Mechanistic investigations reveal that dibromobimane extrudes sulfur from biological sulfhydryl sources other than hydrogen sulfide

Hydrogen sulfide (H₂S) has emerged as an important biological signaling molecule in the last decade. During the growth of this field, significant controversy has arisen centered on the physiological concentrations of H₂S. Recently, a monobromobimane (mBB) method has been developed for the quantification of different biologically-relevant sulfide pools. Based on the prevalence of the mBB method for sulfide quantification, we expand on this method to report the use of dibromobimane (dBB) for sulfide quantification. Reaction of H₂S with dBB results in formation of highly-fluorescent bimane thioether (BTE), which is readily quantifiable by HPLC. Additionally, the reaction of sulfide with dBB to form BTE is significantly faster than the reaction of sulfide with mBB to form sulfide dibimane. Using the dBB method, BTE levels as low as 0.6 pM can be detected. Upon use of the dBB method in wild-type and CSE^–/– mice, however, dBB reports significantly higher sulfide levels than those measured using mBB. Further investigation revealed that dBB is able to extract sulfur from other sulfhydryl sources including thiols. Based on mechanistic studies, we demonstrate that dBB extracts sulfur from thiols with α- or β-hydrogens, thus leading to higher BTE formation than from sulfide alone. Taken together, the dBB method is a highly sensitive method for H₂S but is not compatible for use in studies in which other thiols are present.     

Insights into LiAlH4 Catalyzed Imine Hydrogenation

Commercial LiAlH₄ can be used in catalytic quantities in the hydrogenation of imines to amines with H₂. Combined experimental and theoretical investigations give deeper insight in the mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH₄ is exchanged for Na or K. Exchanging Al for B or Ga also led to dramatically reduced activities. This indicates a heterobimetallic mechanism in which cooperation between Li and Al is crucial. Potential intermediates on the catalytic pathway have been isolated from reactions of MAlH₄ (M=Li, Na, K) and different imines. Depending on the imine, double, triple or quadruple imine insertion has been observed. Prolonged reaction of LiAlH₄ with PhC(H)=NtBu led to a side-reaction and gave the double insertion product LiAlH₂[N]₂ ([N]=N(tBu)CH₂Ph) which at higher temperature reacts further by ortho-metallation of the Ph ring. A DFT study led to a number of conclusions. The most likely catalyst for hydrogenation of PhC(H)=NtBu with LiAlH₄ is LiAlH₂[N]₂. Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal mol⁻¹ (ΔH). The rate-determining step is hydrogenolysis of LiAlH[N]₃ with H₂ with a barrier of +29.2 kcal mol⁻¹. In agreement with experiment, replacing Li for Na (or K) and Al for B (or Ga) led to higher calculated barriers. Also, the AlH₄⁻ anion showed very high barriers. Calculations support the experimentally observed effects of the imine substituents at C and N: the lowest barriers are calculated for imines with aryl-substituents at C and alkyl-substituents at N.     

Soluble Congeners of Prior Insoluble Shape-Persistent Imine Cages

One of the most applied reaction types to synthesize shape-persistent organic cage compounds is the imine condensation reaction and it is assumed that the formed cages are thermodynamically controlled products due to the reversibility of the imine condensation. However, most of the synthesized imine cages reported are formed as precipitate from the reaction mixture and therefore rather may be kinetically controlled products. There are even examples in literature, where resulting cages are not soluble at all in common organic solvents to characterize or study their formation by NMR spectroscopy in solution. Here, a triptycene triamine containing three solubilizing n-hexyloxy chains has been used to synthesize soluble congeners of prior insoluble cages. This allowed us to study the formation as well as the reversibility of cage formation in solution by investigating exchange of building blocks between the cages and deuterated derivatives thereof.     

Theoretical study of the structural and energetic properties of platinum clusters with up to 60 atoms

Structural Chemistry - Applying a theoretical approach that combines an efficient and fast global optimization based on genetic algorithms (GA) to search in structure space and the parameterized...     

