<i>J</i>‑Resonance Line Shape of Magnetic Field-Affected Reaction Yield Spectrum from Charge Recombination in a Linked Donor–Acceptor Dyad

Magnetic field effects (MFEs) allow detailed insight into spin conversion processes of radical pairs that are formed, for example, in all charge separation processes, and are supposed to play the key role in avian navigation. In this work, the MFE of charge recombination in the charge-separated state of a rigid donor–bridge–acceptor dyad was analyzed by a classical and a quantum theoretical model and represents a paradigm case of understanding spin chemistry with unprecedented detail. The MFE is represented by magnetic field-affected reaction yield (MARY) spectra that exhibit a sharp resonance, resulting from S/T level crossing as the Zeeman splitting equals twice the exchange interaction. Although in the classical kinetic model, the spin conversion processes between the four singlet and triplet substates are shown for the first time to obey an identical generalized energy dependence, quantum theory proves that the MARY resonance line is composed of relaxation, coherent hyperfine induced spin mixing, and S/T dephasing contributions

Integrated Workflow for Structural Proteomics Studies Based on Cross-Linking/Mass Spectrometry with an MS/MS Cleavable Cross-Linker

Cross-linking combined with mass spectrometry (MS) has evolved as an alternative strategy in structural biology for characterizing three-dimensional structures of protein assemblies and for mapping protein–protein interactions. Here, we describe an integrated workflow for an automated identification of cross-linked products that is based on the use of a tandem mass spectrometry (MS/MS) cleavable cross-linker (containing a 1,3-bis-(4-oxo-butyl)-urea group, BuUrBu) generating characteristic doublet patterns upon fragmentation. We evaluate different fragmentation methods available on an Orbitrap Fusion mass spectrometer for three proteins and an <i>E. coli</i> cell lysate. An updated version of the dedicated software tool MeroX was employed for a fully automated identification of cross-links. The strength of our cleavable cross-linker is that characteristic patterns of the cross-linker as well as backbone fragments of the connected peptides are already observed at the MS/MS level, eliminating the need for conducting MS³ or sequential CID (collision-induced dissociation)- and ETD (electron transfer dissociation)-MS/MS experiments. This makes our strategy applicable to a broad range of mass spectrometers with MS/MS capabilities. For purified proteins and protein complexes, our workflow using CID-MS/MS acquisition performs with high confidence, scoring cross-links at 0.5% false discovery rate (FDR). The cross-links provide structural insights into the intrinsically disordered tetrameric tumor suppressor protein p53. As a time-consuming manual inspection of cross-linking data is not required, our workflow will pave the way for making the cross-linking/MS approach a routine technique for structural proteomics studies

Integrated Workflow for Structural Proteomics Studies Based on Cross-Linking/Mass Spectrometry with an MS/MS Cleavable Cross-Linker

Cross-linking combined with mass spectrometry (MS) has evolved as an alternative strategy in structural biology for characterizing three-dimensional structures of protein assemblies and for mapping protein–protein interactions. Here, we describe an integrated workflow for an automated identification of cross-linked products that is based on the use of a tandem mass spectrometry (MS/MS) cleavable cross-linker (containing a 1,3-bis-(4-oxo-butyl)-urea group, BuUrBu) generating characteristic doublet patterns upon fragmentation. We evaluate different fragmentation methods available on an Orbitrap Fusion mass spectrometer for three proteins and an <i>E. coli</i> cell lysate. An updated version of the dedicated software tool MeroX was employed for a fully automated identification of cross-links. The strength of our cleavable cross-linker is that characteristic patterns of the cross-linker as well as backbone fragments of the connected peptides are already observed at the MS/MS level, eliminating the need for conducting MS³ or sequential CID (collision-induced dissociation)- and ETD (electron transfer dissociation)-MS/MS experiments. This makes our strategy applicable to a broad range of mass spectrometers with MS/MS capabilities. For purified proteins and protein complexes, our workflow using CID-MS/MS acquisition performs with high confidence, scoring cross-links at 0.5% false discovery rate (FDR). The cross-links provide structural insights into the intrinsically disordered tetrameric tumor suppressor protein p53. As a time-consuming manual inspection of cross-linking data is not required, our workflow will pave the way for making the cross-linking/MS approach a routine technique for structural proteomics studies

