Particle image velocimetry measurements of the separated flow behind a rearward facing step

Particle image velocimetry (PIV) was used to measure mean flow and turbulence characteristics in the separated flow behind a rearward facing step. The confidence limits characterising estimates of mean velocity and turbulence intensity obtained by PIV are discussed. The relevance of background turbulence levels and sample size in determining these limits is considered.     

Determination of triazine pesticides by high-performance liquid chromatography with swept-potential electrochemical detection

A swept-potential electrochemical detector, operating in the square-wave voltammetric mode, is used to detect a mixture of five triazine pesticides separated on a reverse-phase resin column. Limits of detection are below 1 ng injected. Two compounds, not completely separated by the column, are resolved on the potential axis.     

Photon Asymmetry of Deuteron Photodisintegration Between 160 and 410 MeV

 The photon asymmetry Σ of the reaction has been measured across the photon energy range 160–410 MeV at the Mainz Microtron MAMI. Linearly polarized photons were obtained from coherent bremsstrahlung and their energies were determined with the Glasgow tagging spectrometer. The large acceptance detector DAPHNE enabled the outgoing protons to be measured over the complete azimuthal range and for polar angles from 35° to 155° in the c.m. system. The data are compared with existing results and recent theoretical calculations. Received August 22, 1998; accepted for publication February 8, 1999     

2'-O-[2-[N,N-(dialkyl)aminooxy]ethyl]-modified antisense oligonucleotides

[structure] Oligonucleotides with two novel modifications, 2'-O-?2-[N, N-(dimethyl)aminooxy]ethyl? (2'-O-DMAOE) and 2'-O-?2-[N, N-(diethyl)aminooxy]ethyl? (2'-O-DEAOE), have been synthesized. These modifications exhibit high binding affinity to target RNA (and not to DNA) and enhance the nuclease stability of oligonucleotides considerably with t(1/2) > 24 h as a phosphodiester.     

Structural characterization of phosphatidyl-<Emphasis Type="Italic">myo</Emphasis>-inositol mannosides from <Emphasis Type="Italic">Mycobacterium bovis</Emphasis> bacillus calmette gúerin by multiple-stage quadrupole ion-trap mass spectrometry with electrospray ionization. II. Monoacyl- and diacyl-PIMs

The multiple-stage ion-trap mass spectrometric approaches towards to the structural characterization of the monoacyl-PIM (triacylated PIM) and the diacyl-PIM (tetracylated PIM), namely, the PIM (diacylated PIM) consisting of one or two additional fatty acid substituents attached to the glycoside, respectively, were described. While the assignment and confirmation of the fatty acid substituents on the glycerol backbone can be easily achieved by the methods described in the previous article, the identity of the glycoside moiety and its acylation state can be determined by the observation of a prominent acylglycoside ion arising from cleavage of the diacylglycerol moiety ([M - H - diacylglycerol](-)) in the MS(2)-spectra of monoacyl-PIM and diacyl-PIM. The distinction of the fatty acid substituents on the 2-O-mannoside (i.e., R(3)CO(2)H) from that on the inositol (i.e., R(4)CO(2)H) is based on the findings that the MS(3)-spectrum of [M - H - diacylglycerol](-) contains a prominent ion arising from further loss of the fatty acid at the 2-O-mannoside (i.e., the [M - H - diacylglycerol - R(3)CO(2)H](-) ion), while the ion arising from loss of the fatty acid substituent at the inositol (i.e., the [M - H - diacylglycerol - R(4)CO(2)H](-) ion) is of low abundance. The fatty acyl moiety on the inositol can also be identified by the product-ion spectrum from MS(4) of the [M - H - diacylglycerol - R(3)CO(2)H](-) ion, which gives rise to a prominent ion corresponding to loss of R(4)CO(2)H. An [M - H - acylmannose](-) ion was also observed in the MS(2)-spectra and, thus, the identity of the fatty acid substituent attached to 2-O-mannoside can be confirmed. The combined information obtained from the multiple-stage product-ion spectra from MS(2), MS(3), and MS(4) permit the assignment of the complex structures of monoacyl-PIMs and diacyl-PIMs in a mixture isolated from M. bovis Bacillus Calmette Guérin.     

A combined experimental and theoretical study of the kinetics and mechanism of the addition of alcohols to electronically stabilized silenes: a new mechanism for the addition of alcohols to the Si=C bond

The stabilized silene 1,1-bis(trimethylsilyl)-2-adamantylidenesilane (4) has been generated by photolysis of a novel trisilacyclobutane derivative in various solvents and studied directly by kinetic UV spectrophotometry. Silene 4 decays with second-order kinetics in degassed hexane solution at 23 degrees C (k/epsilon = 8.6 x 10(-6) cm s(-1)) due to head-to-head dimerization. It reacts rapidly with oxygen [k(25 degrees C) approximately 3 x 10(5) M(-1) s(-1)] but approximately 10 orders of magnitude more slowly with methanol (MeOH) than other silenes that have been studied previously. The data are consistent with a mechanism involving reaction with the hydrogen-bonded dimer of the alcohol, (MeOH)(2) (k = 40 +/- 3 M(-1) s(-1); k(H)/k(D) = 1.7 +/- 0.2). The stable analogue of silene 4, 1-tert-butyldimethylsilyl-1-trimethylsilyl-2-adamantylidenesilane (5), reacts approximately 50 times more slowly, but via the same mechanism. The mechanism for addition of water and methanol (ROH; R = H, Me) to 4, 5, and the model compound 1,1-bis(silyl)-2,2-dimethylsilene (3a) has been studied computationally at the B3LYP/6-31G(d) and MP2/6-31G(d) levels of theory. Hydrogen-bonded complexes with monomeric and dimeric methanol, in which the Si=C bond plays the role of nucleophile, have been located computationally for all three silenes. Reaction pathways have been characterized for reaction of the three silenes with monomeric and dimeric ROH and reveal significantly lower barriers for reaction with the dimeric form of the alcohol in each case. The calculations indicate that 5 should be approximately 40-fold less reactive toward dimeric MeOH than 4, in excellent agreement with the approximately 50-fold difference in the experimental rate constants for reaction in hexane solution.     

Formation of mixed monolayers of silsesquioxanes and alkylsilanes on gold

The formation of mixed monolayers of hydridospherosilsesquioxane clusters (H(8)Si(8)O(12)) and alkylsilanes (H(2n+1)C(n)SiH(3)) on Au has been investigated using X-ray photoelectron and reflection-absorption infrared spectroscopies and scanning tunneling microscopy. All of the techniques indicate the displacement of the majority of the siloxane clusters from the surface in favor of the alkylsilane.