It is shown that the 50:50 beam splitter can naturally project a suitably encoded matter-photon state to either a desired entangling gate-operated state of the matter qubits or to their initial state when the photon is detected. The recurrence of the initial state permits us to implement the desired entangling gate in a RUS way. To implement a distributed quantum computation we suggest an encoding method by means of the effect of dipole-induced transparency proposed recently [E. Waks and J. Vuckovic, Phys. Rev. Lett. 96, 153601 (2006)]. The effects of the unfavorable factors on our scheme are also discussed.

Constitutive plasma membrane targeting and microdomain localization of Dok5 studied by single-molecule microscopy

In the present study, single-molecule fluorescence microscopy was used to examine the characteristics of plasma membrane targeting and microdomain localization of enhanced yellow fluorescent protein (eYFP)-tagged wild-type Dok5 and its variants in living Chinese hamster ovary (CHO) cells. We found that Dok5 can target constitutively to the plasma membrane, and the PH domain is essential for this process. Furthermore, single-molecule trajectories analysis revealed that Dok5 can constitutively partition into microdomain on the plasma membrane. Finally, the potential mechanism of microdomain localization of Dok5 was discussed. This study provided insights into the characteristics of plasma membrane targeting and microdomain localization of Dok5 in living CHO cells. (C) 2008 Elsevier B.V. All rights reserved.

Control of single attosecond pulse generation from the reflection of a synthesized relativistic laser pulse on a solid surface

The influence of the carrier-envelope phase (CEP) of the driving laser pulse on the generation of single attosecond (as) pulses from surface harmonics by using the polarization gating technique is investigated in detail. It is found that the modulation depth of the high-order harmonic spectrum depends on the CEP, and a strong single 68 as pulse can be generated when the CEP is stable and has the proper value. The physical origin of the influence of the CEP is explained in terms of the oscillating mirror model. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2997342]

Preliminary investigation into implication of a single cell organism in fish feed bouyancy and flotation

Two Isocaloric Isoproteic 30% crude protein diets were formulated for Clariid catfish and Tilapia with wheat grain starch (WGS) and cassava tuber starch (CTS) incorporated at 10 percent as binding agents. Saccharomyces cerevisiae was included at 2% as floating additive. The water stability, nutrient retention and flotation of pelleted feeds were observed for 60 minutes. There were generally decreasing trends in stability and retention at increasing time of immersion in water. The lipid retention was higher (P>0.05) than proteins in both diets. WGS diet was better (P>0.05) than CTS diet in flotation, which has attributed to the presence of gluten protein in wheat products. It is envisaged that a break through in floating feed development in Nigeria aquaculture would save the Nigeria economy from extruded (floating) feed importation

Effects of particle size ratio on single particle motion in colloidal dispersions

The motion of a single Brownian particle of arbitrary size through a dilute colloidal dispersion of neutrally buoyant bath spheres of another characteristic size in a Newtonian solvent is examined in two contexts. First, the particle in question, the probe particle, is subject to a constant applied external force drawing it through the suspension as a simple model for active and nonlinear microrheology. The strength of the applied external force, normalized by the restoring forces of Brownian motion, is the Péclet number, Pe. This dimensionless quantity describes how strongly the probe is upsetting the equilibrium distribution of the bath particles. The mean motion and fluctuations in the probe position are related to interpreted quantities of an effective viscosity of the suspension. These interpreted quantities are calculated to first order in the volume fraction of bath particles and are intimately tied to the spatial distribution, or microstructure, of bath particles relative to the probe. For weak Pe, the disturbance to the equilibrium microstructure is dipolar in nature, with accumulation and depletion regions on the front and rear faces of the probe, respectively. With increasing applied force, the accumulation region compresses to form a thin boundary layer whose thickness scales with the inverse of Pe. The depletion region lengthens to form a trailing wake. The magnitude of the microstructural disturbance is found to grow with increasing bath particle size -- small bath particles in the solvent resemble a continuum with effective microviscosity given by Einstein's viscosity correction for a dilute dispersion of spheres. Large bath particles readily advect toward the minimum approach distance possible between the probe and bath particle, and the probe and bath particle pair rotating as a doublet is the primary mechanism by which the probe particle is able to move past; this is a process that slows the motion of the probe by a factor of the size ratio. The intrinsic microviscosity is found to force thin at low Péclet number due to decreasing contributions from Brownian motion, and force thicken at high Péclet number due to the increasing influence of the configuration-averaged reduction in the probe's hydrodynamic self mobility. Nonmonotonicity at finite sizes is evident in the limiting high-Pe intrinsic microviscosity plateau as a function of bath-to-probe particle size ratio. The intrinsic microviscosity is found to grow with the size ratio for very small probes even at large-but-finite Péclet numbers. However, even a small repulsive interparticle potential, that excludes lubrication interactions, can reduce this intrinsic microviscosity back to an order one quantity. The results of this active microrheology study are compared to previous theoretical studies of falling-ball and towed-ball rheometry and sedimentation and diffusion in polydisperse suspensions, and the singular limit of full hydrodynamic interactions is noted.

