Caging the mercurous ion: a tetradentate tripodal nitrogen ligand enhances stability and J(1H199Hg)

The dimeric mercurous ion has been encapsulated by a pair of the tetradentate tripodal nitrogen ligands tris[(2-(6-methylpyridyl))methyl]amine (TLA). The complex [Hg₂(TLA)₂](ClO₄)₂ (1) was isolated directly from an acetonitrile solution of Hg(ClO₄)₂ 3H₂O and TLA. Complex 1 crystallizes in the triclinic space group with a = 10.537(2) Å, b = 10.751(2) Å, c = 10.907(2) Å, = 75.20(3), = 73.73(3), = 75.73(3), and Z = 1. The cation is located an inversion center. The Hg–Hg and Hg–N_amine bond distances are 2.5469(8) and 2.297(6) Å, respectively, and the average Hg–N_pyridyl bond length is 2.75(7) Å. Complex 1 was stable indefinitely in acetonitrile-d ₃ solution, permitting detection of 13 and 22 Hz heteronuclear couplings between the Hg(I) ions and the methylene protons of the ligand. Comparisons with the structures and spectroscopic properties of related mercuric and mercurous complexes are made.     

Sequence and side-chain specific photofragment (193 nm) ions from protonated substance P by matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Photodissociation at 193 nm (6. 43 eV) of the protonated substance-P, [M + H]⁺ ions, in a delayed extraction matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometer, is reported. The photofragment ion spectrum of substance P contains a complete series of a-type fragment ions and abundant side-chain cleavage ions. This article focuses on the utility of MALDI-TOF photodissociation for peptide sequencing.     

A new estra‐1,3,5(10)‐triene‐3,17β‐diol solvate: estradiol–methanol–water (3/2/1)

The title solvate of the steroid 17β‐estradiol (E₂) with methanol and water, C₁₈H₂₄O₂·0.67CH₄O·0.33H₂O, is the first E₂ derivative to contain three crystallographically independent mol­ecules in the asymmetric unit. The three steroid mol­ecules, along with two methanol mol­ecules and a water mol­ecule, create a three‐dimensional hydrogen‐bonded system. Three‐sided columns are formed, with the estradiol mol­ecules aligned lengthwise parallel to (101), and joined by solvent mol­ecules at both hydro­philic ends. The three estradiol mol­ecules differ slightly in their ring‐bowing angles, i.e. the angle between the mean plane of the A ring and that of the BCD ring; this angle ranges from 7.1 to 12.2°.     

Thermal Expansion of HMX

Abstract  

Thermal expansion of the β-phase of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (β-HMX, C₄H₈N₈O₈) was studied in the temperature range of −150 to 30 °C. β-HMX is monoclinic (a = 6.5255(10) ?, b = 11.0369(18) ?, c = 7.3640(12) ?, and β = 102.67(1)°), space group P2₁/n. On cooling from room temperature to −150 °C the crystal under goes an anisotropic contraction with the a-axis virtually unchanged while the b and c axes contract by approximately 1.8 and 0.6% respectively. The disproportionate change in the a and c axes results in approximately a 0.4% change in the β angle. Despite the large differences in expansion along the different axes no phase change was observed.     

Fluorescence Correlation Spectroscopy with Photobleaching Correction in Slowly Diffusing Systems

Fluorescence correlation spectroscopy (FCS) is a powerful tool to quantitatively study the diffusion of fluorescently labeled molecules. It allows in principle important questions of macromolecular transport and supramolecular aggregation in living cells to be addressed. However, the crowded environment inside the cells slows diffusion and limits the reservoir of labeled molecules, causing artifacts that arise especially from photobleaching and limit the utility of FCS in these applications. We present a method to compute the time correlation function from weighted photon arrival times, which compensates computationally during the data analysis for the effect of photobleaching. We demonstrate the performance of this method using numerical simulations and experimental data from model solutions. Using this technique, we obtain correlation functions in which the effect of photobleaching has been removed and in turn recover quantitatively accurate mean-square displacements of the fluorophores, especially when deviations from an ideal Gaussian excitation volume are accounted for by using a reference calibration correlation function. This allows quantitative FCS studies of transport processes in challenging environments with substantial photobleaching like in living cells in the future.     

