Certification of fat-soluble vitamins,carotenoids, and cholesterol in human serum: Standard reference material 968b

In Standard Reference Material 968b, fat-soluble vitamins and cholesterol in human serum, certified values are provided for cholesterol, retinol, retinyl palmitate, -tocopherol, trans--carotene, total -carotene (trans plus cis isomers), total -carotene, and lutein. Non-certified values are also reported for -tocopherol (includes -tocopherol), -tocopherol, zeaxanthin, -cryptoxanthin, trans-lycopene, trans-lycopene, trans--carotene, total lycopene, 9-cis-carotene, 13- plus 15-cis--carotene, and 15-cis--carotene. Both certified and non-certified values are based on the agreement among results from three different liquid chromatographic analytical procedures developed at NIST and from an interlaboratory comparison exercise among institutions that participate in a NIST-managed Micronutrients Measurement Quality Assurance Program. Cholesterol is certified in this material using the NIST isotope dilution/mass spectrometric definitive method.     

An X-ray absorption study of two VOCl3-modified silicas: evidence for chloride-silica interactions

The structures of the sites formed in the gas-solid reactions of VOCl3 with the surfaces of a fumed silica (Aerosil) and a silica gel (Sylopol) were investigated by using X-ray absorption spectroscopy. XANES and EXAFS analysis at the vanadium K-edge reveal that the sites have a uniform first coordination sphere regardless of the origin or the extent of hydroxylation of the silica support (controlled by thermal treatment in vacuo at 100 and 500 degrees C). Analysis of the second coordination sphere was limited by the lack of structural uniformity. EXAFS curve-fitting confirmed that the sites are [triple bond]SiOVOCl2, but revealed an unexpected asymmetry in the V-Cl bond distances. The latter is suggested to be a manifestation of silicon-chloride interactions.     

Reductive alkylation of beta-alkoxy aziridines: new route to substituted allylic amines

[reaction: see text] A new route to substituted cyclic allylic amines via the reductive alkylation of beta-alkoxy aziridines using excess alkyllithium reagents is described.     

Evidence for the pairwise disposition of grafting sites on highly dehydroxylated silicas via their reactions with Ga(CH3)3

Molecules encountering silica interfaces interact primarily with the hydroxyl groups that terminate the bulk structure. When the nominal surface density is very low, these "silanols" are presumed to be isolated. Nevertheless, silicas that are highly dehydroxylated by pretreatment at 800 °C react with Ga(CH(3))(3) at room temperature to give primarily disilanolate-bridged digallium sites, [(CH(3))(2)Ga(μ-OSi≡)](2). The EXAFS at the Ga K-edge shows a prominent Ga-Ga scattering path, regardless of whether an excess or a limiting amount of Ga(CH(3))(3) is used. Some dimers are formed by the concerted reaction of Ga(CH(3))(3) with an "isolated" silanol and an adjacent siloxane bond. These grafting sites are proposed to be hydroxyl-substituted 2-rings, formed by condensation within a vicinal Q(2)-Q(3) pair. Other dimers are formed by reaction of Ga(CH(3))(3) with vicinal Q(3)-Q(3) pairs which have not condensed, even at 800 °C. In a computational model for the dimer sites, the O-O distance is <2.6 ?, which is far shorter than the calculated mean interhydroxyl separation for the thermally treated silicas (12.2 ?). This highly nonrandom distribution of surface silanols, in combination with the coupled reaction of "isolated" silanols and strained siloxane bonds, accounts for the preferential formation of grafted site pairs rather than isolated grafted sites when silica surfaces are chemically modified.     

Determination of neurotransmitters and their metabolites using one- and two-dimensional liquid chromatography with acidic potassium permanganate chemiluminescence detection

High-performance liquid chromatography with chemiluminescence detection based on the reaction with acidic potassium permanganate and formaldehyde was explored for the determination of neurotransmitters and their metabolites. The neurotransmitters norepinephrine and dopamine were quantified in the left and right hemispheres of rat hippocampus, nucleus accumbens and prefrontal cortex, and the metabolites vanillylmandelic acid, 3,4-dihydrophenylacetic acid, 5-hydroxyindole-3-acetic acid and homovanillic acid were identified in human urine. Under optimised chemiluminescence reagent conditions, the limits of detection for these analytes ranged from 2.5?×?10^?8 to 2.5?×?10^?7 M. For the determination of neurotransmitter metabolites in urine, a two-dimensional high-performance liquid chromatography (2D-HPLC) separation operated in heart-cutting mode was developed to overcome the peak capacity limitations of the one-dimensional separation. This approach provided the greater separation power of 2D-HPLC with analysis times comparable to conventional one-dimensional separations. Figure

2D-HPLC separation and permanganate chemiluminescence detection of neurotransmitter metabolites     

IR spectroscopic investigation of cis-(CH(3))(2)Au(O,O'-acac) and cis-(CD(3))(2)Au(O,O'-acac)

The IR spectrum of cis-(CH(3))(2)Au(O,O'-acac) has been reassigned by comparing frequencies for cis-(CH(3))(2)Au(O,O'-acac) and cis-(CD(3))(2)Au(O,O'-acac), and by analysis of the DFT-calculated normal modes and their frequencies for the isolated molecules. The vibrational intensity in the C-H stretching region arises almost entirely from the cis-(CH(3))(2)Au fragment, while the methyl deformation intensity is largely of acetylacetonato ligand origin. A low frequency mode in the C-H stretching region is the first overtone of the delta(a)(CH(3)) mode of cis-(CH(3))(2)Au. The Au-C stretching modes are affected by deuteration of the cis-(CH(3))(2)Au fragment, while the Au-O stretching modes are not.     

Expedient approach to chiral cyclobutanones: asymmetric synthesis of cyclobut-G

An efficient one-pot asymmetric synthesis of cyclobutanones from chiral enol ethers is described. The approach is illustrated with alkyl- and functionalized alkyl-substituted enol ethers (nine examples). A new enantioselective synthesis of cyclobut-G (Lobucavir) could thus be achieved.     

Simulating electron spin resonance spectra of macromolecules labeled with two dipolar-coupled nitroxide spin labels from trajectories

An efficient method for simulating continuous-wave electron spin resonance spectra (ESR) of molecules labeled with two dipolar-coupled nitroxides from trajectories of the molecular motion is presented. Two approximate treatments of the dipolar spin evolution, resulting in significantly shorter simulation times, are examined in order to determine their range of applicability. The approach is illustrated in the context of a double-helical B-DNA. ESR spectra for DNA undergoing anisotropic global diffusion and internal stretching dynamics are calculated for three different labeling geometries with the spin labels bracketing, respectively, three, two and one base pairs. While multifrequency spectra of all three labeling schemes are very sensitive to DNA tumbling, the last one is found to be most informative about the local DNA dynamics.     

Linear, high molecular weight polyethylene from a discrete, mononuclear phosphinoarenesulfonate complex of nickel(II)

A well-defined, homogeneous catalyst, [(Ph)(2-(2',6'-(OMe)(2)-C(6)H(3))-C(6)H(4))P(2-SO(3)-C(6)H(4))]Ni(Ph)PPh(3), in which a single, bulky ortho-biphenyl substituent on the chelating phosphine blocks one axial position, is very active for formation of linear polyethylene (M(n) = 403,000 g mol(-1), M(w)/M(n) = 1.87).