Hydrogen bis[tris(4‐fluorophenyl)phosphane oxide] triiodide

In the title compound, C₃₆H₂₅F₆O₂P₂⁺·I₃⁻, hydrogen‐bonded [{(p‐FC₆H₄)₃PO}₂H]⁺ dimers assemble along the crystallographic c axis to form channels that house extended chains of triiodide anions. Although the I—I bond lengths of 2.9452 (14) and 2.9023 (15) Å are typical, the inter‐ion I...I distance of 3.5774 (10) Å is unusually short. A posteriori modelling of nonmerohedral twinning about (100) has been only partially successful, achieving a reduction in the maximum residual electron density from 5.28 to 3.24 e Å⁻³. The inclusion of two low‐occupancy I‐atom sites (total 1.7%), which can be interpreted as translational disorder of the triiodide anions along the channels, reduced the maximum residual electron density to 2.03 e Å⁻³. The minor fractional contribution volume of the nonmerohedral twin domains refined to 0.24 and simultaneous refinement of the inversion twin domains showed the crystal to be a 0.5:0.5 inversion twin.     

Label Scrambling During CID of Covalently Labeled Peptide Ions

Covalent labeling along with mass spectrometry is finding more use as a means of studying the higher order structure of proteins and protein complexes. Diethylpyrocarbonate (DEPC) is an increasingly used reagent for these labeling experiments because it is capable of modifying multiple residues at the same time. Pinpointing DEPC-labeled sites on proteins is typically needed to obtain more resolved structural information, and tandem mass spectrometry after protein proteolysis is often used for this purpose. In this work, we demonstrate that in certain instances, scrambling of the DEPC label from one residue to another can occur during collision-induced dissociation (CID) of labeled peptide ions, resulting in ambiguity in label site identity. From a preliminary study of over 30 labeled peptides, we find that scrambling occurs in about 25% of the peptides and most commonly occurs when histidine residues are labeled. Moreover, this scrambling appears to occur more readily under non-mobile proton conditions, meaning that low charge-state peptide ions are more prone to this reaction. For all peptides, we find that scrambling does not occur during electron transfer dissociation, which suggests that this dissociation technique is a safe alternative to CID for correct label site identification. Graphical Abstract

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InspIRED by Nature: NADPH‐Dependent Imine Reductases (IREDs) as Catalysts for the Preparation of Chiral Amines

Imine reductases (IREDs) are NADPH‐dependent oxidoreductases that catalyse the asymmetric reduction of cyclic prochiral imines to amines, with excellent stereoselectivity. Since their discovery, stereocomplementary IREDs have been applied to the production of both (S) and (R) cyclic secondary amines, and the expansion in gene sequences recently identified has hinted at new substrate ranges that extend into acyclic imines and even suggest the possibility of asymmetric reductive amination from suitable ketone and amine precursors. Structural studies of various IREDs are beginning to reveal the complexities inherent in determining substrate range, stereoselectivity and mechanism in these enzymes, which represent a valuable emerging addition to the toolbox of available biocatalysts for chiral amine production.     

Nanotome cluster bombardment to recover spatial chemistry after preparation of biological samples for SIMS imaging

A C₆₀⁺ cluster ion projectile is employed for sputter cleaning biological surfaces to reveal spatio-chemical information obscured by contamination overlayers. This protocol is used as a supplemental sample preparation method for time of flight secondary ion mass spectrometry (ToF-SIMS) imaging of frozen and freeze-dried biological materials. Following the removal of nanometers of material from the surface using sputter cleaning, a frozen-patterned cholesterol film and a freeze-dried tissue sample were analyzed using ToF-SIMS imaging. In both experiments, the chemical information was maintained after the sputter dose, due to the minimal chemical damage caused by C₆₀⁺ bombardment. The damage to the surface produced by freeze-drying the tissue sample was found to have a greater effect on the loss of cholesterol signal than the sputter-induced damage. In addition to maintaining the chemical information, sputtering is not found to alter the spatial distribution of molecules on the surface. This approach removes artifacts that might obscure the surface chemistry of the sample and are common to many biological sample preparation schemes for ToF-SIMS imaging.     