Second, the probe particle in question is no longer subject to a constant applied external force. Rather, the particle is considered to be a catalytically-active motor, consuming the bath reactant particles on its reactive face while passively colliding with reactant particles on its inert face. By creating an asymmetric distribution of reactant about its surface, the motor is able to diffusiophoretically propel itself with some mean velocity. The effects of finite size of the solute are examined on the leading order diffusive microstructure of reactant about the motor. Brownian and interparticle contributions to the motor velocity are computed for several interparticle interaction potential lengths and finite reactant-to-motor particle size ratios, with the dimensionless motor velocity increasing with decreasing motor size. A discussion on Brownian rotation frames the context in which these results could be applicable, and future directions are proposed which properly incorporate reactant advection at high motor velocities.

Heterodimerization of integrin Mac-1 subunits studied by single-molecule imaging

Heterodimerization of integrin Mac-1 (alpha(M) beta(2)) Subunits plays important role on regulating leukocytes adhesion to extracellular matrix or endothelial cells. Here, using total internal reflection microscopy, we investigated the heterodimerization of integrin Mac-1 subunits at the single-molecule level in live cells. Individual alpha(M) subunit fused to the enhanced yellow fluorescent protein (eYFP) was imaged at the basal plasma membrane of live Chinese hamster ovary (CHO) cells. Through analysis of mean square displacement (MSD), diffusion coefficient, the size of restricted domain and fraction of molecules undergoing restricted diffusion, we found that as compared with the diffusion in the absence of beta(2) subunit, the diffusion of single-molecule of alpha(M)-YFP was suppressed significantly in the presence of beta(2) subunit. Thus, based on the oligomerization-induced trapping model, we suggested that in the presence of beta(2) subunit, the am subunit may form heterodimer with it. (C) 2008 Elsevier Inc. All rights reserved.

Theoretical investigation of single attosecond pulse generation in an orthogonally polarized two-color laser field

We theoretically demonstrate the generation of extreme ultraviolet supercontinua in an orthogonally polarized two-color few-cycle laser field. We show that the ionized electrons can be driven back to their parent ion after traveling along curved trajectories in a plane perpendicular to the beam propagation direction, giving rise to a train of attosecond pulses at different polarization angles. A single isolated attosecond pulse can be obtained by blocking the low-order high harmonics, which contribute to the formation of the satellite pulses. (C) 2008 Optical Society of America.

Single sub-100 attosecond pulse generation in a two-colour time-gating laser field

We theoretically propose a method of generating a single sub-100 attosecond (as) pulse with a two-colour time-gating laser field. The field is synthesized by an 8 fs/800 nm (three optical cycles) pulse and a 24 fs/2400 nm (three optical cycles) pulse with an optimal time delay between them. In our simulation, we obtain a supercontinuum with an extremely broad spectrum of 150 eV and generate an isolated attosecond pulse with 96 as pulse duration without any dispersion compensation.