The use of l-tartaric acid in the synthesis of enanthiomerically pure compounds: synthesis of 4-O-benzyl-2,3-dideoxi-L- threo-hex-2-eno no-1,5-lactone.

The title compound was obtained through a seven steps sequence and using - dimethyl L-tartrate as the starting material (30% overall yield). The system 2,3-dideoxy-L-threo-hex-2-enono-1,5-lactone is present in several natural compounds.     

Internet’s critical path horizon

Internet is known to display a highly heterogeneous structure and complex fluctuations in its traffic dynamics. Congestion seems to be an inevitable result of users behavior coupled to the network dynamics and it effects should be minimized by choosing appropriate routing strategies. But what are the requirements of routing depth in order to optimize the traffic flow? In this paper we analyse the behavior of Internet traffic with a topologically realistic spatial structure as described in a previous study [S.-H. Yook et al. , Proc. Natl Acad. Sci. USA 99, 13382 (2002)]. The model involves self-regulation of packet generation and different levels of routing depth. It is shown that it reproduces the relevant key, statistical features of Internets traffic. Moreover, we also report the existence of a critical path horizon defining a transition from low-efficient traffic to highly efficient flow. This transition is actually a direct consequence of the webs small world architecture exploited by the routing algorithm. Once routing tables reach the network diameter, the traffic experiences a sudden transition from a low-efficient to a highly-efficient behavior. It is conjectured that routing policies might have spontaneously reached such a compromise in a distributed manner. Internet would thus be operating close to such critical path horizon.Received: 22 December 2003, Published online: 14 May 2004PACS: 89.75.-k Complex systems - 05.70.Ln Nonequilibrium and irreversible thermodynamics - 87.23.Ge Dynamics of social systems     

Magnetic field assisted fluidization： A modified Richardson-Zaki equation

Magnetic particles can be uniformly fluidized by coupling the gas flow with an externally imposed magnetic field. Interparticle forces generated by the magnetic field cause aggregation of the particles in chain-like structures preferentially oriented along the magnetic field lines. In the present paper, we study the implications of the formation of these special types of aggregates on the empirical Richardson-Zaki （RZ） equation, originally proposed to describe the expansion of fluidized beds of non-aggregated particles. We have addressed two important issues, namely the flow regime, which is a function of the size of the aggregates, and the effect of shape and orientation of the chain-like aggregates with respect to gas flow on fluid drag. We propose a modified RZ equation （MRZE） in which the velocity scale, given by the terminal settling velocity of the individual aggregates, and the RZ exponent are predetermined as a function of the chain length. The chain length depends on the ratio of the magnetic energy to gravitational energy, and is estimated from the magnetic field intensity, and particle magnetization, size and density. Predictions of the MRZE are successfully compared with published results in the literature on the expansion of magnetic particles in the presence of externally applied magnetic fields.     

Column switching liquid chromatography and post-column photochemically fluorescence detection to determine imidacloprid and 6-chloronicotinic acid in honeybees

The determination of imidacloprid and its main metabolite (6-chloronicotinic acid) in honeybees was performed by liquid chromatography with post-column photochemical derivatisation in alkaline medium and fluorescence detection. The compounds were extracted from honeybees with acetone under ultrasound conditions prior to liquid-liquid partition with dichloromethane. The separation of extract components was performed using a 50 mm x 4.6 mm i.d. short column packed with 5 microm Aquasil C(18) using an acetonitrile:water gradient program as mobile phase. Injection of samples in 0.1 mol L(-1) H(3)PO(4)/KH(2)PO(4) buffer solution (pH 3) improved the chromatographic separation between the most polar components of matrix and the 6-chloronicotinic acid. Matrix components were removed to waste using an on-line clean-up method previously to post-column reaction. Limits of quantification were 0.3 and 5.0 microg L(-1) (corresponding to 1.2 and 20.0 microg kg(-1) in the honeybee sample) for imidacloprid and 6-chloronicotinic acid, respectively. The recovery was ranged from 80.2 to 91.7% with a relative standard deviation lower than 9.0%.     