Synthesis and characterisation of the SrxBa1−xFeO3−y-system and the fluorinated phases SrxBa1−xFeO2F

Compounds in the system Sr_xBa_1?xFeO_3?y have been prepared under different partial pressures of oxygen. In this system, different perovskite-type structures are found depending highly on the values of x and y. Fluorination using polyvinylidenedifluoride (PVDF) gives oxyfluoride materials of composition Sr_xBa_1?xFeO₂F, which normally crystallize in the cubic perovskite structure. Rietveld refinement results provide information about the packing density for oxide and oxyfluoride samples and allow a general comparison between these two different types of materials. Furthermore, the determination of the average iron oxidation state also showed that the oxygen deficiency, y, depends significantly on the value of x and the structure determined by the Sr/Ba ratio.     

Geometry of an Isolated Dimer of Imidazole Characterised by Rotational Spectroscopy and Ab Initio Calculations

An isolated, gas‐phase dimer of imidazole is generated through laser vaporisation of a solid rod containing a 1:1 mixture of imidazole and copper in the presence of an argon buffer gas undergoing supersonic expansion. The complex is characterised through broadband rotational spectroscopy and is shown to have a twisted, hydrogen‐bonded geometry. Calculations at the CCSD(T)(F12*)/cc‐pVDZ‐F12 level of theory confirm this to be the lowest‐energy conformer of the imidazole dimer. The distance between the respective centres of mass of the imidazole monomer subunits is determined to be 5.2751(1) Å, and the twist angle γ describing rotation of one monomer with respect to the other about a line connecting the centres of mass of the monomers is determined to be 87.9(4)°. Four out of six intermolecular parameters in the model geometry are precisely determined from the experimental rotational constants and are consistent with results calculated ab initio.     

Cis→Trans Isomerization of Pro<Superscript>7</Superscript> in Oxytocin Regulates Zn<Superscript>2+</Superscript> Binding

Ion mobility/mass spectrometry techniques are employed to investigate the binding of Zn²⁺ to the nine-residue peptide hormone oxytocin (OT, Cys¹-Tyr²-Ile³-Gln⁴-Asn⁵-Cys⁶-Pro⁷-Leu⁸-Gly⁹-NH₂, having a disulfide bond between Cys¹ and Cys⁶ residues). Zn²⁺ binding to OT is known to increase the affinity of OT for its receptor [Pearlmutter, A. F., Soloff, M. S.: Characterization of the metal ion requirement for oxytocin-receptor interaction in rat mammary gland membranes. J. Biol. Chem. 254, 3899–3906 (1979)]. In the absence of Zn²⁺, we find evidence for two primary OT conformations, which arise because the Cys⁶–Pro⁷ peptide bond exists in both the trans- and cis-configurations. Upon addition of Zn²⁺, we determine binding constants in water of K_A = 1.43 ± 0.24 and 0.42 ± 0.12 μM^?1, for the trans- and cis-configured populations, respectively. The Zn²⁺ bound form of OT, having a cross section of Ω = 235 Ų, has Pro⁷ in the trans-configuration, which agrees with a prior report [Wyttenbach, T., Liu, D., Bowers, M. T.: Interactions of the hormone oxytocin with divalent metal ions. J. Am. Chem. Soc. 130, 5993–6000 (2008)], in which it was proposed that Zn²⁺ binds to the peptide ring and is further coordinated by interaction of the C-terminal, Pro⁷-Leu⁸-Gly⁹-NH₂, tail. The present work shows that the cis-configuration of OT isomerizes to the trans-configuration upon binding Zn²⁺. In this way, the proline residue regulates Zn²⁺ binding to OT and, hence, is important in receptor binding.

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