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71.
Rudd DJ Sazinsky MH Merkx M Lippard SJ Hedman B Hodgson KO 《Inorganic chemistry》2004,43(15):4579-4589
The diiron active site in the hydroxylase of Methylococcus capsulatus (Bath) methane monooxygenase (MMOH) has been studied in the oxidized form by X-ray absorption spectroscopy (XAS). Previous investigations by XAS and X-ray crystallography have identified two different distances (3.0 and 3.4 angstroms) between the two Fe atoms in the dinuclear site. The present study has employed a systematic extended X-ray absorption fine structure (EXAFS) fitting methodology, utilizing known and simulated active site and relevant model structures, to determine unambiguously the Fe-Fe separation in the oxidized form of MMOH. Consistent and unique fits were only possible for an Fe-Fe distance of 3.0 angstroms. This methodology was then applied to study potential changes in the active site local structure in the presence of MMOD, a protein of unknown function in multicomponent MMO. Fe K-edge and EXAFS analyses revealed negligible changes in the diiron site electronic and geometric structure upon addition of MMOD to oxidized MMOH. 相似文献
72.
Yi‐Xun Zhang Shauntina H. Jackson Mohamed S. Rajab Frank R. Fronczek Steven F. Watkins 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(4):o219-o221
3β‐Hydroxy‐7‐drimen‐12,11‐olide hemihydrate, C15H22O3·0.5H2O, (I), has two sesquiterpene molecules and one water molecule in the asymmetric unit. The OH groups of both molecules and both H atoms of the water molecule are involved in near‐linear intermolecular hydrogen bonds, having O⋯O distances in the range 2.632 (3)–2.791 (2) Å. 3β‐Acetoxy‐7‐drimen‐12,11‐olide, C17H24O4, (II), has its ring system in very nearly the same conformation as the two molecules of (I). 相似文献
73.
Jackson SN Mishra S Murray KK 《Rapid communications in mass spectrometry : RCM》2004,18(18):2041-2045
Matrix-assisted laser desorption/ionization (MALDI) was used for the on-line analysis of single particles. An aerosol was generated at atmospheric pressure and particles were introduced into a time-of-flight (TOF) mass spectrometer through a single-stage differentially pumped capillary inlet. Prior to entering the mass spectrometer, a matrix was added to the particles using a heated saturator and condenser. A liquid matrix, 3-nitrobenzyl alcohol (NBA), and a solid matrix, picolinic acid (PA), were used. Particles were ablated with a 351 nm excimer laser and the resulting ions were mass-separated in a two-stage reflectron TOF mass spectrometer. Aerosol particles containing the biomolecules erythromycin and gramicidin S were analyzed with and without the matrix addition step. The addition of NBA to the particles resulted in mass spectra that contained an intact molecular ion mass peak. In contrast, PA-coated particles did not yield molecular ion peaks from matrix-coated particles. 相似文献
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US Environmental Protection Agency Method 300.0 specifies the use of an IonPac AS4A anion-exchange column with a carbonate-hydrogencarbonate eluent and suppressed conductivity detection for the determination of inorganic anions in environmental waters by ion chromatography. Hydroxide eluents have not typically been used for the routine analysis of common inorganic anions due to the lack of an appropriate hydroxide selective column and the difficulty in preparing contaminant free hydroxide eluents. The use of ion chromatography with a hydroxide-selective IonPac AS17 column, automated eluent generation and potassium hydroxide gradient represents a new approach to the routine determination of inorganic anions in environmental waters. This new approach, which is a modification of Method 300.0, allows equivalent method performance with improved linearity, precision, and method detection limits. The AS17 column provides superior retention of fluoride from the column void volume and improved resolution from small organic acids, such as formate and acetate, compared to the AS4A column. Quantitative recoveries were obtained for all the common inorganic anions spiked into typical environmental waters using this new approach, and the Performance Based Measurement System Tier 1 method validation quality control acceptance criteria are well within the acceptable ranges defined by Method 300.0. In addition, the EG40 eluent generator eliminates the need to manually prepare eluents, increasing the level of automation and ease-of-use of the ion chromatography system. 相似文献
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78.
A. Poglitsch J. W. Beeman N. Geis R. Genzel M. Haggerty E. E. Haller J. Jackson M. Rumitz G. J. Stacey C. H. Townes 《International Journal of Infrared and Millimeter Waves》1991,12(8):859-884
FIFI is an imaging spectrometer with two or three Fabry-Perot interferometers (FPI) in series for airborne astronomical observations in the far-infrared range (=40...200m). It employs 5×5 arrays of photoconducting detectors and offers spectral resolutions as small as 2km/s. Resolution and bandwidth can be set over a wide range to match a variety of astronomical sources. Cryogenic optics minimizes thermal background radiation and provides for in-flight step tunable spatial resolution. At 158 m wavelength the background-limited NEP is 3 × 10-15W/Hz at 40 km/s resolution and with two FPI's; with three FPI's the expected NEP is 10-15WHz at 5 km/s resolution.The frequency-chopping mode of the high-resolution Fabry-Perot allows for line detection in extended objects. Absolute internal flux calibration ensures adequate flat fielding of the array elements. 相似文献
79.
Nomkoko TE Jackson GE Nakani BS Louw WK Zeevaart JR 《Dalton transactions (Cambridge, England : 2003)》2004,(5):741-749
The thermodynamic equilibria of copper(II), zinc(II), calcium(II) and gadolinium(III) with 3,3,9,9-tetramethyl-4,8-diazaundecane-2,10-dione dioxime (L1) have been studied at 25 degrees C and an ionic strength of 0.15 mol dm(-3). Copper and gadolinium form stable complexes with the ligand while the corresponding zinc species are more than 9 log units less stable. No complexes between calcium and the ligand were detected. The low binding strength of L1 towards zinc is attributed to the square-planar coordination geometry forced on the metal ion by the ligand as revealed by molecular mechanics calculations and molecular dynamics simulations. Speciation calculations, using a computer model of blood plasma, indicate that, despite the high concentration of zinc(II) and calcium(II) in vivo, L1 is able to increase the low-molecular-mass fraction of copper in plasma. Octanol/water partition coefficient of [CuL1H(-1)] indicates that although this species is largely hydrophilic, approximately 6% of the complex goes into the octanol phase and hence may promote dermal absorption of copper by the same amount. The dermal penetration rate is calculated to be 4.0 x 10(-4) mm h(-1). The [CuL1H(-1)] complex, which predominates at pH 7.4, is a poor mimic of native copper-zinc superoxide dismutase. Biodistribution experiments using the 64Cu-labelled [CuL1H(-1)] complex indicate an initial high uptake of this species in the liver followed by redistribution into muscle. Only a small amount is excreted through the urine. 相似文献
80.
A novel [Co(pentaamine)Cl](2+) complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2'-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl](2+) species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl(4)(2)(-) salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH](2+) is base catalyzed (k(OH) 0.70 M(-)(1) s(-)(1), 25 degrees C, I = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two -CH(2)- pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl](2+) and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD](2+) in OD(-) show that each act of deprotonation at the acidic methylene leads to loss of Cl(-). This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (k(H)/k(D) = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism. 相似文献