New method for the photo-chemiluminometric determination of benzoylurea insecticides based on acetonitrile chemiluminescence

The viability of tandem photochemical reaction–chemiluminescence detection has been studied for the determination of five benzoylurea insecticides, namely, diflubenzuron, triflumuron, hexaflumuron, lufenuron and flufenoxuron. The ‘on-line’ photochemical reaction of benzoylurea pesticides provides an enhanced chemiluminescence response of the pesticides during their oxidation by potassium hexacyanoferrate(III) and sodium hydroxide, whose signal increases with the percentage of acetonitrile in the reaction medium. The determination was performed using a photoreactor consisting of a PFA (perfluoroalkoxy) tube reactor coil (5 m × 1.6-mm O.D. and 0.8-mm I.D.) and an 8-W xenon lamp. As the yield of the photoderivatization process and the chemiluminescent signals depend on the percentage of acetonitrile, the chromatographic column (a Gemini C18, Phenomenex 150 mm × 4.6 mm, 5-μm particle size) was chosen with the aim of using high percentages of this organic solvent in the mobile phase. Previous studies showed that the rate of the chemiluminescent reaction was very fast. Therefore, a modification was carried out in the detector in order to mix the analytes and reactants as near as possible to the measure cell. The optimised method was validated with respect to linearity, precision, limits of detection and quantification accuracy. Under the optimised conditions, linear working range extends three orders of magnitude with the relative standard deviation of intra-day precision below 10% and detection limits between 0.012 and 0.18 μg mL⁻¹, according to the compound. The proposed method has been successfully applied to the determination of benzoylureas in cucumber with good results. Figure     

A rheo-optical method for the study of aggregated particles: application to the characterization of ABO blood group monoclonal antibodies

The adhesive energy of monoclonal antibody induced agglutination can be assessed from the shear dissociation of agglutinates. Anti-A, anti-B, and anti-AB monoclonal antibodies agglutinate erythrocytes with variable strength, depending on the affinity of antibodies and on the antigenic density on the cell surface. The time required for an applied shear stress to dissociate cell agglutinates completely is depending on the strength of the agglutination reaction. The dissociation process under constant shear stress can be followed by measuring intensity of light backscattered by suspended particles. The obtained dissociation curves could be adjusted by an exponential expression governed by a parameter called characteristic time of dissociation. This parameter decreases with increasing antibody concentration. Integration of dissociation curves provided an approach of adhesive energy induced by monoclonal antibody. Received: 7 May 1999 Accepted: 30 May 2000     

Single Peptide Backbone Surrogate Mutations to Regulate Angiotensin GPCR Subtype Selectivity

Mutating the side-chains of amino acids in a peptide ligand, with unnatural amino acids, aiming to mitigate its short half-life is an established approach. However, it is hypothesized that mutating specific backbone peptide bonds with bioisosters can be exploited not only to enhance the proteolytic stability of parent peptides, but also to tune its receptor subtype selectivity. Towards this end, four [Y]⁶-Angiotensin II analogues are synthesized where amide bonds have been replaced by 1,4-disubstituted 1,2,3-triazole isosteres in four different backbone locations. All the analogues possessed enhanced stability in human plasma in comparison with the parent peptide, whereas only two of them achieved enhanced AT₂R/AT₁R subtype selectivity. This diversification has been studied through 2D NMR spectroscopy and unveiled a putative more structured microenvironment for the two selective ligands accompanied with increased number of NOE cross-peaks. The most potent analogue, compound 2 , has been explored regarding its neurotrophic potential and resulted in an enhanced neurite growth with respect to the established agent C21.     

A mathematical model for Babesiosis disease in bovine and tick populations

In this paper, we analyze the Babesiosis transmission dynamics on bovine and tick populations. Ticks play a role of infectious agents and vector of the protozoan Babesia hemo‐parasite. In this sense, we set out a mathematical model with constant size population for the evolution of the infected bovines with Babesiosis and analyze its qualitative dynamics. Statistical data are used to estimate some of the parameters of the model. Numerical simulations of the model varying the parameters show different scenarios about the spread of the disease. Copyright © 2011 John Wiley & Sons, Ltd.