Evaluation of a multiple spin- and gradient-echo (SAGE) EPI acquisition with SENSE acceleration: Applications for perfusion imaging in and outside the brain

Perfusion-based changes in MR signal intensity can occur in response to the introduction of exogenous contrast agents and endogenous tissue properties (e.g. blood oxygenation). MR measurements aimed at capturing these changes often implement single-shot echo planar imaging (ssEPI). In recent years ssEPI readouts have been combined with parallel imaging (PI) to allow fast dynamic multi-slice imaging as well as the incorporation of multiple echoes. A multiple spin- and gradient-echo (SAGE) EPI acquisition has recently been developed to allow measurement of transverse relaxation rate (R₂ and R₂^?) changes in dynamic susceptibility contrast (DSC)-MRI experiments in the brain. With SAGE EPI, the use of PI can influence image quality, temporal resolution, and achievable echo times. The effect of PI on dynamic SAGE measurements, however, has not been evaluated. In this work, a SAGE EPI acquisition utilizing SENSE PI and partial Fourier (PF) acceleration was developed and evaluated. Voxel-wise measures of R₂ and R₂^? in healthy brain were compared using SAGE EPI and conventional non-EPI multiple echo acquisitions with varying SENSE and PF acceleration. A conservative SENSE factor of 2 with PF factor of 0.73 was found to provide accurate measures of R₂ and R₂^? in white (WM) (r_R2 = [0.55–0.79], r_R2? = [0.47–0.71]) and gray (GM) matter (r_R2 = [0.26–0.59], r_R2? = [0.39–0.74]) across subjects. The combined use of SENSE and PF allowed the first dynamic SAGE EPI measurements in muscle, with a SENSE factor of 3 and PF factor of 0.6 providing reliable relaxation rate estimates when compared to multi-echo methods. Application of the optimized SAGE protocol in DSC-MRI of high-grade glioma patients provided T₁ leakage-corrected estimates of CBV and CBF as well as mean vessel diameter (mVD) and simultaneous measures of DCE-MRI parameters K^trans and v_e. Likewise, application of SAGE in a muscle reperfusion model allowed dynamic measures of R₂′, a parameter that has been shown to correlate with muscle oxy-hemoglobin saturation.     

A high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform

We report the generation and characterization of a new high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform. Oxygen-atom transfer to [(tpa(Mes))Fe(II)](-) (tpa(Ar) = tris(5-arylpyrrol-2-ylmethyl)amine) in acetonitrile solution affords the Fe(III)-alkoxide product [(tpa(Mes2MesO))Fe(III)](-) resulting from intramolecular C-H oxidation with no observable ferryl intermediates. In contrast, treatment of the phenyl derivative [(tpa(Ph))Fe(II)](-) with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)-oxo complex [(tpa(Ph))Fe(IV)(O)](-) that has been characterized by a suite of techniques, including mass spectrometry as well as UV-vis, FTIR, M?ssbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide signatures for the iron-oxo chromophore, and M?ssbauer and XAS measurements establish the presence of an Fe(IV) center. Moreover, the Fe(IV)-oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity studies show that the high-spin ferryl tpa(Ph) complex is capable of mediating intermolecular C-H oxidation as well as oxygen-atom transfer chemistry.     

Indication of reactor ν(e) disappearance in the Double Chooz experiment

The Double Chooz experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. An observed-to-predicted ratio of events of 0.944±0.016(stat)±0.040(syst) was obtained in 101 days of running at the Chooz nuclear power plant in France, with two 4.25 GW(th) reactors. The results were obtained from a single 10 m(3) fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 flux measurement after correction for differences in core composition. The deficit can be interpreted as an indication of a nonzero value of the still unmeasured neutrino mixing parameter sin(2)2θ(13). Analyzing both the rate of the prompt positrons and their energy spectrum, we find sin(2)2θ(13)=0.086±0.041(stat)±0.030(syst), or, at 90% C.L., 0.017相似文献   

A and the vanishing topology of discriminants

Summary Suppose thatf: ⁿ , 0 ^p , 0 is finitely -determined withnp. We define a Milnor fiber for the discriminant off; it is the discriminant of a stabilization off. We prove that this discriminant Milnor fiber has the homotopy type of a wedge of spheres of dimensionp–1, whose number we denote byµ (f). One of the main theorems of the paper is a = type result: if (n, p) is in the range of nice dimensions in the sense of Mather, then -codium,with equality iff is weighted homogeneous. Outside the nice dimensions we obtain analogous formulae with correction terms measuring the presence of unstable but topologically stable germs in the stabilization. These results are further extended to nonlinear sections of free divisors.Oblatum 15-VIII-1990Partially supported by a grant from the National Science Foundation and a Fullbright Fellowship