Reducing ghosting due to k-space discontinuities in fast spin echo (FSE) imaging by a new combination of k-space ordering and parallel imaging

In multi-echo imaging sequences like fast spin echo (FSE), the point spread function (PSF) in the phase encoding direction contains significant secondary peaks (sidebands). This is due to discontinuities in adjacent k-space data obtained at different echo times caused by T₂ decay, and leads to ghosting and hence reduced image quality. Recently, utilising multiple coils for signal reception has become the standard configuration for MR systems due to the additional flexibility that parallel imaging (PI) methods can provide. PI methods generally obtain more data than is required to reconstruct an image. Here, this redundancy in information is exploited to reduce discontinuity-related ghosting in FSE imaging. Adjacent phase encoded k-space lines are acquired at different echo times alternately in the regions of discontinuity (called ‘feathering’). This moves the resulting ghost artefacts to the edges of the field of view. This property of the ghost then makes them amenable to removal using PI methods. With ‘feathered’ array coil data it is possible to reconstruct data over the region of the discontinuity from both echo times. By combining this data, a significant reduction in ghosting can be achieved. We show this approach to be effective through simulated and acquired MRI data.     

Electron paramagnetic resonance (EPR) study of γ-radiation-induced radicals in 1,3,5-trithiane and its derivatives

Radicals formed in γ-irradiated 1,3,5-trithiane (TT) and its three derivatives, α- and β-2,4,6-trimethyl-1,3,5-trithiane (α-TMT and β-TMT), and 2,4,6-trimethyl-2,4,6-triphenyl-1,3,5-trithiane (TMTPT), were studied by the electron paramagnetic resonance (EPR) method in the solid state. The sulfur radical cations (>S^+•) were identified in all compounds at 77 K. In TT and its two derivatives, α-TMT and β-TMT, the >S^+• decay via deprotonation-forming C-centered radicals. Further increase of temperature up to 293 K results in the appearance of thiyl-type radicals (RS^•). In TMTPT, the >S^+• are stable up to 250 K. They formed the intermolecularly three-electron-bonded dimeric radical cations (S∴S)⁺ while RS^• radicals were not observed. Some of the radical assignments and their EPR parameters (g and a hyperfine splittings) obtained support from the DFT calculations.     

Crystal Structure of Bis(5,17-dihydroxy-3,7,15,19,25,26-hexa-azatricyclo(19.3.1.19,13)hexacosa-1(25),2,7,9(26),10,12,14,19,21,23-decaene-μ1,2-perchlorato barium) diperchlorate: a Schiff-Base Macrocyclic Complex Derived from 2,6-Diformylpyridine and 1,3-Diaminopropan-2-ol

Abstract  

The title complex, [Ba(L1)(μ_1,2-ClO₄)]₂(ClO₄)₂ (where L1 = 5,17-dihydroxy-3,7,15,19,25,26-hexa-azatricyclo(19.3.1.19,13)hexacosa-1(25),2,7,9(26),10,12,14,19,21,23-decaene), was synthesised by a Schiff base condensation of 2,6-diformylpyridine with 1,3-diaminopropan-2-ol in dry methanol using barium(II) as template ion. The complex was characterised by IR, ¹H NMR, mass spectrometry, elemental analysis and X-ray crystallography. The structure was solved in the triclinic space group P [`\text1] {\bar{\text{1}}} with a = 8.4587(9), b = 8.7360(9), c = 17.7722(19) ?, α = 79.786(2), β = 88.780(2), γ = 83.048(2)°, V = 1283.0(2) ?³, Z = 1 and R1 = 0.0433. The macrocycle is folded and the complex dimerises via perchlorate anions which bridge the barium ions, and this interaction is supported by hydrogen bonding between neighbouring alcohol groups. The dimers are further linked into chains via intermolecular O–H···OClO₃ hydrogen bonds.     

Photoreversible DNA condensation using light-responsive surfactants

A means to control DNA compaction with light illumination has been developed using the interaction of DNA with a photoresponsive cationic surfactant. The surfactant undergoes a reversible photoisomerization upon exposure to visible (trans isomer, more hydrophobic) or UV (cis isomer, more hydrophilic) light. As a result, surfactant binding to DNA and the resulting DNA condensation can be tuned with light. Dynamic light scattering (DLS) measurements were used to follow lambda-DNA compaction from the elongated-coil to the compact globular form as a function of surfactant addition and light illumination. The results reveal that compaction occurs at a surfactant-to-DNA base pair ratio of approximately 7 under visible light, while no compaction is observed up to a ratio of 31 under UV light. Upon compaction, the measured diffusion coefficient increases from a value of 0.6 x 10(-8) cm2/s (elongated coil with an end-to-end distance of 1.27 microm) to a value of 1.7 x 10(-8) cm2/s (compact globule with a hydrodynamic radius of 120 nm). Moreover, the light-scattering results demonstrate that the compaction process is completely photoreversible. Fluorescence microscopy with T4-DNA was used to further confirm the light-scattering results, allowing single-molecule detection of the light-controlled coil-to-globule transition. These structural studies were combined with absorbance and fluorescence spectroscopy of crystal violet in order to elucidate the binding mechanism of the photosurfactant to DNA. The results indicate that both electrostatic and hydrophobic forces are important in the compaction process. Finally, a DNA-photosurfactant-water phase diagram was constructed to examine the effects of both DNA and surfactant concentration on DNA compaction. The results reveal that precipitation, which occurs during the latter stages of condensation, can also be reversibly controlled with light illumination. The combined results clearly show the ability to control the interaction between DNA and the complexing agent and, therefore, DNA condensation with light.     