Investigations of the Degenerate Intramolecular Charge Exchange in Symmetric Organic Mixed Valence Compounds: Solvent Dynamics of Bis(triarylamine)paracyclophane Redox Systems

Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and intermolecular electron transfer properties is crucial to the application and optimization of organic hole conductors. Here, we report on the degenerate intramolecular electron exchange reactions of several purely organic mixed valence compounds based on the bis­(triarylamine)­paracyclophane structural unit, which are archetypical molecular wires. Different bridging moieties are compared, and the foremost impact of the solvent environment on the rate of electron transfer is demonstrated. Comparing the rate constants found for many different solvents, we find that surprisingly the electron transfer reaction is limited by the solvent dynamic effect and not strongly impacted by the peculiarities of the bridging moiety, a finding which was not anticipated for this type of long-range, thermally activated intramolecular charge transfer from previous studies. Rate constants are measured by dynamic electron paramagnetic resonance spectroscopy. Our insight was possible using various solvents spanning a wide range of longitudinal relaxation times (0.24 ps ≤ τ_L ≤ 516 ps) and Pekar factors (0.298 ≤ γ ≤ 0.526)

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-<i>tert</i>-butyliminoborane and Alkynes

Di-<i>tert</i>-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl­(P<i>i</i>Pr₃)₂]₂, which gives a rhodium η⁴-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B–C bond coupling en route to the rhodium η⁴-1,2-azaborete complex and the selective oxidative cleavage of the B–N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene

Forest practitioners’ requirements for remote sensing-based canopy height, wood-volume, tree species, and disturbance products

Despite decades of development, the uptake of remote sensing-based information products in the forestry sector is still lagging behind in central and southern Europe. This may partly relate to a mismatch of the developed remote sensing products and the requirements of potential users. Here, we present the results of a questionnaire survey in which we questioned 355 forest practitioners from eight central and southern European countries. We aimed to learn about forest practitioners' technical requirements for four remote sensing-based information products, including information on tree species, canopy height, wood volume/biomass, and forest disturbances. We asked for practitioners’ preferences with respect to thematic and spatial detail as well as the maximal acceptable error and the temporal frequency with which the information layers would be needed. We then examined whether the education, age, and professional background affect the requirements. Preferences with respect to spatial and thematic detail were comparably diverse while more homogenous patterns could be observed for demands with respect to errors and temporal frequency. Our results indicate that for some information products such as canopy height maps, existing remote sensing technology, and workflows can match all demands of practitioners. Remotely sensed information on forest disturbances partly fulfils the demands of the practitioners while for products related to tree species and wood volume/biomass the level of thematic detail and the accuracy of the products demanded by practitioners in central and southern Europe is not yet fully matched. We found no statistically significant differences between the demographic groups examined. The findings of this study improve our understanding of matches and mismatches of the technical requirements of practitioners for remote sensing-based information product

Chelate Cooperativity and Spacer Length Effects on the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes

Homo- and heterodivalent crown-ammonium pseudorotaxanes with different spacers connecting the two axle ammonium binding sites have been synthesized and characterized by NMR spectroscopy and ESI mass spectrometry. The homodivalent pseudorotaxanes are investigated with respect to the thermodynamics of divalent binding and to chelate cooperativity. The shortest spacer exhibits a chelate cooperativity much stronger than that of the longer spacers. On the basis of crystal structure, this can be explained by a noninnocent spacer, which contributes to the binding strength in addition to the two binding sites. Already very subtle changes in the spacer length, i.e., the introduction of an additional methylene group, cause substantial changes in the magnitude of cooperative binding as expressed in the large differences in effective molarity. With a similar series of heterodivalent pseudorotaxanes, the spacer effects on the barrier for the intramolecular threading step has been examined with the result that the shortest spacer causes a strained transition structure and thus the second binding event occurs slower than that of the longer spacers. The activation enthalpies and entropies show clear trends. While the longer spacers reduce the enthalpic strain that is present in the transition state for the shortest member of the series, the longer spacers become entropically slightly more unfavorable because of conformational fixation of the spacer chain during the second binding event. These results clearly show the noninnocent spacers to complicate the analysis of multivalent binding. An approximate description which considers the binding sites to be connected just by a flexible chain turns out to be more a rough approximation than a good model. The second conclusion from the results presented here is that multivalency is expressed in both the thermodynamics and the kinetics in different ways. A spacer optimized for strong binding is suboptimal for fast pseudorotaxane formation