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At least 739 records · Page 41Linked to original sources
PubMed1988

Insoluble organic material of the Orgueil carbonaceous chondrite and the unidentified infrared bands.

Deuterium abundance measurements and the 13C NMR spectrum of the HF/HCl insoluble residue of the Orgueil carbonaceous chondrite indicate the presence of an extensive component of polycyclic aromatic hydrocarbons (PAH) that is of possible interstellar origin or is similar to PAH hypothesized to exist in interstellar space. Infrared spectra have been obtained using an FTIR spectrometer of the acid insoluble residue, the residue after heating in vacuum, and condensate. Bulk acid insoluble residue was pressed into KBr pellets and also heated under high vacuum to sublimate a volatile component onto KBr disks. The remaining non-volatile organic component of the Orgueil meteorite from such evaporations pressed into KBr pellets exhibits a spectral signature similar to that observed in emission from the Orion Nebula and found in Raman spectra of interplanetary dust particles (IDPs). In addition it has an 11.3 microns (885 cm-1) band indicating PAH having single hydrogens per peripheral aromatic ring. We conclude the nonvolatile material is similar to interstellar PAH because the observed 11.3 microns (885 cm-1) unidentified infrared (UIR) emission suggests interstellar PAH have single hydrogens per peripheral aromatic ring. The volatile fraction of the Orgueil shows an aliphatic CH stretch feature and its spectrum in the 2-25 microns range is similar to that of the bulk residue.

Carbon↗
PubMed1988

Complex formation between anionic semiquinoid form of a flavoenzyme D-amino acid oxidase and ligands. Stabilizing mechanism of anionic semiquinoid flavoenzyme.

Picolinate binds to the anionic semiquinoid form of D-amino acid oxidase (DAO), and the complex formed has a broad absorption band in the long-wavelength region extending beyond 800 nm, which is reminiscent of a charge transfer interaction. The binding has a stoichiometry of 1:1 with respect to the enzyme. The dissociation constant at 25 degrees C was 30 microM at pH 7.0. The pH dependence (pH 7.0-8.3) of the dissociation constant indicates that one proton is associated with the complex formation, and suggests that picolinate able to bind to the anionic semiquinoid enzyme is in the cationic form protonated at the nitrogen atom. By adding dithionite to the oxidized DAO solution containing pyruvate and various amines, a similar anionic semiquinoid DAO complex having a broad long-wavelength absorption band, appeared. Resonance Raman spectra with excitation at 623.8 nm of the anionic semiquinoid DAO complex formed in the presence of pyruvate and methylamine indicate that the complex consists of the anionic semiquinoid DAO and N-methyl-alpha-iminopropionate produced from pyruvate and methylamine, and that the imino group must be protonated. This supports the proposal that the presence of a positively charged group in the vicinity of flavin is required for the stabilization of the anionic semiquinoid flavin. The results also suggest that the broad absorption band is derived from the charge transfer interaction between the anionic semiquinoid flavin and the imino acid, in which the flavin C(4a)-N(5) locus and the locus containing (Formula: see text) of the amino acid are important for the interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical Phenomena↗
PubMed2002

Spectrally resolved luminescence of undoped and Dy3+ doped Na2SO4.

Structural phase transitions influence the luminescence characteristics by modifying the spectra, sensitivity and luminescence efficiency in each phase. This has implications for dosimetric materials both for sensitivity and reproducibility. Previous work with dosemeter materials has not documented effects of different phases on thermoluminescence but Na2SO4 exists in several phases in the temperature range of interest for thermoluminescence dosimetry. Therefore spectrally resolved TL and radiothermoluminescence analyses of undoped and Dy3+ doped Na2SO4 have been performed. Dramatic differences between TL of slow cooled and quenched Na2SO4:Dy are explained in terms of the initial phase of the samples at room temperature. For undoped material the effect of phase conversions during heating shift the wavelengths of the emission bands. A model of Dy in small precipitate phases is considered.

Dysprosium↗
PubMed1996

The mutation Met121-->His creates a type-1.5 copper site in Alcaligenes denitrificans azurin.

The Cu ligand Met121 in azurin of Alcaligenes denitrificans was mutated to His. The spectroscopic and mechanistic properties of [M121H]azurin appear to be pH dependent with a pKa of 3.8 due to the ionization of His121. The [M121H]azurin mutant exhibits two major distinct metal-site-coordination geometries which coexist in solution according to pH-dependent equilibrium. Both species have been spectroscopically characterized by ultraviolet-visible, EPR and resonance Raman spectroscopies. At neutral pH, His121 is deprotonated and acts as the fourth ligand of the Cu; the spectroscopic characteristics of the Cu site at this pH are halfway between those of a type-1 and a type-2 Cu site, and the site is referred to as a type-1.5 or intermediate Cu site. The spectral data are compatible with a tetrahedral geometry of this site. At low pH, the spectroscopic data indicate that [M121H]azurin has a trigonal type-1 rhombic Cu site.

Alcaligenes↗
PubMed2003

Fiber optic probes for biomedical optical spectroscopy.

Fiber optic probes are a key element for biomedical spectroscopic sensing. We review the use of fiber optic probes for optical spectroscopy, focusing on applications in turbid media, such as tissue. The design of probes for reflectance, polarized reflectance, fluorescence, and Raman spectroscopy is illustrated. We cover universal design principles as well as technologies for beam deflecting and reshaping.

Equipment Design↗
PubMed2003

Structure-based carbon nanotube sorting by sequence-dependent DNA assembly.

Wrapping of carbon nanotubes (CNTs) by single-stranded DNA (ssDNA) was found to be sequence-dependent. A systematic search of the ssDNA library selected a sequence d(GT)n, n = 10 to 45 that self-assembles into a helical structure around individual nanotubes in such a way that the electrostatics of the DNA-CNT hybrid depends on tube diameter and electronic properties, enabling nanotube separation by anion exchange chromatography. Optical absorption and Raman spectroscopy show that early fractions are enriched in the smaller diameter and metallic tubes, whereas late fractions are enriched in the larger diameter and semiconducting tubes.

Anions↗
PubMed2004

The structure of the first coordination shell in liquid water.

X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the molecular arrangement in the first coordination shell of liquid water. The local structure is characterized by comparison with bulk and surface of ordinary hexagonal ice Ih and with calculated spectra. Most molecules in liquid water are in two hydrogen-bonded configurations with one strong donor and one strong acceptor hydrogen bond in contrast to the four hydrogen-bonded tetrahedral structure in ice. Upon heating from 25 degrees C to 90 degrees C, 5 to 10% of the molecules change from tetrahedral environments to two hydrogen-bonded configurations. Our findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liquid water. Serious discrepancies with structures based on current molecular dynamics simulations are observed.

Chemical Phenomena↗