Optical, redox, and NLO properties of tricyanovinyl oligothiophenes: comparisons between symmetric and asymmetric substitution patterns

A series of tricyanovinyl (TCV)-substituted oligothiophenes was synthesized and investigated with a number of physical methods including UV/Vis, IR, and Raman spectroscopy, nonlinear optical (NLO) measurements, X-ray diffraction, and cyclic voltammetry. Mono- or disubstituted oligomers were prepared by the reaction of tetracyanoethylene with mono- or dilithiated oligomers. The comparative effects of the symmetric and asymmetric substitutions in the electronic and molecular properties have been addressed. These oligomers display dramatic reductions in both their optical and electrochemical band gaps in comparison with unsubstituted molecules. The analysis of the electronic properties of the molecules was assisted by density functional theory calculations, which are in excellent agreement with the experimental data. TCV substitution influences the energies of the frontier orbitals, especially with respect to the stabilization of LUMO orbitals. X-ray structural characterization of a monosubstituted oligomer exhibits pi-stacking with favorable intermolecular interactions. NLO results agree with the role of the intramolecular charge-transfer feature in the asymmetric samples. These results furthermore exalt the role of conformational flexibility in the disubstituted compounds and reveal an unexpected nonlinear optical activity for symmetric molecules. Regarding the electronic structure, the interpretation of the vibrational data reflects the balanced interplay between aromatic and quinoid forms, finely tuned by the chain length and substitution pattern. The electronic and structural properties are consistent with the semiconducting properties exhibited by these materials in thin film transistors (TFTs).     

Electrospray ionization quadrupole time-of-flight tandem mass spectrometric analysis of hexamethylenediamine-modified maltodextrin and dextran

A combined methodology for obtaining at the preparative scale and characterization by nanoelectrospray ionization (nanoESI) quadrupole time-of-flight (QTOF) mass spectrometry (MS) and tandem MS (MS/MS) of linear polysaccharides modified at the reducing end is presented. Two polydisperse maltodextrins (1000 and 3000 Da) and a high molecular weight polydisperse dextran (6000 Da) were coupled with hexamethylenediamine (HMD). The coupling products were analyzed by nanoESI-QTOF-MS in the positive ion mode and MS/MS using collision-induced dissociation (CID) at low energies. In the HMD-M1000 mixture, the polysaccharide chains containing from 2 to 8 Glc residues were detected, while in HMD-M3000 we identified a complete series of chains containing from 8 to 21 Glc moieties. The employed ESI conditions enhanced the detection of chains with up to 46 Glc residues in the HMD-D6000 sample. By optimized MS/MS, HMD-modified polysaccharides of 3, 4, 5, 12 and 46 degrees of polymerization yielded product ion spectra exhibiting the whole set of Y- and B-fragment ions. The MS structural data were obtained within a few minutes of signal acquisition, with a sample consumption situating the analysis sensitivity in the picomolar range.     

1JCH correlates with alcohol hydrogen bond strength

The strength of the H-bond donation by alcohols is reflected in the carbon-hydrogen bond of the H-C-O-H functional group. The one-bond 13C-1H spin-spin coupling constant of hexafluoroisopropanol (HFIP) correlates with the strength of the H-bond in various HFIP-amine complexes with a slope of approximately -0.2 Hz in 1JCH per approximately 1 kJ mol(-1) increase in the H-bond enthalpy. The decrease in 1JCH is attributed to an increased overlap of the H-bonding sigma orbital with the antibonding sigma orbitals of the vicinal C-H bonds.     

The effects of electron and chemical Ionization Modes on the MS Profiling of Whole Bacteria

Free fatty acid profiling of whole bacteria [Francisella tularensis, Brucella melitensis, Yersinia pestis, Bacillus anthracis (vegetative and sporulated), and Bacillus cereus] was carried out with direct probe mass spectrometry under 70-eV electron ionization (EI) and isobutane chemical ionization in both the positive (CI+) and negative modes (CI-). Electron ionization produced spectra that contained molecular ions and fragment ions from various free fatty acids. Spectra acquired with isobutane chemical ionization in the positive mode yielded molecular ions of free fatty acids as well as ions from other bacterial compounds not observed under EI conditions. Spectra obtained with negative chemical ionization did not contain as much taxonomic information as EI or CI+; however, some taxonomically significant compounds such as dipicolinic acid and poly(3-hydroxybutyrate) did produce negative ions. All ionization modes yielded spectra that could separate the bacteria by Gram-type when observed with principle components analysis (PCA). Chemical ionization in the positive ion mode produced the greatest amount of differentiation between the four genera of bacteria when the spectra where examined by PCA.     

Improving Li-ion battery charge rate acceptance through highly ordered hierarchical electrode design

In Li-ion technology, increasing electrode loading (thickness) is one approach to improve performance; however, this approach typically compromises power density and safety. To achieve the goal of decoupling energy and power density, a novel electrode architecture is proposed. The electrode design enhances uniform ionic current, especially in thick electrodes. A highly ordered and hierarchical (HOH) graphite anode concept was designed, fabricated, and tested for efficacy. The HOH electrodes consisted of ordered arrays of macro-scale line-of-sight linear channels made through laser ablation. SEM and Raman spectroscopy demonstrated that laser ablation is a feasible approach to fabricate HOH electrodes without affecting the graphite anode chemistry, respectively. A 65–120% improvement in charge rate acceptance (5.5 mAh/cm²) was achieved in the HOH electrodes compared to conventional electrodes. A restricted diffusion direct current polarization test determined that the HOH design improved ionic flow throughout porous electrodes. Altogether, the results of this study suggest that improved charge rate acceptance can be achieved by engineering electrode porosity to mitigate the effects of concentration polarization in high energy density graphite anodes. These findings can facilitate the development of higher energy and power density Li-ion batteries, while improving resilience against Li plating under severe charge conditions.