Application of Atmospheric Solids Analysis Probe Mass Spectrometry (ASAP-MS) in Petroleomics: Analysis of Condensed Aromatics Standards,Crude Oil,and Paraffinic Fraction

Atmospheric solids analysis probe mass spectrometry (ASAP-MS) is a powerful tool for analysis of solid and liquid samples. It is an excellent alternative for crude oil analysis without any sample preparation step. Here, ASAP-MS in positive ion mode, ASAP(+)-MS, has been optimized for analysis of condensed aromatics (CA) standards, crude oil, and paraffinic fraction samples using a Synapt G2-S HDMS. Initially, two methodologies were used to access the chemical composition of samples: (1) using a temperature gradient varying from 150 to 600 °C at a heating rate of 150 °C min^–1, and (2) with constant temperature of 300 and 400 °C. ASAP(+)-MS ionized many compounds with a typical petroleum profile, showing a greater signals range of m/z 250–1300 and 200–1400 for crude oil and paraffin samples, respectively. Such performance, mainly related to the detection of high molecular weight compounds (>1000 Da), is superior to that of other traditional ionization sources, such as ESI, APCI, DART, and DESI. Additionally, the CA standards were identified in both forms: radicals, [M]^+?, and protonated cations, [M + H]⁺, with minimum fragmentation. Therefore, ASAP was more efficient in accessing the chemical composition of nonpolar and polar compounds. It is promising in its application with ultrahigh resolution MS instruments, such as FT-ICR MS and Orbitrap, since molecular formulas with greater resolution and mass accuracy (<1 ppm) would be assigned.

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Analysis of moisture sorption in lyocell-polypropylene composites

The preparation of composites by thermoforming of intermingled fibre slivers is an efficient method to receive high performance and lightweight materials. Cellulosic fibres have benefits like low density and sustainability but the sorption of water due to the high hydrophilicity of the cellulose requires attention. The swelling of the wet fibres changes the fibre-matrix adhesion and as a consequence, the mechanical strength of the composite is influenced negatively. In this study, the thermoplastic polypropylene was combined with lyocell fibres as reinforcement. Moisture sorption isotherms of cellulose/polypropylene composites were recorded as function of relative humidity. Additionally, the specific surface area was analysed by the Brunauer–Emmett–Teller model. It has been found, that the moisture sorption is influenced by the polypropylene (PP) ratio in the composites. At 60% relative humidity the moisture uptake of the lyocell fibres was reduced from 10.8 to 5.8% for lyocell embedded in a composite with 50% polypropylene. Besides the hysteresis between moisture sorption/desorption cycles was found to be proportional to the increased content of PP. The “Parallel Exponential Kinetics” (PEK) model was used to analyse the kinetics of moisture sorption of these composites in more detail. With the help of the PEK model the sorption/desorption kinetics were described by a fast and slow moisture sorption/desorption process. The capacity for rapid moisture sorption is reduced by the formation of PP layers on the lyocell surface. The share of slow moisture sorption increased with increasing PP content in the composite. The results support understanding of the interaction of water with cellulose containing composites.     

Time‐Resolved Energetics of Photoprocesses in Prokaryotic Phytochrome‐Related Photoreceptors

Time‐resolved photoacoustics (PA) is uniquely able to explore the energy landscape of photoactive proteins and concomitantly detects light‐induced volumetric changes (ΔV) accompanying the formation and decay of transient species in a time window between ca. 20 ns and 5 μs. Here, we report PA measurements on diverse photochromic bilin‐binding photoreceptors of prokaryotic origin: (1) the chromophore‐binding GAF3 domain of the red (R)/green (G) switching cyanobacteriochrome 1393 (Slr1393g3) from Synechocystis; (2) the red/far red (R/FR) Synechocystis Cph1 phytochrome; (3) full‐length and truncated constructs of Xanthomonas campestris bacteriophytochrome (XccBphP), absorbing up to the NIR spectral region. In almost all cases, photoisomerization results in a large fraction of energy dissipated as heat (up to 90%) on the sub‐ns scale, reflecting the low photoisomerization quantum yield (<0.2). This “prompt” step is accompanied by a positive ΔV₁ = 5–12.5 mL mol^?1. Formation of the first intermediate is the sole process accessible to PA, with the notable exception of Slr1393g3‐G for which ΔV₁ = +4.5 mL mol^?1 is followed by a time‐resolved, energy‐conserving contraction ΔV₂ = ?11.4 mL mol^?1, τ₂ = 180 ns at 2.4°C. This peculiarity is possibly due to a larger solvent occupancy of the chromophore cavity for Slr1393g3‐G.